Hot Water Supply 1. Hot water supply system

The provision of hot water in any premises usually involves the installation of one system, or the combination of systems of hot water generation and distribution. The system can be: • Central storage plant with distribution pipeworks serving the whole

building • Remote storage plants, heated from a common heat source, with

distribution pipework serving zones of the building • Local storage vessels or combination units heated by independent

sources of heat • Local instantaneous units

2. Choice of system

The type of system will depend on the type of building: • Large block of flats

To provide each flat with individual local water heater, heated either by gas or electricity. The tenant then pays what has been consumed through the gas or electricity meter.

• Office blocks and factories

If the lavatories are few and widely separated, the advantage will be using small separate local hot water supply units using electrical or gas heaters. If, however, the sanitary fitments are arranged in a compact manner, central system will be more economical and will give greater economy in running cost.

• Hospitals, hotels and institutions building

The central system is almost exclusively adopted for the reason that it can give a great reserve for heavy demands, and the maintenance of a large number of smaller units could be avoided. In building complexes of this type the heat is often derived from a central boiler plant which provides all the heating requirements, cheaper fuels could be used and therefore a greater economy in running cost is achieved with such system.

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3. Types of fuels

• Commonly used fuels: gas, oil and electricity • Solid fuels such as coal or coke may be used in special occasions • Water heaters can be classified according to types of fuel used, nature of

heat source supply or storage capacity 4. Storage Vs Instantaneous heaters 4.1 Storage type heaters

• With a reserve/storage of hot water • Generally used where:

- large quantities of water required at intervals - large fluctuation of required quantity - reliable supply desired

4.2 Instantaneous type heaters

• Water heated almost instantaneously as it flows through the heating medium

• No storage is allowed • Suitable for continuous flow of hot water with minimum fluctuation • Heating element/medium has to be designed to satisfy maximum

demand • Sudden increase or decrease in demand may result in undesired

temperature drop or rise 5. Types of water heaters 5.1 Gas heaters

(a) Direct gas –fired storage heater (Fig.5.1)

• Can be used to serve a single outlet or several outlets in a flat • May have atmospheric burners or forced-draft burners, the latter are

for larger installations • Adequate supply of combustion air is important • Properly installed and sized gas vent or chimney is important for

proper operation and safety

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(b) Direct gas-fired instantaneous heater (Fig.5.2)

• Water flows through a venturi which produces a differential pressure across a diaphragm to open the gas valve

• Some devices require a minimum water pressure (~3m) to operate the gas valve

• Pilot flame for ignition of burner is required 5.2 Cistern-type electric storage heater (Fig.5.3)

• This type of heater consists of a storage cistern of about 9 to 11 litres capacity and is designed for use as a local heater

• Water temperature is controlled by means of an adjustable thermostat. The electricity is automatically cut off when the pre-determined temperature is reached

• Should the thermostat malfunction and the water boils, steam will escape through the vent pipe and then the overflow pipe

5.3 Free-outlet type electric storage heater (Fig.5.4)

• Available in capacities from 6 to 136 litres • Generally for small installations, directly above basins, baths or sinks

5.4 Pressure type electric storage heaters 5.4.1 Vented heaters (Fig.5.5)

• This is fed from a high-level cistern which provides the necessary pressure at outlets

• Accommodates expansion due to heated water, and is fitted with an open safety vent pipe to permit the escape of air or steam, and to prevent explosion without the need for any mechanical device

• The vented hot water storage system provides: - constant low pressure - reserve water supply

• but needs: - protection against the entry of contaminants to cistern

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5.4.2 Unvented heaters (Fig. 5.6)

• This is usually fed direct from the supply pipe under mains pressure • It has no vent pipe and relies on mechanical devices for the safe control

of heat energy and hot water expansion • The mechanical devices include a thermostat, a thermal cut-out, a

temperature relief valve and a pressure relief valve • Waterworks regulations permit the installation of unvented electric

thermal storage type water heater of storage capacity not exceeding 200 litres. Details of the technical and safety requirements are given in Chapter 5 (Non-centralized hot water system) of ‘Hong Kong Waterworks Standard Requirements for Plumbing Installation in Buildings’

• Features of the unvented hot water storage system are: - eliminates the need for cold water storage and risk of frost damage - may require a larger supply pipe but eliminates some duplication of

pipework - contains no reserve supply in case of supply failure - eliminates cistern refill noise - relies on mechanical controls which need regular inspection and

maintenance - gives better pressure at outlets, particularly at showers - allows quicker installation than vented system but involves more

costly components 6. Instantaneous electric heater (Fig.6.1)

• electrical input ranging from 3 to 9 kW • pressure switch to ensure water flow before actuation of heating element

7. Hot water distribution system 7.1 Localised hot water supply system (Fig. 7.1)

• With local systems, the water is heated adjacent to the outlet usually by gas or electricity

• This may involve some storage but often an instantaneous heater is employed

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7.2 Central hot water supply system

A central system comprises a boiler or water heater of some form, coupled by circulating piping to a storage vessel, the combination of the two being so proportioned so as to permit all the normal demands for hot water being satisfied.

7.2.1 Direct system (Fig.7.2)

• Water is normally heated within a boiler fired by oil or gas or electricity and circulated to a hot water storage cylinder or calorifier

• This is often carried out in conjunction with space heating • The difficulties with such systems is that oxygen and scale formation

products are constantly being introduced to the system and cause corrosion and scale formation

• An alternative to this system is the indirect system 7.2.2 Indirect system (Fig.7.3)

• It is similar in principle to the direct system except that water from the boiler is circulated through a coil in the storage vessel via the primary flow and return

• This is a completely separate circuit and the water in this circuit does not mix in any way with that drawn off from the storage vessel

• This circuit has its own vent pipe and also a separate small cold water feed and expansion tank, from which a cold feed pipe to fill the primary circuit is taken

• Heat is transferred from the water in the coil to the water surrounding it in the storage cylinder

• The water is therefore heated “indirectly” and the purpose of this is to protect the boiler from scaling

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7.2.3 Pumped Circulation (Fig.7.4)

• In the case of larger buildings such as blocks of flats, offices or colleges, the solution is to have a central boiler house providing a primary circulation of hot water (or steam) to a calorifier (hot water storage cylinder)

• Form the calorifier, a secondary circulation of hot water is conveyed through a ring main system by means of a circulation pump

• Pumped circulation in the secondary circuit creates better circulation, hence reduces water waste due to dead leg problem (Fig. 7.5), but may increase heat loss in the distribution piping

• Hence, thermal insulation are required for hot water piping to reduce heat loss

7.2.4 Combined system (Fig.7.6)

• It has become common practice to combine the duties of heating and hot water supply for one boiler or group of boilers

• This can be done by using indirect system • During summer, while the heating is off, the boiler is of course

oversized • Hence, a large boiler running intermittently on a light load may lead to

lower efficiency • Therefore, it is preferable to provide small boilers to share the loading

especially during part load operation 7.3 Comparison between centralised and localised systems

Centralised systems Localised unit heaters Large bulk storage Fitted close to fittings

Long lengths of secondary pipework, possibility of large heat losses

Saving of boiler house and fuel storage

One central plant implies simpler and less maintenance

A no. of heaters means more connection points

Reduction in flue construction Greater risk of fire

Better control in energy consumption

Structural or architectural aspects may restrict the use of cheaper fuel

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8. Local regulations 8.1 Individual heaters

• All water fittings and pipework shall comply with the relevant Waterworks Regulations

• Heaters should be installed in accordance with the ‘Hong Kong Waterworks Standard Requirements for Plumbing Installation in Buildings’, Chapter 5

8.2 Centralised hot water systems

• All water fittings and pipework shall comply with the relevant Waterworks regulations

• Centralised hot water system should be installed by a licensed plumber in accordance with the ‘Hong Kong Waterworks Standard Requirements for Plumbing Installation in Buildings’, Chapter 6

9. Hot water supply to high rise buildings (Fig 9.1)

• Hot water system is usually divided into two or more zones to prevent excessive pressures on the lowest draw-off points

• Intermediate-height calorifier and break pressure cistern to limit pressure head at the lowest fittings

• It is essential that the head acting on these points does not exceed 45m 10. Use of pneumatic system in hot water circuit (Fig 10.1)

• In case of insufficient head available from the roof storage tank, a pneumatic booster system is utilized to serve the top three or four floors

• This will produce sufficient head to achieve satisfactory water pressure Problem: • Motion of hot water depends on operation of pneumatic system • No hot water flow, no motion and hot water will not circulate in the

loop and heat loss cause temperature drop in the hot water system • Pressure cannot be maintained at water outlet unless the circuit is a

closed one

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Improvements: • Add a thermostat and a solenoid valve to the return path, adjust the

thermostat so the solenoid valve will open when temperature in the return loops drops below 50oC

• When the solenoid valve opens system pressure will drop (pressurized circuit), motion of hot water begins. Temperature in the hot water system can be maintained

• When pump runs, solenoid valve should be arranged to close, otherwise pressure cannot be maintained in the system

11. Planning of hot water installations

The following considerations should be taken into account in planning of hot water installations: • Total consumption • Peak demands • Type of installation, whether local or central • Storage capacity • Methods of heating water • Insulation of hot pipes and vessels • Use of circulating system, where appropriate • Choice of materials for installation in relation to nature of water • In connection with the pipes and fittings

- mechanical and chemical properties to resist failure - prevention of air locks, water hammer and noise - number of fitments and probable demand - available head and loss of head due to friction in pipes and fittings - provision for isolating part of the system - accessibility for maintenance

12. Hot water recovery and storage capacity(Fig12.1)

• When sizing hot water storage and the hot water boiler power, it is

important to determine not only the correct size but also the correct relationship between recovery and storage

• A pattern of hot water usage can be projected for a building, the actual usage being largely a function of the building population and the type of activity that take place

• From the demand pattern histogram, a series of sequential peak hour loads should be calculated for groups of hours, from 1 hour through to the maximum number of hours in the operating period. The total hour

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loads should be recorded with the average hourly load for each group of hours under consideration

• A graph can be drawn and a curve is formed which establishes the relationship between storage and recovery that satisfies the system requirements

• Figure 12 shows the relationships between recovery and storage for the various building categories

• The higher the recovery rate, the greater the heating capacity and the smaller the storage capacity required

• Applicable to all types of hot water storage vessels including off-peak electric heated vessel, but not applicable to non-storage generators because the max. performance of these is determined by the peak system requirement per minute

13. Hot water pipe sizing

• Sizing hot water supply pipes involves the same principles as sizing of

cold water supply pipes • The piping system should be capable of meeting the probable maximum

demand at an acceptable pressure drop and water flow velocity • The secondary return pipe is sized on the basis of the flow rate required

to offset the heat emission from the pipework including the flow and return lines

14. Circulating pump sizing

• Hot water return flow is obtained from the following formula:

Q = q / ρ CP ∆t Where Q = pump capacity, L/s q = heat loss, W ρ = density of water = 0.99 kg/L (50oC) CP = specific heat of water =4180 J/kgK ∆t = allowable temperature drop, K

• Flow and return temperatures of 65C and 55C are usually adopted • The calculated flow and the resistance head of the circulation system are

then used to select a suitable circulating pump

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15. Energy saving in hot water system

• Limit water tap temperature to as low as possible 55oC to 58oC instead

of 60oC to 65oC • Reduce heat loss from the pipework by installing good quality

insulation materials • Reduce water flow rate by using flow restricted control valve • Efficient utilization of waste heat and solar energy • Proper secondary return system • Preheated cold water if possible • Improve system efficiency by improving maintenance program • Use semi-storage type instantaneous heater

16. Legionnaires’ disease (Legionnaires’ pneumophila)

• Legionnaires’ disease appears as a form of pneumonia, with patients presenting symptoms of muscle pains, cough, breathlessness, headache and fever, often culminating in respiratory failure

• Found in many hot water systems, particularly large complex systems such as those in hospitals, hotels, office blocks and factories

• Legionella may colonise water storage tanks, calorifiers, pipework and plant, filters and certain unsuitable fittings and materials

16.1 Conditions that encourage its proliferation:

• Presence of sludge, scale, algae and organic particulates can provide nutrients for growth

• Water temperature in the range of 20 oC to 45 oC favours growth • Segments of service water systems in which the water stagnates (e.g.

shower head and certain sections of storage-type water heaters) provide ideal breeding locations

16.2 Prevention of Legionnaires’ Disease 16.2.1 Design precautions:

• The hot water storage device of the system shall be designed to operate at 60 oC to effectively kill the bacteria

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• Water within the hot water storage device shall have reach 60 oC for at least 5 minutes prior to the discharge to the distribution system

• Drain outlets shall be provided at the lowest point of hot water storage devices for flushing away settled sludge

• Secondary pumped circulation shall be provided where necessary to reduce temperature stratification within the hot water storage devices

• Deadlegs and stagnant corners in the hot water pipework shall be avoided

• All piping systems and associated hot water storage devices should be flushed clean upon commissioning prior to bringing them into operation

• Avoid the use of natural rubber, porous and organic matters (eg. leathers) as part of pipework since these materials provide nutients and a favoured environment for the growth of micro-organisms

• Hot water storage devices should be well insulated to prevent heat losses to a temperature at which legionella may survive

• Cold make up water should not be able to short circuit through the hot water storage device and the system should be designed to ensure that all water is adequately heat disinfected prior to leaving the storage devices

16.2.2 Operation and maintenance:

• Operate hot water storage devices at 60 oC and maintain the tap outlet

temperature at 50 oC • Drain and clean the hot water storage device regularly to avoid

accumulation of oxides, rust, scales and sludge • Carry out the following modifications/improvements as necessary:

- remove redundant pipework containing stagnant water - retrofit existing hot water storage devices so as to provide drains at

the lowest point of the devices - provide secondary pumped circulation to reduce temperature

stratification • Hot water outlets which are infrequently used or are connected to

stagnant water supply pipework shall be flushed at full flow for a minimum period of one minute at least on a monthly basis

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Fig. 5.1 Direct gas-fired storage heater

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Fig. 5.2 Instantaneous gas water heater

Fig. 5.3 Cistern type electric heater

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Fig. 5.4 Open-outlet electric heater

Fig. 5.5 Vented heater

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Fig. 5.6 Unvented system

Fig. 6.1 Instantaneous electric water heater

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Fig. 7.1 Local hot water system

Fig. 7.2 Direct hot water system

Fig. 7.3 Indirect hot water system

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Fig. 7.5 Dead leg

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Fig 9.1 Hot water supply to high-rise building

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Fig 10.1 Use of pneumatic system in hot water circuit

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Fig 12.1 Relation between hot water storage & recovery

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Fig.12.2 Hot water storage and recovery curves

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Fig.16.1 Legionnaires’ Disease

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