Warming Up to Heat Pumps

Inspecting central air conditioning is hard enough, but inspecting heat pumps is even more challenging. Not only is it an air conditioning system that also can act as a heating system, there may be a backup heating system to inspect as well. Water-source heat pumps also include the complexity of circulating pumps, piping and water-to-refrigerant heat exchangers.

To understand water-source heat pumps, it makes sense to first understand air-source heat pumps.

The Air-Source Heat Pump Concept
If we can take heat from a house and throw it outside when we want to keep the house cool, why can we not run it backward in the winter, taking heat from outside and throwing it inside?

In much the same way that a refrigerator can grab heat from its cool interior and throw the heat out into the kitchen, an air-source heat pump grabs heat from the outside air and dumps it into the house.

How can we get heat from cold air? Even though the temperature is low, there is heat in the air. If we can put something even colder outside, the heat will flow out of the outdoor air and into that “colder thing.” That “colder thing” turns out to be the same refrigerant used in the summer for air conditioning.

If we want to grab heat from outdoors when it is 45БЛF (7БЛC) outside, we have to pass a colder liquid through an outside coil. This sounds similar to what we did indoors during the air conditioning season. We are going to use an expansion device to create a low-pressure, low-temperature liquid that can enter the evaporator coil. This cold liquid will take the 45БЛF (7БЛC) outside air being blown across the coil and pick up some heat from it.

The latent heat of vaporization is important, as the liquid entering the outdoor coil is boiled off to a low-pressure, low-temperature gas. The gas moves out of the outdoor coil and moves into the house. Now we have a cool, low-pressure gas coming into the house, but we want to get the heat out of the gas.

We move the gas through a compressor to raise its pressure and temperature. We now have a hot, high-pressure gas that we pass through the indoor coil. In this situation, the indoor coil is the condenser coil. The hot gas passes through the coil and the house air blows across the coil. As the gas inside the coil is cooled, it condenses back to a liquid, giving off its heat to the house air.

This is how we grab heat from the outdoors and dump it indoors. To complete the refrigerant loop, we take the warm, high-pressure liquid back outside, passing it through an expansion device to create a cool, low-pressure liquid that boils off in the outdoor coil (acting as an evaporator). Then we go through the cycle again.

The process is almost the same as the air conditioning process, using a change of state, and pressurizing and depressurizing the gas at the appropriate points. With air conditioning, we move heat from the indoors to the outdoors.

A reversing valve is used to change the direction of the refrigerant flow when changing from cooling mode to heating mode. The indoor and outdoor coils remain the same, the compressor remains the same and the expansion devices perform the same function, although there may be two instead of one. Depending on the climate and installation, both of the refrigerant lines may be insulated.

The problem with air-source heat pumps in cold climates
The ability of an air conditioner or a heat pump to move heat reduces as the outdoor temperature gets further away from the desired indoor temperature.

Although it is unusual to have a summer outdoor temperature that is more than 30БЛF (16БЛC) higher than what we would like indoors, it is not at all unusual to have an outdoor temperature in the winter that is 30БЛF to 60БЛF (16БЛC to 32БЛC) cooler than the inside of the house. The amount of heat that a heat pump can deliver drops as the outdoor temperature drops.

This means that, in some northern climates, the heat pump cannot always deliver enough heat. Auxiliary heating (electric or fossil fuel) often needs to be added to air-source heat pump systems.

What if we could find a heat source with a more constant temperature than the outside air?

Water-Source Heat Pumps
In the heating season, heat can be captured from water in wells, ponds, rivers or lakes and transferred to the household air. Deep water from these sources is a constant 40БЛF to 50БЛF (4БЛC to 10БЛC) year-round, providing a stable heat source in winter and a heat sink in summer. During the summer, heat from the house is dumped into the water.

By passing the refrigerant through a water-to-refrigerant heat exchanger (also known as a water jacket), heat can be pulled out of (or dumped into) the water source. This heat exchanger replaces the outdoor coil and fan found with air-source heat pumps. The relatively constant temperature of the water source means that water-source heat pumps are not subject to the same seasonal inefficiencies as air-source heat pumps. Backup heating systems often are not needed.

Water-source heat pumps may be open-loop or closed-loop.

Open-Loop Systems
Open-loop systems draw water from a source and discharge it somewhere else. For example, the system may draw water from one well and dump it into another. Some authorities require these wells to be at least 100 feet apart.

Because the water runs directly into the water jacket, open-loop systems are highly dependent on good water quality and quantity. Iron content and acidity can damage the heat exchanger. Mineral deposits can clog the heat exchanger, so regular maintenance is recommended. These systems can use up to 10,000 gallons of water per day; therefore, it is crucial that the well can provide enough water at all times of the year. Water disposal needs to be done in an environmentally acceptable manner.

Closed-Loop Systems
Closed-loop systems circulate a liquid (typically antifreeze, such as propylene glycol, calcium chloride or methyl alcohol) through a loop submerged in a lake, for example. This eliminates the water quality and quantity concerns found with open-loop systems. In many ways, closed-loop systems are very similar to ground-loop systems. (Note: Ground-source heat pumps are a topic for another article.)

Closed-loop systems can be more expensive than open-loop systems, depending on several factors, although the antifreeze pumps tend to draw significantly less electricity than the well pumps that are required for open-loop systems.

Water-source heat pumps and related equipment, such as circulating pumps, are contained within the home. Water or antifreeze piping typically is run underground to the source. There will be little, if any, evidence of a closed-loop, water-source heat pump on the exterior of the home.

Water-source heat pumps in condominium buildings
These systems operate in the same manner as closed-loop heat pump systems (described above) except that they use the hot water risers in the building as a heat source, a heat sink or both. Condominium heat pumps tend to be smaller—both physically and in heating and in cooling capacity—than heat pumps in houses. They usually are mounted within wall or ceiling cavities in the living space, although larger stand-alone units also can be found.

Components
Typically, the indoor components of a water-source heat pump include the following:

• a coil to transfer heat to or from the house air (similar to any heat pump or air conditioner)

• refrigerant lines

• an expansion device

• a compressor

• a reversing valve

• a second coil, a heat exchanger, or both; usually, a water jacket that transfers heat between the refrigerant and the antifreeze or water pump(s) to move the antifreeze or water

It is also common to use the heat pump to heat domestic water, helping the conventional water heater. By passing domestic water through the “de-super-heater,” which is another heat exchanger located ahead of the condenser coil, heat is pulled from the hot refrigerant. This typically works best in mild weather and when the heat pump is running in cooling mode, as there is extra heat that can be captured from the refrigerant.

Coefficient of Performance
Heat pumps typically use electric energy to drive the compressor and move air across the coil(s). When a heat pump is grabbing some heat (from the outdoor air, water loop, ground loop or another source) and delivering it into the house, it is consuming electrical energy. If the amount of heat delivered to the house is just equal to the energy used to capture that heat, the coefficient of performance (COP) of the heat pump is 1.0. To put it another way: The COP is greater than 1.0 when we get more energy from our heat source than it costs us to collect the energy. The COP is less than 1.0 when we get less energy than we spend to get it. The COP is 1.0 when the cost equals the benefit. Because electric resistance heat operates at a COP of roughly 1.0, it is better to shut off the heat pump and use electric heat when the COP drops below 1.0.

Unlike air-source heat pumps, water-source heat pumps do not have the COP affected by exterior temperatures. Typically, they are able to maintain a COP greater than 3.0.

This concludes our introduction to water-source heat pumps. It was not possible to cover all the details of all the problems that can occur, but we provide descriptions of these in the ASHI@Home Training Program (https://www.homeinspector.org/ASHIHOME-Training-System).

Carson Dunlop is a consulting engineering firm devoted to home inspection since 1978 (www.carsondunlop.com).