Portland 503 231 4009
San Diego 619 823 1573
Seattle 206 933 9651
© 2010, 2011 Western Environmental Services Corporation. All rights reserved.
Permission granted to reproduce for personal and educational use only.

Hybrid Water Source Heat Pump Systems

Summary

Traditional water source heat pump systems are designed for applications where there is a simultaneous call for heating and cooling throughout much of the year. This paper explains how water source heat pumps work and compares traditional and hybrid water source heat pumps. Hybrid water source heat pump systems outperform traditional systems by combining the best of water-cooled air conditioning and hydronic heating for greater efficiency and quieter operation.

How traditional water source heat pump systems work

A traditional water source heat pump system’s biggest selling point is its ability to move heat from zones that need cooling to zones that need heating. These systems are designed for applications that need coincident heating and cooling for most of the year, such as:
  • high-rise office buildings
  • hotels
  • condos
  • retirement facilities
  • schools and universities
In each of these applications, when the outside air temperature falls below about 55°, some rooms or offices in the building’s interior or its south-facing side require cooling while exterior or north-facing rooms require heating. The most efficient way to simultaneously cool and heat is to move the heat from the spaces needing cooling to the spaces needing heating.

A traditional water source heat pump system was designed to do just that. In areas that need cooling, the heat pumps serving those areas extract heat from the air, add the heat of compression, and reject the heat to a water loop. The water source heat pumps serving the areas that need heat extract heat from the water loop, add the heat of compression, and reject the heat to the air. Figure 1 is an overview of how a traditional water-source heat pump system works.

Figure 1 Traditional water source heat pump system operation

A water source heat pump system is made up of multiple units. At any given time, some units will be operating in the cooling mode and some units will be operating in the heating mode to meet the simultaneous cooling and heating demands of buildings such as office towers, retirement facilities, and schools.

Figure 2 shows a traditional water source heat pump operating in the cooling mode.
Figure 2 Traditional water source heat pump in the cooling mode

Figure 3 shows a traditional water source heat pump working in the heating mode.
Figure 3 Traditional water source heat pump in the heating mode

In the ideal scenario, the units doing the cooling would add the exact amount of heat to the loop as required by the units taking the heat out of the loop. In an unbalanced scenario, either more heat is put into the loop than the heat pumps in heating mode need or more heat is taken out of the loop than is being put in by the heat pumps in cooling mode.

Unbalance between heat generated and heat required is why a water source heat pump loop includes a boiler and a fluid cooler. When it is necessary to add heat to the loop (that is, when more heat pumps are in the heating mode than are in the cooling mode), the boiler is activated to add heat to the water loop. When heat must be rejected from the loop (more heat pumps are in the cooling mode than are in the heating mode), the fluid cooler is activated and heat is rejected to the outside. A central controller senses water temperatures and automatically controls the boiler and cooling tower.

High COP

Because most of the heat being supplied to the areas that require heat is heat that is transferred from the areas needing cooling, a water source heat pump system is considered one of the more optimum designs for buildings that need coincident cooling and heating. In fact, typically the traditional water source heat pump provides approximately four times more energy than it consumes when it is in the heating mode, giving a water source heat pump a coefficient of performance (COP) of 4. In comparison, a high-efficiency gas boiler or gas furnace has a COP in the range of .90 to .95.

Rating a system’s COP is a common way of comparing heating systems. The COP is determined by dividing the amount of energy delivered from a heating system by the amount of energy consumed by that heating system.

Technically, no system can deliver more energy than it uses to create that heat. The first law of thermodynamics states that energy can be neither created nor destroyed—it merely changes form. So how can a typical water-source heat pump have a COP of around 4? Because 75% of energy output comes from the fluid loop (free energy) and only 25% comes from compressor and fan heat (purchased energy).

The equation below shows how a COP of 4 for the traditional water source heat pump is arrived at.



Energy out (heating)
3 parts fluid heat +
1 part compressor & fan heat
COP = ----------------------------= ------------------------------------------=4
Energy in (kW/hr)

1 part compressor & fan
(purchased energy)

Traditional water source heat pump systems' weaknesses

As good as they are at transferring heat from areas needing cooling to areas needing heating, traditional water source heat pump systems have four main weaknesses.

water loop temp limited to 90°
Water loop temperatures are generally designed to not exceed 90º. When water source heat pumps are in the heating mode, they are extracting heat from the water and putting it into the refrigerant in the condenser. If the water temperature is above 90º, the suction gas leaving the evaporator may be too high to effectively cool the compressor.

In the cooling mode the water temperature technically could go as high as 125° without the condenser refrigerant temperature getting too high. However, the traditional water source heat pump system’s water loop is always kept at 90° or below because heat pumps are operating in both the cooling and heating modes. The 90º temperature ceiling limits the capacity of the water loop to hold heat and too often causes the loop to reject heat from the water loop while some of the building zones are in the heating mode and could use the free heat that is being rejected.
not optimized for either heating or cooling
Because the traditional water source heat pump is designed for both heating and cooling, it cannot be optimized for either. For example, manufacturers of water source heat pumps typically use a coaxial heat exchanger for the cooling-side condenser. If the units were designed only for cooling, a more efficient shell and tube heat exchanger could be used, and the pumping requirement per ton of energy exchanged could be decreased substantially, reducing both the size of the pumps required to maintain the water loop and the energy cost to run the pump.
electricity is more expensive than gas
As explained earlier in discussing COP, three quarters of the energy provided by a water source heat pump either comes from the water loop and thus is heat recovered by the units in the air conditioning mode or it is energy supplied to the loop from the boiler plant. The remaining ¼ is supplied by the compressor and fan energy. The fan and compressor are powered by electricity, a more expensive energy source than a heat source powered by gas, such as a boiler.
waste heat rejected rather than used for heating
A heat pump cannot handle the temperature extremes of untreated outside air being brought into a building, so a traditional water source heat pump system almost always uses a separate air handler with gas heating to meet the building’s fresh air requirement. Therefore, with a traditional water source heat pump system it is not unusual to see the fluid cooler rejecting heat to the outside (because the fluid loop has reached 90°) while at the same time the outside air system is using gas to heat code-required make-up air.

Why hybrid water source heat pump systems outperform traditional systems

Hybrid water source heat pump systems work like a traditional heat pump system in cooling mode and like a fan coil unit in heating mode to deliver greater economy and comfort. The following sections explain how hybrid heat pump systems work and their advantages over traditional heat pump systems.

Hybrid heat pump systems in the cooling mode

In the cooling mode, the hybrid heat pump works like a traditional water-source or geothermal heat pump, as shown in Figure 4.
Figure 4 Hybrid heat pump in the cooling mode

Hybrid heat pump systems in the heating mode

Unlike a traditional reversing water source heat pump which uses a compressor in the heating mode, the hybrid heat pump uses a hydronic heating coil, as shown in Figure 5.

Figure 5 Hybrid heat pump in the heating mode

Just like a traditional water source heat pump, the hybrid water source heat pump ejects the heat from cooling into the water loop. However, unlike the traditional system, the water loop is run through a hot water coil, and the coil heats the supply air stream.

Hybrid water source heat pumps advantages

Here’s how hybrid water source heat pump systems overcome the traditional water source heat pump system’s weaknesses.

water loop temp to 125º
The hybrid water source heat pump does not have a reversing valve and does not use the compressor in the heating mode. Instead, the temperature of the water loop is allowed to rise above 90°, because the compressor comes on only when there is a call for cooling. (Note that in the Pacific NW the water loop temperature is generally in the 110° to 120° range during the heating season.) This lets the system retain more of the heat generated by the heat pumps that are in the cooling mode without rejecting that heat through the fluid cooler. The higher the water loop temperature, the less energy is needed to generate heat. This saves on basic energy cost, because it is less expensive to move heat than to create heat.
optimized for both heating and cooling
The hybrid unit is designed to maximize both heating and cooling. The cooling efficiency of the refrigerant system is greater than traditional systems, because hybrid systems don’t use the refrigerant system for heating. By using a shell and tube heat exchanger, pumping requirements are reduced from the traditional three gpm per ton to two gpm per ton, thus saving considerably on pumping costs.

Instead of using the compressor to generate heat, the hybrid water source heat pump has a hot water coil in its unit, so it operates as a fan coil in the heating mode. Because the water loop temperature can run at a higher water temperature than a traditional water source system, more free heat from the units in the cooling mode is available. When additional heat is required in the loop, the hybrid system uses the boiler to add heat to the loop.
gas is cheaper than electricity
As discussed earlier, when there is a call for heating,a traditional water source heat pump uses heat from the compressor and fan (which run on electricity) to supply ¼ of the heat delivered. In contrast, the hybrid water source heat pump does not use its compressor in heating mode. Instead, the hybrid system acts like a fan coil and allows the loop temperature to increase, keeping more of the free heat rejected into the loop by the units in air conditioning mode. When more energy is needed than is being provided by the units in cooling mode, a high-efficiency gas boiler delivers more heat to the loop. For example, if electricity costs $.10/KWH and gas costs $1.50/therm, it costs $29.31 to generate one million BTUs with the compressor versus $16.05 with a 93%-efficient gas boiler.
waste heat used rather than rejected
Unlike a traditional water source heat pump system, a hybrid water source system does not need a gas-fired make-up air system. Instead, the make-up air is brought in through an air handler with a hot water coil fed by the building’s fluid loop, the same loop that feeds the hybrid heat pump system. Because loop temperature is allowed to move above 90°, all of the make-up air requirements for heating can be met without using any more gas heat than that provided to the water loop. Therefore you will never have a time when the fluid cooler is rejecting heat while gas heat is being added to the system to meet the make-up air requirements, as happens with a traditional water source heat pump system. Nor will you ever have the fluid cooler rejecting heat while you are using energy to create heat, as can happen when traditional systems’ compressors are operating in the heating mode.

Additional hybrid heat pumps advantages include:
  • Quiet operation, especially in the heating mode.
  • No reversing valve.
  • Longer compressor life because the compressor doesn’t operate in the heating mode.
  • Lower maintenance costs.

FAQs about hybrid water source heat pumps

What is shoulder season?

Shoulder season is the time of year during which a building requires heating and cooling at the same time. For example, offices along the windows of a high-rise office building may need heat while at the same time interior offices and the computer room need cooling. In some climates, shoulder season may last from fall through spring. The longer the shoulder season, the greater the cost benefit of a hybrid heat pump system.

My state’s energy code requires an economizer. Does a hybrid water source heat pump system meet code?

Many states have energy codes that require economizers when temperatures drop to a specified temperature, such as 45º F. Most state codes have an exception for high-efficiency water source heat pumps. For example, the Oregon Energy Code specifies an exception for "systems that use heat recovery between interior and exterior zones." The reasoning is that "an economizer would reduce the amount of time when the compressor would operate in cooling mode during cool outdoor conditions. This would reduce the effectiveness of heat recovery." There may be other exceptions—check your state’s code.

What kind of buildings are good candidates for hybrid water source heat pump systems?

Buildings that have large system diversity such as single-occupancy space on the exterior and high-occupancy space on the interior are excellent candidates for hybrid water source heat pump systems. Office towers, condos, retirement facilities, and educational facilities are generally excellent applications.

Do I have to insulate the pipes?

No. ASHRAE 90.1-1989, section 9.4.8.2 Piping insulation says “all HVAC system piping shall be thermally insulated.” The section also lists exceptions to this requirement. The relevant exceptions include:
  • (b) “piping that conveys fluids having a design operating temperature range between 55 F and 105 F.”
  • (d) “where it can be shown that the heat gain or heat loss to or from the piping without insulation will not increase building energy costs.”
ASHRAE 90.1,2007…6.4.4.1.3 adds exception (c) “where heat gain or heat loss will not increase energy usage.”

These exceptions apply to the hybrid water source hybrid system, because it reduces the amount of energy consumed.

Does cooling efficiency fall with the elevated fluid loop temperatures?

Although cooling efficiency does fall with the elevated fluid loop temperature, the drop is more than made up from the efficiencies gained by the condenser being designed for cooling-only operation. The shell and tube condenser plus other efficiencies made in the refrigerant tubing design gives the hybrid unit a very high efficiency rating.

Does the additional static pressure caused by the hot water coil add an operating energy penalty?

It might. However, the static pressure across the coil is only .05 inch, and the use of the variable speed drives makes the system fan power most likely significantly less than the system it is being compared against. The slight energy penalty that might be due to the .05-inch static pressure pales in comparison to the energy saved by not running the compressor in the heating mode.

CGC Group is the only manufacturer of hybrid water source heat pump systems. How large a heat pump can they supply?

CGC Group supplies hybrid heat pump systems in various sizes:
  • Vertical stack units: ¾ ton up to 5 tons
  • Ceiling-mounted units: up to 5 tons
  • Classroom-style units: up to 5 tons
  • Vertical units: up to 26.5 tons
  • Roof-top units: up to 56 tons

Can the roof-top units recover heat from the building exhaust air?

Yes, the roof-top units have exhaust air recovery capability.

Does the boiler plant need to be sized larger than with a typical water source heat pump system?

The fluid cooler will be smaller, and the boiler plant may need to be larger. However, the boiler plant can be downsized substantially if a heat recovery system is used to recover heat from the exhaust air stream or if an air-to-water heat pump is used as a first-step heat source for the fluid loop.

Is a gas fired boiler always the most cost-effective way to provide back-up heat for the fluid loop?

No. In fact, using an air-to-water heat pump will almost always be a more energy-efficient way to provide back-up heat for the fluid loop. Plus utility rebates and tax credits are available in some areas. Ask your local Wescor office about the incentives available for air-to-water heat pumps.

Hybrid water source heat pump systems--the best way to simultaneously heat and cool
Contact Wescor for additional information on hybrid water source heat pump systems
Oregon, SW Washington, and Idaho
Dave Baasch
503 231 4009
DaveB@wescorhvac.com
Southern California
Dan Salter
619 823 1573
DanS@wescorhvac.com
Washington and Alaska
Spence Braden
206 933 9651
SpenceB@wescorhvac.com