fdus 888 final draft uganda standard€¦ · fdus 888: 2009 iv © unbs 2009 – all rights reserved...

FINAL DRAFT UGANDA

STANDARD

FDUS 888

First Edition2009-mm-dd

Reference numbe

rFDUS 888: 2009

© UNBS 2009

Code of practice – Solar heating systems for swimming pools

FDUS 888: 2009

ii © UNBS 2009 – All rights reserved

Compliance with this standard does not, of itself confer immunity from legal obligations

A Uganda Standard does not purport to include all necessary provisions of a contract. Users are responsible for its correct application

© UNBS 2009

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying and microfilm, without prior written permission from UNBS.

Requests for permission to reproduce this document should be addressed to

The Executive Director Uganda National Bureau of Standards P.O. Box 6329 Kampala Uganda Tel: 256 41 4505995 Fax: 256 41 4286 123 E-mail: [email protected] Web: www.unbs.go.ug

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© UNBS 2009 – All rights reserved iii

Contents Page

Foreword………………………………………………………………………………………………..…………iv

1 Scope....................................................................................................................................................1 2 Normative references...........................................................................................................................1 3 Definitions .............................................................................................................................................3 4 Relevant statutory requirements .......................................................................................................5 5 Components.........................................................................................................................................6 6 System design .....................................................................................................................................9 6.1 General .................................................................................................................................................9 6.2 Design considerations........................................................................................................................9 6.3 Typical system designs ....................................................................................................................12 6.4 Collector location ..............................................................................................................................14 7 Thermal performance........................................................................................................................15 8 Electrical considerations..................................................................................................................19 9 Installation..........................................................................................................................................20 9.1 General ................................................................................................................................................20 9.2 Pre-installation checks ......................................................................................................................20 9.3 Plumbing and pipework considerations ..........................................................................................21 9.4 Connections to existing filtration system........................................................................................21 9.5 Special considerations ......................................................................................................................22 9.6 Heat loss mechanisms......................................................................................................................22 9.7 Passive pool heating..........................................................................................................................23 9.8 Active pool heating ............................................................................................................................25 9.9 Plumbing schematics.........................................................................................................................269.10 Piping…………………………………………………………………………………………………………..43 9.11 Flow control and safety devices .......................................................................................................44 9.12 Instructing the homeowner ...............................................................................................................46 10. Commissioning, handover and documentation..............................................................................47 Annex A Details of the model system referred to in clause 6 for thermal performance...........................50 Bibliography....................................................................................................................................................503

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iv © UNBS 2009 – All rights reserved

Foreword

Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry of Tourism, Trade and Industry established under Cap 323, of the Laws of Uganda. UNBS is mandated to co-ordinate the elaboration of standards and is (a) a member of International Organisation for Standardisation (ISO) and

(b) a contact point for the WHO/FAO Codex Alimentarius Commission on Food Standards, and

(c) the National Enquiry Point on TBT/SPS Agreements of the World Trade Organisation (WTO).

The work of preparing Uganda Standards is carried out through Technical Committees. A Technical Committee is established to deliberate on standards in a given field or area and consists of representatives of consumers, traders, academicians, manufacturers, government and other stakeholders.

Draft Uganda Standards adopted by the Technical Committee are widely circulated to stakeholders and the general public for comments. The committee reviews the comments before recommending the draft standards for approval and declaration as Uganda Standards by the National Standards Council.

Committee membership

The following organisations were represented on Subcommittee on Solar Energy, UNBS TC 13/SC 3, in the preparation of this standard:

• Renewable Energy Department, Ministry of Energy and Mineral Development

• Makerere University

• Incafex Solar Systems

• Battery Masters (U) Limited

• Equator-sun (U) Limited

• Ultratec (U) Limited

• Sonnerkraft Solar Systems

• Solar Masters

• Uganda National Plumbers Association

• Uganda Institute of Professional Engineers

• Uganda National Bureau of Standards

FINAL DRAFT UGANDA STANDARD FDUS 888: 2009

© UNBS 2009– All rights reserved 1

Code of practice – Solar heating systems for swimming pools

1 Scope

This Uganda Standard code gives recommendations and guidance for the design, performance, installation and commissioning of solar heating systems for indoor and outdoor swimming pools.

Brief consideration is given to the thermal properties of pool covers.

The code does not deal with the filtration systems for swimming pools to which solar heating systems are often connected.

2 Normative references

The following referenced documents are indispensable for the application of this Uganda Standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 9059:1990, Solar energy — Calibration of field pyrheliometers by comparison to a reference pyrheliometer

ISO 9060:1990, Solar energy — Specification and classification of instruments for measuring hemispherical solar and direct solar radiation

ISO 9459-1:1993, Solar heating — Domestic water heating systems — Part 1: Performance rating procedure using indoor test methods

ISO 9459-2:1995, Solar heating — Domestic water heating systems — Part 2: Outdoor test methods for system performance characterization and yearly performance prediction of solar-only systems

ISO 9459-5:2007, Solar heating — Domestic water heating systems — Part 5: System performance characterization by means of whole-system tests and computer simulation

ISO 9488:1999, Solar energy — Vocabulary

ISO 9553:1997, Solar energy — Methods of testing preformed rubber seals and sealing compounds used in collectors

ISO 9806-1:1994, Test methods for solar collectors — Part 1: Thermal performance of glazed liquid heating collectors including pressure drop

ISO 9806-2:1995, Test methods for solar collectors — Part 2: Qualification test procedures

ISO 9806-3:1995, Test methods for solar collectors — Part 3: Thermal performance of unglazed liquid heating collectors (sensible heat transfer only) including pressure drop

ISO 9808:1990, Solar water heaters — Elastomeric materials for absorbers, connecting pipes and fittings —Method of assessment

ISO 9845-1:1992, Solar energy — Reference solar spectral irradiance at the ground at different receiving conditions — Part 1: Direct normal and hemispherical solar irradiance for air mass 1.5

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2 © UNBS 2009 – All rights reserved

ISO 9846:1993, Solar energy — Calibration of a pyranometer using a pyrheliometer

ISO 9847:1992, Solar energy — Calibration of field pyranometers by comparison to a reference pyranometer

ISO/TR 9901:1990, Solar energy — Field pyranometers — Recommended practice for use

ISO/TR 10217:1989, Solar energy — Water heating systems — Guide to material selection with regard to internal corrosion

FDUS 854-1:2009, Thermal solar systems and components — Solar collectors — General requirements

FDUS 854-2:2009, Thermal solar systems and components — Solar collectors — Test methods

FDUS 855-1:2009, Thermal solar systems and components — Factory made systems —General requirements

FDUS 855-2:2009, Thermal solar systems components — Factory made systems — Test methods

FDUS 885:2009, Standard guide for on-site inspection and verification of operation of solar hot water systems

ASTM B42:2002, Standard specification for seamless copper pipe, standard sizes

ISO 7598:1988, Stainless steel tubes suitable for screwing in accordance with ISO 7-1

ISO 49:1994, Malleable cast iron fittings threaded to ISO 7-1

ISO 4144:2003, Pipework — Stainless steel fittings threaded in accordance with ISO 7-1

ASTM A126:2004, Standard specification for gray iron castings for valves, flanges, and pipe fittings

IEC 60364 (All parts), Low-voltage electrical installations

ASTM E119:2007, Standard test methods for fire tests of building construction and materials

ASTM E861:1994(2007), Standard practice for evaluating thermal insulation materials for use in solar collectors

ISO 9774:2004, Thermal insulation for building applications — Guidelines for selecting properties

ISO 13787:2003, Thermal insulation products for building equipment and industrial installations —Determination of declared thermal conductivity

CISPR 14-1, Electromagnetic compatibility — Requirements for household appliances, electric tools and similar apparatus — Part 1: Emission

CISPR 14-2, Electromagnetic compatibility — Requirements for household appliances, electric tools and similar apparatus — Part 2: Immunity — Product family standard

ISO 9906:1999, Rotodynamic pumps — Hydraulic performance acceptance tests — Grades 1 and 2

ISO 15783:2002, Seal-less rotodynamic pumps — Class II — Specification

ISO 9908:1993, Technical specifications for centrifugal pumps — Class III

ISO 9905:1994, Technical specifications for centrifugal pumps — Class I

BS 1565-2, Galvanized mild steel indirect cylinders, annular or saddle-back type — Part 2: Metric units

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© UNBS 2009 – All rights reserved 3

BS 1566-1, Copper indirect cylinders for domestic purposes — Part 1: Specification for double feed indirect cylinders

BS 1566-2, Copper indirect cylinders for domestic purposes — Part 2: Specification for single feed indirect cylinders

BS EN 1057, Copper and copper alloys — Seamless, round copper tubes for water and gas in sanitary and heating applications

BS 3198, Specification for copper hot water storage combination units for domestic purposes

ISO 4427-1:2007, Plastics piping systems — Polyethylene (PE) pipes and fittings for water supply — Part 1: General

ISO 4427-2:2007, Plastics piping systems — Polyethylene (PE) pipes and fittings for water supply — Part 2: Pipes

ISO 4427-5:2007, Plastics piping systems — Polyethylene (PE) pipes and fittings for water supply — Part 5: Fitness for purpose of the system

EAS 205, Controls for household appliances

ISO 16422:2006, Pipes and joints made of oriented unplasticized poly(vinyl chloride) (PVC-U) for the conveyance of water under pressure — Specifications

BS 4814, Specification for expansion vessels using an internal diaphragm, for sealed hot water heating systems

ISO 15874-1:2003, Plastics piping systems for hot and cold water installations — Polypropylene (PP) — Part 1: General

ISO 15874-2:2003, Plastics piping systems for hot and cold water installations — Polypropylene (PP) — Part 2: Pipes

ISO 15874-5:2003, Plastics piping systems for hot and cold water installations — Polypropylene (PP) — Part 5: Fitness for purpose of the system

ISO 6242-1:1992, Building construction — Expression of users' requirements — Part 1: Thermal requirements

ISO 6242-2:1992, Building construction — Expression of users' requirements — Part 2: Air purity requirements

ISO 15493, Plastics piping systems for industrial applications — Acrylonitrile-butadiene-styrene (ABS), unplasticized poly(vinyl chloride) (PVC-U) and chlorinated poly(vinyl chloride) (PVC-C) — Specifications for components and the system — Metric series

IEC 61000-3-2, Electromagnetic compatibility (EMC) — Part 3-2: Limits — Limits for harmonic current emissions (equipment input current ≤ 16 A per phase)

3 Definitions

For the purposes of this code the following definitions apply.

3.1 collector (solar collector, solar panel) the general term for a device in which solar radiation is absorbed and converted to heat which is removed by the heat transfer fluid.

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3.2 flat plate collector a collector that employs no concentration of the incident solar radiation and in which the absorbed plate is essentially planar.

3.3 embedded collector a collector in which the fluid passages are embedded either in the ground or within a covering such as paving slabs, asphalt or concrete

3.4 trickle collector a flat plate collector in which the heat transfer fluid is not contained within passageways in the absorber plate but flows down the plate surface

3.5 absorber plate (absorber) the element of a collector that receives and absorbs the solar radiation and converts it into heat

3.6 absorber plate surface coating a coating whose principal function is to absorb solar radiation

3.7 selective surface an absorber plate surface coating that will decrease the radiative emission from the absorber plate whilst maintaining a high absorptance for solar radiation

3.8 unglazed collector a collector with the front surface of the absorber plate exposed to the surrounding air. The rear surface may or may not be insulated

3.9 glazed collector a collector with an absorber plate covered by a translucent glazing material. The rear of the absorber plate will normally be insulated within a weatherproof envelope

3.10 direct system a system in which the pool water passes through the solar collectors

3.11 indirect system a system in which a fluid other than the swimming pool water passes through the solar collectors

3.12 integrated circuit a system in which the solar collectors form part of the same pipework circuit as the pool filtration plant

3.13 separate circuit a system in which solar heating circuit is completely separated from the pool filtration circuit

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3.14 drainback system a system in which as part of the normal working cycle the collector is automatically drained and refilled

3.15 draindown system a system within which heat transfer fluid is retained until manual draining takes place

3.16 differential temperature controller a device that is able to detect a small temperature difference and control pumps and other electrical devices in accordance with this temperature difference

3.17 pool inlet the point at which water from the filtration circuit is returned to the pool, generally by means of an inlet nozzle

3.18 pool outlets the points at which water is drawn from the pool to be filtered, generally from a sump outlet at the lowest point in the pool and from a skimmer outlet at the pool surface

4 Relevant statutory requirements

4.1 General

There are statutory requirements that have to be observed before the installation of any swimming pool. As these requirements may vary slightly between different parts of the country, the relevant local authority should always be consulted regarding planning and building regulations and, likewise, the local supply authority regarding water supply requirements.

4.2 Planning

As a general rule, permission has to be obtained from the local planning authority before carrying put any development. Consequently, it should be ascertained from the local planning authority as to whether or not the proposed installation constitutes development.

4.3 Building regulations

General regulations have been enacted concerning the design and construction of buildings. Solar heating systems incorporated into buildings have to comply with these regulations in so far as they affect matters of construction, roof loading, weather tightness, fire resistance, insulation, etc.

Advice on some of these matters is given elsewhere in this code. However, responsibility for the application of the regulations in a particular area rests with the local authority, which may require plans to be deposited, showing how it is proposed to comply with the regulations.

4.4 Water supply

Water supply byelaws for preventing waste, undue consumption, misuse or contamination of water supplies have been made by the Water Authorities and Companies. These byelaws require that written notice is given to the local water authority before installing or altering (except for repair or replacement) any water fitting used or to be used in connection with an existing supply of water from the undertaking.

It should be noted that whilst the various sets of byelaws are identical except for a few minor respects, the interpretation and enforcement of byelaws rests with the particular water supply authority concerned. Bearing

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this in mind, it is considered that to attempt to give meaningful detailed guidance on the application of water byelaws in this code could be misleading. However, it can be assumed that where a solar heating system is to be used to heat water for domestic use, as well as to heat swimming pool water, then the recommendations contained in US 853 should be taken into account.

The application of the water byelaws will depend on for example, whether the solar heating system (for pool water only) is direct or indirect, whether the heat transfer fluid is pool water, potable water or a non-aqueous fluid and how the make up water to the pool and/or solar heating system is supplied. It is therefore recommended that early contact is made with the local water undertaking to discuss the proposed installation and to seek advice.

4.5 Other actions

In addition to complying with the legal requirements detailed in 4.1 to 4.4, it is recommended that the occupier/ owner inform the lessors, mortgagors, insurers, etc. of the property as applicable.

5 Components

5.1 General

This clause describes the principal components used in solar heating systems for swimming pools.

5.2 Collectors

5.2.1 Types of collector

Solar collectors intended for swimming pool applications are commonly of the flat plate variety but they may or may not be glazed and insulated.

Embedded and trickle collectors may be used.

Collectors are designed so that a heat transfer fluid, often the swimming pool water itself, can pass through the collector in close thermal contact with a matt black or similar heat absorbing surface. When the material used between the fluid passages is a good conductor of heat, e.g. copper, the fluid passages can be spaced apart. When a poorer heat conductor is used, it is important to bring the fluid passages closer together.

Table 1 – Some relevant for components and fittings

Feed and expansion cisterns and expansion vessels BS 417, BS 4213, BS 4814

Tanks and cylinders BS 1566-1, BS 1565, BS 3198

Pumps ISO 9906

Valves BS EN 12288

Pipework and fittings

Standards applicable to pipes and pipe fittings include the following

Form and material Designation Type and application

Tubes Copper

Plastics

BS EN 1057

Hot and cold water services

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ABS PP Unplasticized PVC HDPE

Stainless steel

ISO 15493 ISO 15874 ISO 4422 ISO 4427

ISO 7598

Cold water services Hot and cold water services Cold water services Cold water services

Hot and cold water services Fittings Copper and copper alloy

Malleable cast iron and cast copper alloy Unplasticized PVC ABS

BS 864-2

ISO 49 and ISO 4144

ISO 16422 ISO 727-1

(Capillary and compression) Hot and cold water services (compression) Cold water services (screwed pipe fittings) Hot and cold water services Cold water services Cold water services

5.2.2 Selection of collector type

Solar collectors used for pool heating can be installed without glazing or a similar translucent cover in front of the absorber if they operate close to the ambient temperature.

Glazing reduces the radiation incident on the absorber and this effect may outweigh the reduction in heat loss from the front of the collector. Similarly rear insulation may marginally improve the performance but the benefit may be too small to warrant the additional cost.

Unglazed and uninsulated collectors preferably should be mounted in a position that is sheltered from strong prevailing winds. If such a site is not available consideration should be given to the use of glazed and insulated collectors. In the case of pools maintained at temperatures appreciably above the ambient temperature the incorporation of front glazing gives better thermal performance for the same collector area, but the improvement may not justify the extra cost involved. The use of double glazing on the absorber is not likely to be worthwhile.

In indirect systems the use of glazed and insulated collectors may be appropriate because of the temperature differential across the heat exchanger. For the same reason the use of a selective surface may be advantageous.

Trickle collectors may have the advantage of being cheap but if glazed may have a reduced performance due to the build up of algae. Evaporative heat loss from unglazed trickle collectors may substantially reduce their performance.

NOTE Other forms of collector, e.g. concentrating, evacuated and tracking collectors, are not dealt with in this standard because these types are primarily used for higher temperature applications and insufficient experience is currently available about their use for pool heating.

5.2.3 Selection of materials

In direct systems it is important to select materials for the fluid passages that are suitable for contact with swimming pool water. The materials used should neither contaminate the pool water nor should they become corroded under normal service conditions.

Plastics materials, such as polypropylene, polyethylene or EPDM, are generally suitable for contact with pool water and solar collectors manufactured from these materials are available. Black pigmented material is normally utilized and it should be ensured that the material is stabilized against degradation by ultraviolet light.

Copper can also be used for the fluid passages in direct circulation solar collectors but it is important to maintain the pH of the pool water at between 7.2 and 7.6 or above in order to prevent corrosion. This, together with the correct total alkalinity and free residual chlorine level, will prevent undue corrosion/erosion of copper pipework at flow velocities up to 1.5 m/s.

Iron and carbon steel are unsuitable for the fluid passages because there may be rapid corrosion resulting in the failure of the collectors and rust staining of the pool walls and fittings. It is important to recognize that not all grades of stainless steel will resist corrosion in these applications and grade 316 type is recommended.

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Some aluminium alloys are not suitable for direct contact with pool water because of the probability of corrosion.

Adequate steps should always be taken to guard against bimetallic corrosion and pitting corrosion. For example, in indirect drainback systems (see Figure 3) it is important not to use galvanized steel cisterns combined with copper pipework since pitting corrosion of the steel may occur. The risk is particularly high in systems where the heat transfer fluid is highly aerated.

Reference should be made to US 853 regarding corrosion protection and the use of heat transfer fluids other than pool water in indirect swimming pool solar heating systems.

5.3 Controls

5.3.1 A control system is an important part of the swimming pool solar heating installation. The purpose of the control is to ensure that fluid is pumped through the collectors only when heat can be gained.

5.3.2 In direct systems it is usual to incorporate the solar collector circuit into the existing filtration circuit. This minimizes the total pumping power used for pool filtration and for solar collection. It is often cheaper and more convenient than using a separate circuit for the solar collectors (see 6.2.1).

The filtration pump is normally required to run during times when there may be no solar heat gain. It is therefore preferable to be able to run the filtration plant either continuously or on a time switch and to install the means of diverting the pool water through the solar collector circuit whenever a heat gain is available. This can be achieved by using a valve to divert the flow through the solar collector after filtration. Thus the water has to pass through the solar collectors and then back to the pool.

Alternatively, an additional pump may be used in the solar collector circuit which, when activated, draws filtered water and returns it to a point further downstream from where it can return in the normal way to the pool.

5.3.3 Whether a diverting valve or a pump is used, a control system is required to ensure that the pool water is only circulated through the collectors when there is a net heat gain available. An electronic differential temperature controller incorporating a pair of temperature sensors is usually used to activate the solar pump or diverting valve. One of these sensors detects the pool temperature while the other detects the temperature of a section of the absorber plate which is exposed to solar radiation but is thermally insulated and remote from a fluid passage. A collector sensor mounted in the fluid outlet pipe from a swimming pool solar collector may not be satisfactory because it may not adequately detect the relatively small temperature rises which can be considered as useful in a high flow rate system. The pool temperature sensor is normally mounted in close thermal contact with the water in the filtration circuit prior to the branch to the solar collectors (see 7.7).

NOTE In this context the net heat gain is achieved when the value of energy collected exceeds the cost of energy expended by a separate circulating pump. The temperature differential at which the controller turns the system off should therefore take account of any pump energy consumption. Since the relevant temperature differential is likely to be small it is important to select a controller with limited temperature drift characteristics.

5.4 Pool covers

5.4.1 Heat losses from swimming pools occur mainly from the water surface and various types of cover are available to reduce these losses in both indoor and outdoor pools. Covers can be regarded as a useful energy conservation measure with any type of pool and will enable many pools to function more efficiently as natural collectors of solar radiation.

NOTE Manufacturers, suppliers and installers should advice the swimming pool owners on the use and benefits of the pool cover.

5.4.2 Various types of floating pool cover can be used including the following types:

a) single skin plastics film;

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b) double skin plastics film with encapsulated air bubbles;

c) closed cell plastics foam.

d) liquid bio-degradable foam

All of these types are available in either translucent or opaque grades but the plastics foams are frequently supplied laminated onto an opaque woven material. Other covers are available which are stretched across the surface of the pool above the water level.

Covers are moved on and off the pool many times each season. Any pool cover should be sufficiently tough to allow necessary handling. Materials used for covers for open air pools should be adequately resistant to both ultraviolet radiation and to chemicals normally present in swimming pools.

The main function of a cover is to reduce or eliminate evaporation from the surface of the pool. All of the cover types mentioned are effective in this respect since they form a vapour barrier across the top surface of the pool. Any water lying on the top of the cover will reduce its effectiveness. With covers that are suspended above the water it is important to ensure that the edges are reasonably airtight since otherwise water vapour will escape.

The second function of the cover is to prevent heat loss by convection. The single film covers are the least effective in this respect.

The third function is to reduce the radiation heat loss from the swimming pool. This is the least significant heat loss from the surface of the pool.

For a pool that receives sunshine and where the cover may be in place for even only a few daylight hours it is advantageous to select a translucent cover. With such covers there can still be a very significant radiation heat gain to the pool. Sunlight that passes through the cover is largely absorbed by the pool water itself. The water can thus be heated naturally in the same way as with an uncovered pool but with the great advantage that the heat losses from the top surface are substantially reduced.

5.4.3 Safety is an important consideration as most covers cannot support the weight of a child or pet animal. Due to the risk of drowning, no one should swim beneath a cover. This is particularly important with floating covers.

6 System design

6.1 General

Principal design features of swimming pool solar heating systems are often determined by:

a) the type of pool (indoor or outdoor);

b) the intended period of use;

c) possible locations for the collectors, in particular their height relative to the pool surface.

In addition, the choice between a direct or indirect system is fundamental.

Whereas satisfactory system design details may in principle be determined by calculation, it is considered helpful to summarize the features that have been found to be crucial to successful operation and to give a brief description of the most popular system types.

6.2 Design considerations

6.2.1 Interaction with existing equipment

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Any connection between swimming pool solar heating systems and existing filtration equipment has to be such as to ensure continuing satisfactory performance under all operating conditions.

A reduced flow rate through the filtration system may result in inadequate filtration as well as poor mixing of the water in the pool. This could contribute towards increased thermal stratification in the pool with a resultant increase in heat loss from the pool. Moreover care should be taken to ensure that there is no short circuiting of water between the inlets and outlets in the pool which could be caused by a reduced flow rate at the pool inlet nozzles.

6.2.2 Collector drawback and draindown

Many swimming pool solar heating systems need to be designed to ensure that the collectors (and often other exposed components also) may be fully drained under some conditions. Failure of components to be drained satisfactorily may, depending upon their design, result in extensive damage. It is therefore important both that the initial system design is correct and in accordance with manufacturer's recommendations and that users are aware of what constitutes satisfactory behavior.

Two common situations are as follows.

a) Systems in which collectors are located above pool level and drain whenever heat transfer fluid (usually pool water) is not delivered to them under pressure. This is the "drainback" mode of operation and is used typically to provide automatic frost protection or, in the case of some glazed collectors, protection against boiling.

b) Systems in which heat transfer fluid may be retained within collectors for long periods but in which manual draining to preclude frost damage is necessary usually at the end of each swimming season (see 10.7).

In order to achieve satisfactory drainback careful system design is required and the following points should be considered.

1) A suitable air admittance device has to be fitted (see 6.2.3).

2) There has to be an unobstructed route for water to return to the pool by gravity. The closed port of a 3-way valve can prevent water draining back from the collectors.

3) Water should be prevented from returning to the pool by means of reverse flow through the filter since this may backwash part of the debris collected by the filter into the pool. Unless the filtration pump is already fitted with a device to prevent reverse flow, a non-return valve should be fitted in the filtration circuit.

4) The pressure head in the filtration circuit may prevent complete drainback of the solar collector circuit. Any parts of the solar circuit that will not be drained as a result of this should be otherwise protected against frost damage (e.g. by being located indoors).

5) Means should be provided for checking whether the system drains back as intended, e.g. by fitting a drain valve which would show the presence of water if opened [see 10.2 f].

Whether automatic drainback or manual draindown is intended all pipes including collector header pipes and fluid passages within the collectors should be laid to adequate falls to allow complete draining e.g. a minimum of 1 in 200. Drain-off valves should be provided at any low points in the circuit that will not drain by gravity and stop valves should be provided to isolate any parts of the circuit that are designed to be left drained during extremely low temperatures from parts that may be left operational.

6.2.3 Pressures in direct circuits

There are particular considerations regarding pressures in direct circulation systems when solar collectors are installed above the pool water level. The water at the pool surface will be at atmospheric pressure so direct solar heating circuits above pool water level will be at sub-atmospheric (negative) pressures unless

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maintained at a positive pressure by a pump. The potential reduction in pressure below atmospheric pressure is dependent on the height of the circuit above the pool.

When collectors are installed up to about 1 m above pool water level the entire circuit is likely to be maintained at positive pressure when the system circulating pump is running and will only be at negative pressure when the pump is switched off. In such cases the circuit may be designed to drain back into the pool whenever the pump is switched off if an air admittance device, such as a suction relief valve, is incorporated into a high point of the circuit. Automatic air vents used for this purpose may not be relied upon to open, especially under low negative pressure, if of a type held closed by flotation or by spring action.

Where collectors are mounted at higher levels there may be negative pressures in the upper parts of the circuit even when the circulating pump is operating and there is a satisfactory flow rate through the solar collectors. In order to prevent air entrainment which may lead to air locking these parts of the circuit have to be airtight.

In such cases drainback cannot be facilitated by means of a suction relief valve unless it can be ensured that the point in the circuit at which this device is fitted is always maintained at a positive pressure while the circulating pump is operating. This may be achieved by fitting the air admittance device upstream of a restrictor valve positioned close to the highest point in the circuit but a significantly higher pump loading may result.

Alternatively, an air admittance device in the form of an electrically operated "normally open" valve wired in parallel with the pump may be used in a circuit that is otherwise airtight. In this case particular care has to be taken to ensure that air in the circuit will be expelled on refill by virtue of water velocity and the return pipework should be sized accordingly.

If frost protection is to be achieved by manual draining an automatic air admittance device need not be fitted but the system components have to be specified to withstand sub-atmospheric (negative) pressures.

6.2.4 Circulation of heat transfer fluid

In direct solar heating circuits the heat transfer fluid (pool water) may be circulated through the collectors by either the filtration pump or a separate pump. In the latter case the solar heating circuit may either be connected to the filtration circuit or be remote from it with separate inlet and outlet connections to the pool.

In indirect systems optimum flow rates will depend much on the detailed efficiency and pressure loss characteristics of the heat exchanger. Whilst the figures given can serve as a guide, manufacturers' recommendations should be studied, also with a view to ensuring that neither the collector nor heat exchanger efficiency is unduly compromised under typical working conditions. Similarly, pumping power should be considered at an early stage in design.

The efficiency of all thermal solar collectors decreases with increasing operating temperature and this is particularly severe for unglazed units. It is therefore important that the flow rate of heat transfer fluid is sufficiently high to help ensure efficient operation. In practice a flow rate for water of 0.04 kg/(m2.s) of collector is usually satisfactory; flow rates above 0.06 kg/(m2.s) produce little additional benefit and will incur higher pumping energy requirements. Glazed collectors can work with little loss of efficiency at lower flow rates, typically 0.02 kg/(m2.s) to 0.04 kg/(m2.s) for water.

6.2.5 Pipe sizing and distribution

It is particularly important to ensure balanced flow distribution to all collectors within a pool heating system, especially if unglazed collectors are used. In addition to the usual calculations for pressure loss along flow and return pipes the following special factors should be considered at the design stage:

a) alternative collector interconnection schemes such as reverse return pipework layouts, having regard to manufacturers' recommendations;

b) the possible need to install balancing (restrictor) valves especially if banks of collectors are to be sited at different heights in a drainback system;

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c) the need to ensure positive subsequent filling of all collectors following draining, e.g. by ensuring that air cannot be trapped in one or more banks of collectors;

d) the need to size pumps for drainback systems to overcome the total static lift in addition to overcoming the frictional losses in the circuit.

6.2.6 Direct circuits

Pool water can be contaminated with suspended solids and other debris which could block solar collectors and associated pipework. It is therefore important to ensure that only filtered water is passed through the solar collector circuit. This is easily achieved in the case of solar circuits connected to the filtration system, provided that water is diverted to the collectors after the filter. However, in circuits not integrated with the filtration system it is necessary to provide an adequate level of filtration prior to circulation through the solar collectors.

This may be achieved by fitting a suitable mesh strainer over the outlet connection from the pool or by fitting an in-line strainer elsewhere in the circuit but the design has to allow for access for cleaning or provision be made for back-washing. Such pool outlet connections are best kept clear of the sump of the pool or the water surface since suspended matter and debris tends to accumulate at these locations.

6.2.7 Indirect circuits

In indirect circuits the pool water is passed through the secondary side of a heat exchanger located in the filtration circuit. The solar collectors are connected to the primary side of the heat exchanger and heat transfer fluid is circulated by means of a separate pump.

The heat transfer fluid used in the primary circuit may contain a suitable corrosion inhibitor and/or anti-freeze solution to provide frost protection. In sealed circuits heat exchange oils may also be used. Manufacturers' advice regarding the use of dissimilar metals and the suitability of components such as pumps should be sought particularly when using non-aqueous fluids.

It is recommended that the corrosion inhibitor and/or anti-freeze solution should be non-toxic and also contain non-toxic biocide compounds to prevent bacteria and algae growth in the primary heat transfer fluid.

6.3 Typical system designs

6.3.1 Direct circulation with separate pump not connected to filtration circuit

Flow and return connections to the pool should be positioned to ensure good mixing with the pool water. The pump may be either located below the pool water level so that it is kept full of water at all times (and protected from frost by being located indoors or by manual draining in extremely low temperature period) or be self-priming.

Since these systems are not connected to the filtration circuit automatic drainback may be achieved conveniently in most situations.

6.3.2 Direct circulation with separate pump integrated with filtration circuit

A typical circuit is shown schematically in Figure 1. The solar pump is positioned so as to draw filtered water from the filtration circuit and therefore the pump does not normally need to be self-priming. The connection from the solar collector circuit should be positioned so as to introduce solar heated water ahead of any chemical dosing equipment or auxiliary heating plant. Drainback for frost protection may be achieved provided that the pressure head in the filtration circuit is insufficient to hold water in exposed parts of the solar heating system.

6.3.3 Direct circulation with flow diversion by 3-port valve

A typical circuit is shown schematically in Figure 2. A 3-port valve may be installed in the pool filtration circuit with one of the two outlet ports connected to allow circulation through the solar collectors.

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The pipe from the solar collectors should be connected into the filtration circuit such that flow is maintained through any dosing equipment or auxiliary heater.

In order to provide automatic control 3-port valves are normally fitted with an actuator motor connected to the solar heating control system. Since the outlet port to the solar collectors will normally be closed when the filtration pump is switched off (often by a time switch) a drainback system will only function satisfactorily if a means to bypass the closed port of the motorized valve is introduced.

Figure 1- Direct circulation with separate pump integrated with filtration circuit

6.3.4 Indirect circuits

Three typical types of indirect circuit are as follows. In each case the solar collectors should be connected to the primary side of a suitably sized heat exchanger with the pool water passing through the secondary side (see 7.6).

a) Drainback circuits. These incorporate a drainback cistern situated in a location secure from frost and below collector level but above the level of the circulating pump (see Figure 3). An air admittance device which may take the form of an air break at the drainback cistern should be installed in the circuit. A float operated valve usually controls the level of the water in the cistern and it should be recognized that the float may become submerged during the drainback condition. It is important that the cistern has sufficient reserve capacity that the level does not reach either the cistern overflow or the inlet valve. Such systems can provide frost and boiling protection to the solar collectors.

b) Sealed and pressurized circuits. These incorporate conventional pressurized sealed circuit equipment such as expansion vessels and pressure relief valves. Such circuits allow great flexibility to the designer regarding acceptable collector positions in terms of height relative to the pool. The use of a suitable heat transfer fluid may remove the risk of freezing or boiling.

c) Feed and vent cistern circuits. These may be designed in a similar way to conventional forced circulation central heating systems. The cistern should be located above the solar collectors,; so this type

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of system is particularly suitable where solar collectors are mounted at low level. The use of a suitable heat transfer fluid can offer frost protection. The venting arrangement should be designed to allow for the discharge of any generated steam.

6.4 Collector location

6.4.1 General

The most suitable location for the solar collectors should be determined by considering the implications on thermal performance, pipework connections and the visual appearance of alternative available positions but having regard to the need for access for inspection and maintenance.

Figure 2- Direct circulation with flow diversion by 3-port valve

NOTE The pipe circuit requirements are described in 6.2 and 6.3

The effect on thermal performance of different collector locations will depend on their relative exposures to both solar radiation and to wind. Generally collectors should be installed in unshaded positions orientated and inclined to intercept a maximum amount of solar radiation. Guidance on the optimum orientation and angle of tilt is given in Clause 7. The effect of exposure to wind will be more pronounced for unglazed collectors and a sheltered position is therefore to be preferred.

Pipework lengths should be kept to the minimum possible so as to reduce both pumping power requirements and heat losses. The latter will be unimportant with pipework operating at low temperatures but will become more significant as the temperature rises. Standard calculation methods are available to estimate pipework heat losses and these should be used to help determine the optimum thickness of any insulation. The thermal insulation and method of installation should comply with ISO 9774.

6.4.2 Collector fixing

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The method of fixing solar collectors has to be considered carefully taking into account the considerable forces caused by wind lift to which collector fixings may be subjected. Manufacturers' recommendations regarding fixing systems should be followed and where such fixings are to be fastened to other building structures, special attention should be paid to the design of the fixings and the load that they may place on the building structure.

Fixings should not be liable to corrode, cause rainwater leaks or work loose because of wind vibration. The advice of a suitably qualified person should be sought where appropriate.

Where fixing battens or similar items are to be used on flat or sloping roofs, these should always be spaced off the roof or otherwise arranged so that they do not interfere with the normal drainage of rainwater on the roof.

Reference should be made to US 853 for collector installation practices.

Figure 3 – Typical indirect drainback circuit

7 Thermal performance

7.1 General

The predictions of performance are based on computer simulations which are described in detail in Annex A. In these simulations, the central assumptions are that:

a) the collectors are part of a direct system; that is, the pool water passes through the collectors;

b) the collectors face south at 45° to the horizontal;

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c) there is auxiliary heating to maintain the pool water at a fixed temperature so that the temperature of the pool water entering the collector is constant.

The amount of collector area to be chosen for a given application will depend upon the heating requirements for the pool concerned. These requirements will be determined by the size of the pool, the desired water temperature and the degree to which the pool cover is used. For outdoor pools the heating requirements are also strongly affected by the location of the pool, particularly its exposure to wind. For indoor pools the temperature and humidity levels of the air in the pool hall will strongly influence the pool heating requirements.

Pool temperature (oC)

Data set: Kew 1959 to 1979

Energy integrated over January to December

NOTE The performance of collectors, particularly those of type 3, will depend on many factors and these curves should be interpreted with reference to Clause 6.

Figure 4 — Average energy output from collectors for a typical year

7.2 Collector type

It is the characteristics of the collector employed that are of primary importance in determining the thermal performance of the system. Typical values of these are given in Annex A, for three types of flat-plate collectors commonly used for solar heating swimming pools: a single-glazed insulated selective collector (collector 1), a single-glazed insulated matt black collector (collector 2); and an unglazed, uninsulated collector (collector 3). When these collectors are used, the energy output, or heat transferred to the pool water, over a typical year is as shown in Figure 4. Figure 5 shows the output over a typical swimming season, taken here to be the period May to September inclusive.

NOTE In this context a typical year is one in which the weather corresponds closely to the long-term average weather.

7.3 Pool water temperature

The performance of a solar collector, especially if it is unglazed and uninsulated, is strongly dependent on the temperature of the pool water. The effect of pool water temperature on typical energy outputs of the different types of collector is shown in Figure 4 and Figure 5.

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7.4 Positioning of the collector

The heat output varies with the orientation and tilt of the collector, which will often be determined by the site. However, the predicted variation is slight. It can be assumed, all other factors being equal, that the output will be at least 90 % of that shown in the figures if the collector faces anywhere between 30° east and 40° west of true south and is tilted from the horizontal between 20° and 50°.

Pool temperature (oC)

Data set: Kew 1959 to 1979 Energy integrated over May to September


Figure 5 — Average energy output from collectors for the period May to September inclusive

Shading from trees, buildings, etc., can produce a significant decrease in system performance, and collectors should be positioned to minimize this. Undue exposure to wind will also reduce the performance, particularly of unglazed collectors. Conversely, if unglazed collectors are mounted in a very sheltered position, energy output may be increased above that indicated.

7.5 Climate

The heat output figures quoted are annual averages calculated with the meteorological data for Kew (UK) over the 21 year period from 1959 to 1979. The change in weather from year to year may cause variations of up to 15% from the long-term averages for glazed collectors. For unglazed collectors there will be an equal or greater variation in performance from year to year.

Even in a given year, differences in performance may occur between similar systems in the same locality because of variations in local conditions, for example, exposure to wind.

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7.6 Indirect systems

For an indirect system the heat transferred to the pool water will be reduced because the temperatures in the collector circuit will be higher than for a direct system, to provide a temperature differential to operate a heat exchanger. The actual reduction for a given system would depend on the effectiveness of the heat exchanger used and will be larger if unglazed collectors are used than if glazed collectors are used. It should be noted that heat exchangers not specifically designed for low temperature differentials will prove unsuitable.

7.7 Other factors

The flow rate through the collector should be fairly high so that the temperature rise across the collector is kept low and thus the heat losses are minimized. This is especially important if collectors are unglazed. Recommendations for flow rates are given in 6.2.4.

The temperature differential between the pool temperature sensor and the collector sensor at which circulation through the collector is allowed to occur can affect the amount of energy supplied. If the flow rates recommended in 6.2.4 are used, the temperature differential settings should not be critical. However, it is suggested that the temperature difference at which circulation starts should not exceed 2 K and the temperature difference at which it is stopped should not exceed 1 K (see 5.3.3).

7.8 Collector sizing

7.8.1 General

Methods for calculating the heat requirement for indoor pools have been developed. Caution should be exercised when applying these methods for the calculation of heat losses from outdoor pools. The effect of wind speed is most significant but it is not easy to quantify due to its dependence on the amount of shelter provided around the pool. Wind breaks such as hedges or fencing improve the comfort of bathers and reduce heat losses from the pool.

7.8.2 Indoor or outdoor pools with auxiliary heating

Whilst the installed area of the collector may often be influenced by available space, a convenient starting point is to calculate the area necessary to provide all the heat required in the month for which the requirement is lowest. It can then be assured that the system will rarely produce heat that is surplus to requirements. When the average rate of heat loss from the pool is known, perhaps from previous fuel bills, Figure 6 may be used to determine an appropriate collector area. This figure refers to the calculated long-term average performance of collectors operated at Kew-UK for the month of July.

For months other than July, the heat supplied by the collector will be less than that needed to maintain the required temperature. The auxiliary heating system is used to keep the pool temperature at the design value. In that case, the contribution provided by the collector towards the heat requirement may be determined from Figure 4 or Figure 5, according to the period over which the pool is in use.

It may happen that the collector heat output is required to be known for individual months other than July, for example where systems are operated at schools or holiday camps. This may be estimated from additional figures given in Annex A.

For pools in use only from May to September there may be a significant further collector output obtained during the warm-up period (typically during the month of April). The collector should be allowed to pre-heat the pool before the pool is brought into use, with the pool cover left in position to reduce heat loss. The cost of running any pump should be considered when determining the most appropriate length of pre-heat period. The auxiliary heating system should then be turned on as late as possible, working at its full capacity to bring the pool water quickly up to the desired temperature.

7.8.3 Outdoor pools without auxiliary heating

When auxiliary heating is not provided, the pool temperature will vary both from day to day and from month to month. The variation for a given pool will depend on the local weather conditions and the amount of shelter

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provided. If outdoor swimming is desired over the May to September season, the following ratios of collector area to pool area appear to be satisfactory, provided that a pool cover is used.

Location Ratio of collector area to pool area Sheltered 0.5 Exposed 0.8

The area required for this application does not depend much on the type of collector employed, since the pool temperature is normally in the region for which the collector energy output is approximately the same for all collector types.

Pool temperature (oC) [Data set: Kew 1959 to 1979]


Figure 6 – Average energy output from collectors for July

8 Electrical considerations

8.1 General

The electrical component of the system shall be designed and installed in accordance with relevant regulations and standards. Due care and consideration should be given to the environmental conditions when selecting equipment for use.

8.2 Electrical installation

All wiring and apparatus shall be installed in accordance with the requirements of IEC 60364. All plant and equipment shall comply with relevant Uganda Standards.

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8.3 Electrical safety

The Uganda Wiring Regulations require that any wiring and apparatus are properly installed and protected to ensure safety, particularly from the effects of fire and shock. This protection is achieved by adequate insulation of all conductors and apparatus and the provision of effective earthing arrangements. It should be ensured that, in the event of a fault, the installation is automatically disconnected from the supply within 0.4 s. Additionally, protection may be provided by the installation of an appropriate type of residual current circuit breaker.

8.4 Controls

Controls shall be in accordance with the requirements set out in EAS 205.

8.5 Avoidance of electrical interference

Where applicable, the requirements given in IEC CISPR 14 and IEC 61000-3-2 shall be complied with, to avoid interference with other systems.

8.6 Testing

All electrical wiring and apparatus associated with a swimming pool solar heating system has to be inspected and tested to confirm the correct polarity, the effectiveness of the earthing and the adequacy of the insulation. Residual current circuit breakers should be tested regularly and an advisory notice to this effect prominently displayed.

9 Installation

9.1 General

Designers of solar heating systems for swimming pools should supply sufficient information to the installer to enable satisfactory installation and commissioning. It should not be assumed that the installer has specialist knowledge beyond that of general plumbing and heating practice.

A record should be kept by the installer of any necessary design or layout changes agreed with the purchaser.

9.2 Pre-installation checks

The client or his agent should confirm to the installer that clearance of the statutory requirements has been met prior to commencement of installation. The installation contractor should ensure that he has all necessary system design information including:

a) the required location of the solar collectors together with mounting or fixing details as appropriate;

b) design details relating to the height of the collectors above pool level and any necessary arrangements for air purging and/or draining of the collectors;

c) details of the pipework layout, with particular regard to interconnection to any existing system and to the factors considered in 6.2.5;

d) instructions for setting up any balancing valves so as to ensure an evenly distributed flow of fluid through the collectors;

e) in an indirect system, the specified type, source of supply and required concentration of heat transfer fluid, together with any special system cleaning, testing or filling procedures;

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f) details of electrical works including control and earthing arrangements for both existing and new equipment.

It should be checked that the proposed collector arrangement and pipework routes are practicable and that all components for the installation are available. It should also be checked that the existing pool filtration and circulation equipment are in good working order.

9.3 Plumbing and pipework considerations

The manufacturer's recommendations regarding the interconnection of collectors should be followed. All pipework and collector interconnections should be designed to accommodate thermal movement having regard especially to the high stagnation temperatures that may be attained in bright sunlight. These temperatures are not expected to exceed the following:

selective glazed surfaces 200 °C black glazed surfaces 150 °C black surfaces sheltered from wind 90 °C

The installation of the pipework and fittings should be carried out in accordance with good plumbing practice, particular attention being given to the following.

a) Adequate support and fixing should be provided for the pipework to ensure that all levels and falls are maintained, and the spacing of the fixings are such as to limit sagging of the pipe between the supports. Means should be provided to accommodate thermal expansion and contraction of the pipework.

b) Individual support should be provided to heavy components such as pumps, and motorized valves. Pipework used to support other components should be adequately secured.

c) Unions or flanged joints should be provided on each connection to pumps and motorized valves to allow the removal and replacement of the device without the need to cut the pipework.

d) The fall of the pipes should be arranged to allow the installation to be reliably drained and vented. Low points, when unavoidable, should be fitted with drain valves and high points should be fitted with air vents.

e) Where collectors are mounted directly on the ground, e.g. paving with embedded circulating pipes, or are formed beneath the ground surface, e.g. pipes embedded in asphalt, the foundation should provide sufficient support to prevent movement which might damage pipework connections and be capable of supporting the dead weight of the covering and any expected traffic load without damage.

9.4 Connections to existing filtration system

If the solar heating system is to be integrated with the existing pool filtration system, the following recommendations apply.

a) To minimize alteration to the existing pipework, the flow and return connections to the

b) The connection to the solar collector is fitted to the existing pipework after the filter and before the auxiliary heater (if one is fitted).

c) The isolating valves on the flow and return pipes for the solar system should be fitted close to the connection points into the filtration circuit.

d) It should be ensured that pumps, motorized valves and non-return valves are mounted in an acceptable plane with the flow in the correct direction.

e) Where pumps or motorized valves have to be installed in an exposed position, suitable weather protection should be provided unless they are specifically designed for outdoor use.

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f) No thermal insulation should be applied to an installation before an adequate test is carried out to ensure that the system functions correctly without leakage.

9.5 Special considerations

The provisions for fixings and the foundations for any support frame should be inspected and supervised by a competent person.

Wherever possible an opaque heat resistant covering material, e.g. a tarpaulin, should be applied temporarily to collectors to avoid the high temperature rise which can occur should the unfilled collector be exposed to direct sunshine during installation. Care should be taken to avoid burns which can occur when the bare arms or other exposed skin come into contact with any metal or other parts of the collector which may become heated by solar radiation during installation. Apart from the burns, the shock may be sufficient to cause distraction and possible loss of foothold on the roof or supporting frame.

9.6 Heat loss mechanisms

9.6.1 Evaporative losses

The rate of evaporation from a pool surface is dependent upon wind velocity, air temperature, relative humidity and pool water temperature.

Warm water evaporates more rapidly than cool water. Up to 70 % of a swimming pool's heat energy loss results from evaporation of water from its surface. Evaporative losses are directly proportional to wind velocities at the pool surface and are higher from warm pools than from cooler pools.

Because most of the heat loss from a swimming pool is caused by evaporation of water from the surface, every effort should be made to reduce the evaporation process. Air temperature and relative humidity (both of which influence the rate of evaporation) are beyond our control.

9.6.2 Convective losses

Convective losses occur when air cooler than the pool water blows across the pool surface. The layer of air that has been warmed by contact with the water is carried away by the wind and replaced with cooler air — a process that continues as long as the air is in motion. Detailed observations show the heat energy lost from a pool in this fashion is directly proportional to the wind speed at the surface — doubling when the air velocity doubles.

Windbreaks such as hedges, trees, solid fences, buildings and mounds should be placed so as to shield the pool from cool winds.

9.6.3 Radiative losses

Swimming pools radiate energy directly to the sky, another important energy loss mechanism.

Even with a small difference in temperature between the pool surface and the sky, radiative losses may exceed 10 % of the total swimming pool energy losses.

9.6.4 Conductive losses

Since a swimming pool is in direct contact with the ground or air around it, it can lose heat energy by conduction. The amount of energy transferred even from above ground pool walls to the air is quite small compared to the amount lost from the pool surface to the air. Dry ground and concrete are relatively good insulators, so the energy lost through the sides and bottom of an in-ground pool is also small. In fact, much of the energy conducted into the ground during the day is recovered when the pool temperature drops slightly during the night. In general, conductive losses through the walls of in-ground pools may be ignored.

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However, pools immersed in groundwater that is influenced by tidal motion will lose an increased amount of energy through their walls. Heat flows from the pool to the ground-water surrounding it. As the groundwater is moved by the tides, it will be replaced periodically by cooler water. The quantity of heat loss in this situation is higher than for pools in dry ground and is not negligible. This loss is still low compared to losses through evaporation, convection and radiation.

9.7 Passive pool heating

The use of passive techniques is the simplest and most cost-effective method of keeping swimming pools warm. A passive solar system is one in which the heat flows naturally — without the assistance of pumps and fans. Every effort should be made to incorporate the following three features in new pool construction to minimize the expense of supplementary energy for pool heating:

1. Place the pool in a sunny spot.

2. Reduce the wind velocity at the pool surface with suitable windbreaks.

3. Use a pool cover when the pool is not in use to minimize evaporation losses.

Swimming pools themselves are very effective solar energy collectors. The water absorbs more than 75 percent of the solar energy striking the pool surface (Figure 8). If possible, locate the swimming pool so it receives sunshine from about three hours before until three hours after solar noon. During this time period, the sun's rays travel through a relatively short atmospheric path and thus are at their maximum intensity. Additionally, there is less tendency for the sun's rays to be reflected from the pool surface during midday than during early morning and late afternoon, because they strike the pool surface at a small angle of incidence.

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Figure 8 – The swimming pool as a solar collector

9.7.1 Screen enclosures

Screen enclosures reduce the amount of solar energy that strikes the pool surface. When the sun shines perpendicularly to the screen material, only about 15 % of the energy is obstructed since the screen area is 85 % open air space. However, when the sun strikes the material at an angle, much less of the radiation gets through, and the amount available to warm the pool is reduced by as much 30-40 % on a clear day. More auxiliary energy will be required to maintain comfortable swimming temperatures if the pool has a screen enclosure.

9.7.2 Wind speed reduction

Reducing wind velocity at the water surface reduces convective and evaporative losses. Solid fences or tall hedges located close to the pool perimeter are effective windbreaks. Buildings, trees and mounds also protect the pool from the cooling effect of prevailing winter winds. Locate the pool to take maximum advantage of these obstructions, being careful they do not shade the water surface from the sun. Windbreaks are particularly desirable near the ocean or adjacent to lakes, where the average wind speed is higher than in more sheltered locations. Figure 9 shows an example of a well-shielded pool.

Figure 9 – A well-protected pool

9.7.3 Pool covers

Pool covers are effective in reducing heat losses. There are two basic types of pool covers on the market today: opaque and transparent. By reducing evaporation they reduce the quantity of chemicals needed, and they help to keep dirt and leaves out of the pool. Pool covers also reduce pool maintenance costs.

9.7.3.1 Transparent pool covers

Transparent covers will not only reduce evaporative losses but they will also turn the pool into a passive solar collector. Sunlight passes through the cover material and is absorbed by the pool water.

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Because evaporation accounts for about 70 % of pool heat loss, the beneficial effect of pool covers can be dramatic.

Transparent pool covers are made from a variety of materials, such as polyethylene-vinyl copolymers, polyethylene and polyvinylchloride (PVC).

Attention to a few details will extend the life of transparent pool covers. They should not be left folded or rolled up on a hot deck or patio. The sunlight will overheat the inner layers and may even burst the air pockets in bubbled covers. When removing or installing a pool cover, avoid dragging it over the pool deck or any rough surface or sharp obstruction. Although it is recommended that a single, continuous pool cover be used whenever possible, the use of sectioned covers can ease handling in the case of larger pools.

9.7.3.2 Liquid films

Materials like cetyl alcohol spread to form a layer only a few molecules thick on a water surface. They can reduce evaporation by nearly 60 percent. Of course the materials offered for this purpose are not toxic but they are fairly expensive and must be re-dosed frequently (usually at the close of the daily swimming period). The chemical films do not reduce convective or radiation losses, but they do allow solar gains.

9.7.3.3 Opaque covers

Opaque covers are useful for pools that must remain uncovered during daylight hours. Most commercial pools fall into this category. The following types of opaque covers are the most common: woven, plastic safety covers; skinned, flexible foam covers; and rigid or semi-rigid closed cell foam blocks or blankets. The woven safety covers will reduce evaporation losses (if they float and are waterproof) though not as well as a continuous film type cover. Skinned foam covers vary in thickness from less than 1/8 inch to more than ½ inch. In common insulating terms, their effectiveness in reducing heat losses ranges from R-1 to R-4. If they fit snugly to the edges of the pool, they will virtually eliminate evaporation losses during the periods when they are in place. Foam block covers such as expanded polystyrene have insulating values between R-4 and R-12, depending on their thickness. If properly fitted and placed on the pool surface, they, too, will nearly eliminate evaporation losses during the hours they are used. Their effectiveness in reducing convective and radiative losses increases directly with their R-value.

9.8 Active pool heating

Many types of solar collectors are suitable for pool heating. The temperature difference between the water to be heated and the surrounding air is small, so expensive insulating boxes and transparent covers that reduce collector heat loss are not often required. Cool winds above 10 km/h substantially reduce the efficiency of unglazed collectors.

9.8.1 Low-temperature collectors

Types of low-temperature collectors include black flat-plates, black flexible mats (both with passages for pool water) and black pipes.

9.8.2 Flat-plate collectors

Several types of flat-plate collectors, specifically designed for pool heating, are available in both plastic and metal.

Flat-plate collectors for pools feature large-diameter headers at each end and numerous small fluid passageways through the plate portion. The header's primary function is to distribute the flow of pool water evenly to the small passageways in the plate. The header is large enough to serve as the distribution piping, which reduces material and installation labor costs. The fluid passageways, which collect energy from the entire expanse of the surface, are small and are spaced close together across the plate (if it is made of plastic) so most of the collector surface is wetted on its back side. Representative cross sections of plastic collectors are shown in Figure 10. EPDM flexible mat collectors have the same general cross section, as do plastic collectors.

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Figure 10 - -Typical collector designs

High water flow rates also tend to keep collector-to-air-temperature differences low. The total amount of energy delivered to the pool (the most important variable) is the product of the amount of water flowing through the collector multiplied by the water's temperature rise. Five hundred gallons of water raised 1°F contains as much energy as 10 gallons of water raised 50°F, but the collector operating at 10°F above pool temperature will operate less efficiently. Thus, high flow rates increase collector efficiency. Many manufacturers frequently recommend a flow rate as high as one gallon per minute for each 10 square feet of collector area. But such high flow rates are not needed to keep the temperature rise in the collectors below 10°F for best efficiency. Higher flow rates result in high-pressure drops across the collector array. This requires an increase in the horsepower of the circulating pump. Thus, flow rates are usually limited to about one gallon per minute for each 10 square feet of collector area for the configurations shown in Figure 10.

Because even plastic pool heating collectors are expensive, the plastic used must withstand years of exposure to sunlight. The ultraviolet portion of sunlight can break chemical bonds in most plastics and will eventually destroy the material if the process is not retarded. Collector manufacturers use several proprietary combinations of additives or stabilizers and UV inhibitors in the chemical mix of the collector material. These stabilizers and UV inhibitors provide protection from the damaging radiation and retard degradation of the plastic in addition to improving the collector's ability to absorb and conduct the sun's energy. Accurate estimation of plastic durability is difficult; therefore, explicit warranties are desirable. Most manufacturers currently offer a five-year or longer limited warranty. Some plastic collectors are expected to last 25 years.

Plastics are available in numerous formulations and types, many of which are relatively immune to attack from common chemicals. Polypropylene, acrylonitrile-butadiene-styrene (ABS), polyethylene, polybutylene, polyvinylchloride (PVC) and ethylene-propylene with diene monomer (EPDM) are frequently used collector materials. Some have been used to make pool collectors for more than 20 years and have demonstrated their ability to withstand attack by swimming pool chemicals and sunlight for at least that period of time.

Flat-plate collector designs utilizing metals are slightly different from plastic configurations. Metal is a better heat conductor, so relatively long fins can separate the tubes without causing excessive operating temperatures on portions of the collector surface.

9.9 Plumbing schematics

9.9.1 Flow control devices

Solar pool heaters are generally connected to existing pool plumbing systems. This section explains how to make the connections.

A schematic of a frequently used pool filtration loop is shown in Figure 11a. The pump draws the water from the skimmer and main drain, forces it through the filter and returns it to the pool through the conventional heater. Lint, hair and leaf catching strainers are usually installed ahead of the pump.

Solar systems designed to operate with small pressure losses can be added as shown in Figure 11c. A spring-loaded check valve is installed downstream from the filter to prevent collector water from backwashing

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through the filter and flushing trash into the pool from the strainer when the pump is shut down. A manually operated or automatic valve is placed in the main line between Ts that feed the collector bank and return the solar heated water (Figures 11b and 11c). Ball valves may be placed in the feed and return lines for isolating the solar system from the pool filtration system when the filter is being backwashed or when adjustments are being made to the solar system. When solar heating is desired, the pump timer is adjusted to operate during daylight hours, and the valve in the main line is closed somewhat to restrict or fully interrupt the flow and force water up through the collectors. Valves on the lines to and from the solar system should be fully open.

Closing the valve in the main line may increase flow through the collectors. It may seem logical to reduce the flow rate through the solar array to make the return water warmer, and this can be done; however, it is not logical — the collectors will be forced to operate at higher temperatures, their efficiencies will drop, and less solar energy will be delivered to the pool. The temperature rise through the collectors should be kept low, less than 10°F on warm, sunny days, unless the manufacturer's specifications call for a higher temperature differential.

Forcing water through the solar system uses some of the pump's power, thus reducing the flow rate through the pool filtration system. As the main line valve is closed, pressure on a gauge mounted on the filter or discharge side of the pump will rise slightly. If the valve is closed entirely, all of the flow is diverted through the solar array and the collection efficiency increases. If the pressure at the filter does not rise unduly, the solar system should be operated in this way. However, the more the pressure rises, the slower the flow through the filtration system. This will increase the length of time required for the entire pool's contents to be filtered. Thus, it may be necessary to allow some of the flow to bypass the collectors. An inexpensive plastic flow meter can be used on the main line connection to monitor flow rates through the filtration system. Check with local building officials to determine minimum filtration flow rates or pool turnover times required in your area

When the existing pool pump lacks enough power to circulate sufficient flow through the solar system and the filtration system, a booster pump may be required. It should be installed as shown in Figure 11d. Common pool-circulating pumps with or without the strainer basket are suitable for this application.

The booster pump should be placed in the line feeding the solar collectors, not in the main circulation line. In this position it will operate (consuming electricity) only when circulation through the solar collectors is wanted. Of course, the booster pump may be operated by the same time clock as that for the filter pump, but more often it will have a separate control. If both pumps operate from the same timer, it should be set so the pumps come on during daylight hours. In this case, the timer must be rated for the sum of the circulation pump and the solar booster pump.

If the booster pump is separately controlled, the filter pump may run for a longer portion of the day, and the booster pump should turn on during appropriate periods but only when the filter pump is operating.

Manual flow control or control with time clocks is simple and inexpensive but has drawbacks. Since clocks do not sense weather conditions, the circulating pump may be running when there is insufficient solar energy available to warm the pool water. Collectors may lose energy rather than gain it if weather conditions are unfavorable. Automatic flow controls overcome this difficulty. The most common plumbing schematic for systems using these devices is shown in Figure 12.

FDUS 888: 2009


Figure 11 – Plumbing schematics

FDUS 888: 2009


Figure 12 – Automatic control plumbing schematic

Accurate differential temperature control is difficult to achieve because of the small temperature rise that takes place in solar pool heaters. A sensor, tapped into the piping at a convenient place ahead of the collector return line, measures the pool water temperature. Another sensor is housed in a plastic block and placed near the solar collectors, so its temperature parallels that of the collector (or it may be attached to the collector outlet). When the pool water temperature exceeds the collector temperature, the control valve remains in the open position and the flow bypasses the collector loop. When the collector temperature exceeds the pool water temperature, the valve is closed, forcing the flow through the collectors. In practice it has proven equally effective to control the flow through the collectors with a single solar sensor, which turns on the solar pump and/or activates the diverting valve above a fixed solar intensity level.

When operating properly, a differential controller automatically adjusts to changing conditions, monitoring variations in collector temperature caused by clouds, other weather factors and the approach of evening. When collector temperature drops, the control de-energizes the valve and flow bypasses the collector. Maximum pool temperature limits can be programmed into some controls.

Control valves may be actuated hydraulically or electrically. One of the earliest valves used was a hydraulically operated pinch valve consisting of a cylinder with an expandable bladder inside. A high-pressure line connected to the discharge side of the pump is used to expand the bladder, pinching off the flow and diverting it through the solar system. A low-pressure line connected to the suction side of the pump deflates the bladder and allows the flow to pass unimpeded. An automatic controller accomplishes switching between the high- and low-pressure lines.

Electrically operated valves are also used. A differential controller may be used to operate a solenoid that, in turn, activates the main valve in much the same way the pinch valve is activated. Be sure the valve you select is specifically designed and constructed for use on pool systems. Automatic control schematics, taken from the installation diagrams of two low-temperature collector manufacturers are shown in Figures 15 and 16.

The most common method to divert the flow of water to the

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