operation and maintenance of hvac water system

7/31/2019 Operation and Maintenance of HVAC Water System

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Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance

BOpEqSysWaterDistributionSystem.doc1

Operation and Maintenance of HVAC WaterDistribution Systems

Topics:

Introduction

Types of Water-Distribution Systems

Key Components of Water-Distribution Systems

Safety Issues

Best Practices for Efficient Operation

Best Practices for Maintenance

Maintenance Schedule for Water-Distribution Systems

References

IntroductionMany large buildings and campuses have HVAC water-distribution systems, also called hydronicsystems. Water is heated or cooled in a central plant, then pumped to air handlers, where ventilation air isconditioned as needed.

Maintaining and optimizing the performance of pumps and valvesthe key mechanical components ofwater-distribution systemscan be challenging. These systems often have hidden performance problemsthat waste energy and cause excessive wear on equipment.

As with other mechanical components of the HVAC system, a preventive and predictive maintenanceplan is an excellent way to maintain an efficient system, save energy dollars, prevent costly breakdowns,and extend equipment life.

f Learn more about establishing a Best Practice Operation-and-Maintenance Program.

Types of Water Distribution SystemsWater-distribution systems are either closed-loop (cooling- or heating-system water does not come incontact with outside air) or open-loop (condenser water is exposed to outside air, usually in the coolingtower). Water in a closed loop requires less treatment than water in an open system.


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Distribution systems for chilled water and heating water

Hydronic System TypesChilled water: Chilled-water distribution systems transport water used for air conditioning from the chillerto the air handlers, where the chilled water flows through coils in the ventilation air stream. Chilled watercan also flow directly to terminal units located in the occupied space. Another variation is radiant coolingor chilled beams where chilled water flows through a radiant cooling device. Passive chilled beamsprovide cooling separate from the ventilation-air system. Active chilled beams are integrated into aventilation-delivery system.

f Learn more about Operation and Maintenance of Chillers.

Heating water: Heating-water systems move water from the boiler, typically located in a central plant, toair handlers or perimeter terminal units. Heating-water systems are closed loops.

f Learn more about Operation and Maintenance of Boilers.

Condenser water: Condenser water loop moves water between the chillers condenser and the coolingtower. These systems are usually open loops.

f Learn more about Operation and Maintenance of Cooling Towers.

Water-loop heat pump: This hydronic system circulates water to individual water-to-air heat pumpsthroughout the building. Most heating and cooling is done by individual heat pumps serving the zones inwhich theyre located. These heat pumps either take heat from the water loop or reject heat to the loop. Inmoderate weather, the water loop self-balances within a controlled range, usually 6590F. In hotweather, if the temperature of the loop exceeds 90F, water in the loop will be diverted to a cooling tower.


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In cold weather, if the loop temperature drops below 65F, the water will be diverted through a boiler toincrease the water temperature in the loop. Hydronic heat pumps are typically closed-loop systems.

Ground-source water-loop heat pumps: This is a water-loop heat-pump system (as described above)with the addition of an extensive piping loop buried in the ground which acts as a heat sink. Heat isabsorbed from the ground by the water in the piping loop in heating season, and rejected into the groundin cooling season.

Constant- and Variable-Flow Pumping Systems

All hydronic systems are either constant-flow or variable-flow. Generally, older buildings have constant-flow systems and buildings or systems built after about 1990 use some form of variable-flow control forpumping chilled and heating water.

Constant flow: A constant flow rate is maintained throughout the hydronic loop whenever the system isoperating. Flow rates are established for a peak design condition, which represents a small fraction oftotal operating hours.

Variable flow: Variable-flow pumping can be accomplished in several ways:

Mechanical or magnetic clutch: This type of device controls the torque to the pump from the motorin response to an external input from the DDC system, typically the differential pressure between thesupply and return piping.

Variable-speed drive: A VSD alters the frequency of the electrical input to the motor, allowing the

motor to speed up or slow down as required to maintain a setpoint.

Modulating control valves: There are two valve types, two-way and three-way. Either type can be usedfor both constant-flow and variable-flow pumping.

Two-way valves: These can be used in a constant-flow pumping system in conjunction with abypass valve that controls pressure. They allow unused water to return to the central plant if thesupply pressure gets too high due to a number of the valves closing. They can also be used invariable-flow pumping systems where the pump output is controlled directly by the differentialpressure.

Three-way valves: These are traditionally used in constant-flow pumping systems, where the unusedwater is bypassed at the coil. They are also used in variable-flow pumping systems. In special cases,several may be installed at the end of the longest piping runs. The sum of their flows equals the

minimum flow the pump can produce in stably. This also allows the piping system to stay at thetemperature setpoint in case another air handler requires immediate chilled or heating water.

Pumping System ArrangementsThere are several ways to combine the above system of pumps and valves to provide energy savings ascompared to a single-loop constant-flow system.

Primary-secondary variable-flow systems: Water flows through the chiller or boiler primary loop at aconstant rate, and water flows through the secondary loop, which serves air handlers or fan coils, at avariable rate. The decoupled section (shown as common piping in the diagram below) isolates the twosystems hydraulically. Primary-secondary variable-flow systems are more energy efficient than constant-flow systems, because they allow the secondary variable-speed pump to use only as much energy asnecessary to meet the system demand.


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Schematic of primary-secondary loop system

Primary-loop variable-flow systems: These are chilled-water or heating-water systems with a singlevariable-flow loop. A two-way bypass valve is typically used to maintain a minimum specified flow rate

through the chiller or boiler. Primary-loop variable-flow systems are more efficient than primary-secondaryvariable-flow systems.

Key Components of Water Distribution SystemsPumps: HVAC water-distribution systems use centrifugal pumps. Centrifugal pumps have a rotating anda stationary component. The rotating section consists of an impeller and shaft; and the stationary sectionis the housing, which includes a casing, casing cover, and bearings. A motor spins the shaft and impeller,creating centrifugal force to increase the velocity of the water and push it through the volute (the curvedfunnel increasing in area) to the pump outlet.

Centrifugal pump

Valves: Manual or automatic valves regulate the flow of water throughout the piping system. Valvesperform four basic functions: 1) starting, stopping, and directing flow; 2) regulating or throttling flow;3) preventing backflow; and 4) relieving or regulating pressure.


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Balance valves, also called circuit setters, are a special type of regulating valve that limit flow at designconditions. This allows the system to be balanced so that every coil in the system has adequate flow.Excessive pressure drop across these valves can waste a considerable amount of energy.

Valve bodies are available in many configurations according to type of service. Regardless of function,the basic valve components are the body, seat, stem, and packing or gasket. Valve actuators controlwater flow through automatic valves in response to electronic or pneumatic control signals.

Either two-way or three-way valves control the flow of heating or cooling water. Two-way valves reduceflow and are commonly found in variable-flow systems. Three-way valves maintain a single flow rate bydiverting the water around the coil (or other equipment such as a chiller or boiler) when necessary. Three-way valves are typically used in constant-flow systems.

Valve components

Pipe and other system components: Pipes connect all components and, with the possible exception ofhydronic-heat-pump loops, should be insulated per local codes. Other system components that need tobe operated and maintained properly are expansion tanks, strainers, and gauges for pressure andtemperature.

Safety IssuesWorking near rotating pump shafts and couplings can be dangerous. Control panels for electrical motorsand electrical connections on pump motors present a shock hazard. Heating-water systems aredangerous if not fully insulated, so be careful not to touch uninsulated piping without first checking itstemperature.

Always observe lock-out and tag-out procedures during maintenance.


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Best Practices for Efficient OperationThese best practices will help improve the performance of your water-distribution system and reduceoperating costs:

Monitor the make-up water meter: A quick way to spot a leak in a closed-loop hydronic system is tomonitor the meter on the system make-up water inlet. Reading the meter daily or weekly and comparingthe readings to historical averages can help identify problems.

Investigate valve chatter: Valve chatter or water hammer coming from control valves may indicate

overpressurization at the valve. High pump pressure or system imbalance can exceed the valvesactuator shut-off rating and cause water to pass through the closed valve and thereby cause the chatter.Water passing through a closed valve causes overcooling or overheating, thereby destabilizing controland wasting energy. Generally, the shut-off pressure limit for 2-way valves with vertical actuators shouldbe 25% greater than maximum pump head. Two-way valves with rotating actuators use a much lowertorque. Three-way valves do not need as high a limit as vertical 2-way valves since they simply divertflow. Rebalancing or, as a last resort, replacing the valve actuator with one with a suitable higherpressure limit may fix the problem. Confirm the correct operating parameters of the valves in your systembefore making any changes.

Confirm correct control setpoints: Pumps in a variable-flow system are controlled to maintain aminimum differential pressure across the coil for a critical zone or for the coil farthest from the centralplant. Make sure the differential pressure setpoint for this coil (there could be more than one) is at theproper design value. Occasionally these setpoints are raised in an attempt to fix an unrelated problem

and then forgotten, wasting pump energy.

Investigate parallel pumping: Most variable-flow systems have one on-line (or lead) pump and onestandby (or lag) pump. At higher flow rates, it can be more energy efficient to operate both pumps inparallel. A typical control sequence is for the lead pump to operate alone until twice its minimum flow rateis reached, then the lag pump cycles on and the two pumps operate together in parallel on the samespeed-control signal. An engineering professional should determine the best strategy for your particularsystem.

Best Practices for MaintenancePumps are a vital part of HVAC and process-load applications. Pump efficiency directly affects theefficiency of other system components.

Inspect system weekly: At least once a week, a building engineer should walk the entire system andcheck piping, valves, and pumps for leaks and unusual noise. Noise often indicates hidden valve or pumpproblems. Sources of noise include turbulence, cavitation, release of entrained air, and water hammer.

Ensure adequate water treatment: Scale and sludge deposits reduce flow and impair heat transfer. Anongoing water-treatment program is critical to efficient equipment operation throughout the distributionsystem.

Inspect insulation: System efficiency is compromised if the distribution piping has inadequate, damaged,or wet insulation. Proper insulation can reduce radiant energy loss by as much as 90 to 95%. Removablelagging pads or snap-on insulation are available for parts such as valves that need periodic maintenance.

Test valve stroke: Test each automatic valve annually. Conduct a test where the valve actuator movesthe valve stem through the entire range of the stroke. Failure to move smoothly through the entire rangeindicates a problem.

Maintain optimal condenser water flow: Most condenser-water systems include a strainer to removelarger material picked up in the open cooling tower. In areas with high particulate counts in the air, a side-stream mechanical filtration system is recommended to constantly filter the water and removecontaminants.

Test pump efficiency annually: Testing the efficiency of large pumps and comparing it to previousbenchmarking data will help spot developing problems.


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ReferencesASHRAE Handbook, HVAC Systems and Equipment, 2000

US Department of Energy, Improving Pumping System Performance, Second Edition, May 2006www1.eere.energy.gov/industry/bestpractices/pdfs/pump.pdf

Harold R. Colen, PE, HVAC Systems Evaluation, 1990

FEMP O&M Best Practices Guide 2.0, July 2004

For additional information on pumps, visit these sites sponsored by the Hydraulic Institute:www.pumplearning.org, www.pumps.org, and www.pumpsystemsmatter.org.

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