Water Loop Heat Pump (WLHP) Systems

Many buildings have large internal zones that require cooling only during occupied hours, or cooling, year round due to internal heat gains. The chiller condenser water heat is recovered, instead of being rejected to the outside, and used as a heat source for a water-source heat pump system serving the perimeter spaces.

The closed water loop provides condenser water to the interior zone cooling units, picking up the heat normally rejected to a cooling tower, and transferring it as the heat source (evaporator load) for the perimeter zone heat pumps. This substantially increases the coefficient of performance (COP) and lowers the operating cost of the heat pumps over conventional air-source heat pumps. The water loop temperature is typically maintained between about 65 and 90° F and therefore piping can be uninsulated. Loop circulation is typically between 2 to 3 gpm per ton of cooling capacity.

During peak winter conditions when the heat pumps cool the loop below 65° F, a boiler (electric, gas, oil) is used to maintain the 65° F temperature. During peak cooling conditions when all or most of the units are rejecting heat to the loop and its temperature tends to rise above 90° F, a closed-circuit evaporative cooler is used to reject the unneeded heat. As the loop temperature rises, the dampers on the tower (evaporative cooler) open to allow gravity circulation over the loop water coil. If the loop temperature continues to rise, the water spray circulator is turned on. If the loop temperature still rises, then the fans are turned on for full capacity performance.



Closed water loop water source heat pump systems have these key advantages:

  • Meet Construction Budgets
  • Increase Leasable Space
  • Lower Operating Costs
  • Architectural Flexibility
  • Improve Cash Flow

There are also specific advantages for Architects, Owner/Developers, and Consulting/Design Engineers.

Advantages for Architects:

  •  Aesthetics - Most equipment is not visible. Roof is cleaner with less possibility of leaks.
  • Flexibility - The system can be subdivided or expanded into new zones to fit building remodeling or additions easily and inexpensively.
  • Ceiling height to floor can be smaller and building height lower.
  • Smaller mechanical rooms (50% or less in new construction).
  • Less duct shaft space (saves on fire dampers) than air systems.
  • Small building allows construction dollars to be used for other items.

Advantages for Developers and Owners:

  • First and Operating Costs Can Be Lower. Water Source Heat Pumps can save money and lower energy consumption by:
    • recovering heat from building interior zones and "pumping" it to the perimeter of the building.
    • isolating and shutting down unoccupied areas of the building.
  • Service Life Compares to Central Station Systems. Heat pump units are installed inside the building; they aren't exposed to the weather. This is especially important, for example, in coastal areas were the atmosphere can be very corrosive
  • Tenant Satisfaction. Individual control of occupied spaces. This system is well-suited to "tenant finish" installations.
  • More Leasable Space on New Construction. Smaller mechanical rooms (50% or less in new construction). Units can be installed above dropped ceilings, under windows, in closets.
  • Spread Out and Lower Construction Costs. Can used phased construction methods. While the loop piping and accessories are installed as the building progresses, the heat pumps can be installed as tenant space is leased or scheduled for occupancy.
  • Options such as integrating with the fire sprinkler system can further reduce first costs.

Advantages for Consulting/Design Engineers:

  • Simple to design
  • Satisfied clients with many advantages for Owner/Developers
  • WLHP units available from reliable manufacturers and are covered by ARI Certification programs.
  • Service life of commercial Water-to-air heat pumps is 19 years; compares to central station equipment in the 20-year range. (ASHRAE 1999 Applications Handbook page 35.3)
  • System can be integrated with the fire sprinkler system.
  • Other heat recovery and enhancement options are available.


While WLHP systems have many advantages, these are some of the trade offs that should be considered:

  • Accessibility to terminal units, particularly units located above the ceiling, is important. Architects and mechanical and structural designers must carefully coordinate their work.
  • A loop circulating pump must operate continuously and these pumping costs can be high. They are not tenant meterable
  • If core areas do not require many hours of cooling during the heating season, (such as apartment houses, or hotels with infrequently used public spaces), higher boiler use can offset the high heat pump COP
  • Maintenance and filter changing is required in tenant spaces and there are compressors, fans and fan motors located throughout the building
  • Access to ceiling mounted units can be difficult; needs careful design consideration
  • Water piping is installed throughout building with possibility of leaks (but no more than with under-window fan-coil or wet sprinkler systems)
  •  Design of system is no more sensitive to error than a central station system
  • Requires careful coordination among the design and construction team to avoid future problems
  • Increased internal electrical distribution system to serve each heat pump (could cost 2 to 5% more)
  • Requires separate ventilation system to supply fresh air to all zones
  • Secondary or backup heat sources are required in cooler climates

Application Considerations 

The water source heat pump is, by definition, a heat recovery system. It is best applied to buildings that have simultaneous cooling and heating loads. This is the case during winter months during which some units cool interior zones while others heat perimeter zones. The heat rejected by cooling units is used to warm the zones calling for heat. A boiler is used to warm condensing water during the peak heating periods, if necessary. Also, a cooling tower is required to reject the heat energy from the condenser water loop during periods of high cooling demand.

Water source heat pumps can be suspended in the ceiling plenum, floor mounted behind walls or placed directly in the occupied space as a console unit. There are also rooftop and unit ventilator type water source heat pumps.

Water source heat pump systems generally cost less to install than a central built-up system. They offer individual zone control with the added flexibility of being able to accommodate changes in location and sizes as thermal zones or zone occupants change. This system is often installed in ceiling plenums, which frees up valuable floor space.

Another valuable benefit of water source heat pumps is that they can accommodate simultaneous calls from zones requiring heating or cooling. Depending on the climate, outside air may require preheat or cooling prior to being introduced to the unit. In the example of ceiling mounted water source heat pumps, put outside air ducts near each unit to improve indoor air quality.

On the negative side, this system often experiences a higher maintenance cost and shorter replacement life than other systems because of continuous fan and compressor operation during heating and cooling modes. The system makes some noise since the compressor and fan are commonly located close to the zone occupant. The noise can be minimized by placing units away form the occupied space and ducting the supply air to the zone.

Often, multiple units serve an occupied space, so, if one were to fail, the other units could back it up until the unit is repaired. The packaged design of most unit types allows quick change-out by service personnel so maintenance can be performed off site.

Performance Ratings 

The Air-conditioning & Refrigeration Institute (ARI) publishes the Directory of Certified Applies Air-conditioning Products every six months, with updated standard ratings of all models certified under ARI Standard 320 - Water-Source Heat Pumps. These standard ratings are based on tests performed at Standard Rating Conditions. These standard conditions are shown in this table:

Outdoor Surface
Test Indoor Coil -Air Entering Temperature Water Source Temperature Air Temp Surrounding Unit
Standard Rating Conditions 80 °F 67 °F 85 °F 95 °F 80 °F
Standard Rating Conditions Heating Cycle 70 °F 60 °F (max) 70 °F - 70 °F
Maximum Operating Conditions Cooling Cycle 95 °F 74 °F 95 °F - 95 °F
Maximum Operating Conditions Heating Cycle 80 °F - 90 °F - 80 °F
Low Temperature Operations Cooling 67 °F 57 °F 65 °F - -
Insulation Efficiency 80 °F 75 °F 80 °F - -
Water flow as determined in Standard Rating Cooling Test

See the ARI Directory for the details of these tests.

The Directory shows, for each model listed, the Btuh cooling capacity and EER, and the Btuh heating capacity and COP Standard Rating. The cooling EER (Energy Efficiency Rating) is the ratio of Btuh cooling capacity per watt of power input, expressed in Btuh per watt. The heating COP (Coefficient of Performance) is 3.412 Btuh per watt times the ratio of the total Btuh heating capacity provided by the refrigeration system including circulating fan heat (but excluding supplementary resistance heat) divided by the total electrical input in watts. It is a dimensionless number.

Power consumed by the loop water pump and cooling tower is not included. To determine the total system performance, such power consumption must be included.


Individual model efficiencies vary widely between family models and manufacturers. A brief review of a recent ARI Directory showing multiple manufacturers and sizes indicated these ranges:

ARI Ranges
ARI-rated Cooling ARI-rated Heating
Capacity Range Btuh EER Range Average EER Capacity Range Btuh COP Range Average COP
Single Package - Ducted
6,200 to 124,000 10.2 to 16.1 11.1 to 14.5 8,200 to 168,000 3.8 to 5.4 3.8 to 4.9
Single Package - Free Air Delivery (no duct)
6,600 to 32,400 9.6 to 15.8 11.5 to 13.1 7,900 to 40,000 3.8 to 5.1 4.0 to 4.4
Split System - Ducted
14,000 to 78,000 11.0 to 14.0 11.0 to 14.0 18,500 to 92,000 3.8 to 4.8 3.8 to 4.8

At ARI conditions, the heating capacity is typically 25 to 35% greater than the Btuh cooling capacity

Applications to Avoid

The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Standard 90.1 specifies minimum efficiency standards for water source heat pumps. The current Standard 90.1-1989 is in the process of revision and it is expected these values will be increased to those under ISO 13256-1 effective approximately early 2002.

Standard 90.1 Minimum Efficiencies
Current Proposed Revision
Less than 17,000 Btuh 9.3 EER 11.2 EER
17,000 up to 65,000 Btuh 9.3 EER 12.0 EER
65,000 Btuh up to 135,000 Btuh 10.5 EER 12.0 EER
Heating - -
Up to 135,000 Btuh (11.25 tons) 3.8 COP 4.2 COP

There are no efficiency requirements for 135,000 Btuh cooling capacity and larger. Available units range from 11.0 to 12.9 EER and 3.8 to 4.2 COP.

Higher Efficiency Models 

Some manufacturers have been developing higher efficiency units and this trend is expected to continue. As shown above, current units have a cooling efficiency range of 11.9 to 14.5 EER and heating range of COP of 3.8 to 4.9. It is expected available efficiencies will soon approach an EER of 16 and a COP of 5.5.

Application Ratings

Many applications of WLHPs require ratings under other than the ARI Standard Rating Conditions. Manufacturers publish applications ratings on their specifications sheets at various ranges of conditions at which their models will perform, along with the standard ARI ratings.


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