Heat Pumps

By Frank G. Ross

When temperatures are dropping outdoors it’s especially nice to be indoors in a warm environment.  The heating system, along with the cooling system, is checked as a part of a home inspection.  Homes with heat pumps generally prompt two main questions:  How do heat pumps heat and how do they work in air-conditioning mode?  The best way to answer these questions is to start by explaining how an air-conditioner works. 

Forced air units that have central air-conditioning, or a heat pump, are a two-part unit, partly inside and partly outside the home. Forced air systems consist of distribution ductwork that spreads the conditioned air throughout the house, and a fan to help this process located inside some type of furnace or air-handler. Between the furnace or air-handler and the ductwork is an evaporator coil that all the air blows through before it is distributed to the house. This is the inside part of system.

   Attached to the evaporator coil are two copper refrigerant lines which go outside to the compressor. The compressor is in either the air-conditioner or the heat pump unit, depending on which one you have. The outside unit has the compressor on the inside, and surrounding the compressor is a condenser coil (it looks like a radiator). In this unit there is a fan that blows on the coil to extract the heat it produces. It appears that an air conditioner cools your home’s air, but actually it removes heat from the indoor air and transfers that heat to the outdoor air. Heat is extracted from the home by passing indoor air across the evaporator coil in the indoor unit. The refrigerant lines then carry the heat to the outdoor unit to the condenser coil where it is released into the outside air. The cooling cycle continues until the indoor temperature reaches the thermostat setting. So in the cooling mode the coil on the inside of the house is cold and the coil on the outside of the house is hot. A heat pump works exactly like an air-conditioner in the cooling mode. It extracts heat from inside the home and transfers it to the outdoor air. The heating cycle is what makes a heat pump different from an air conditioner.

Unlike an air conditioner, a heat pump has a reversing valve that reverses the process, so in the heating mode the unit collects heat from the outdoor air and transfers it inside to heat your home. (Even when the air outside feels cold, it still contains some heat.)  Heat is extracted from the outside air by passing the air across the condensing coil in the outdoor unit. The refrigerant lines then carry the heat to the indoor unit to the evaporator coil where it is released into the indoor air through the use of a fan. As with the cooling cycle, the heating cycle continues until the indoor temperature reaches the thermostat setting. So in the heating mode, the opposite condition of the cooling mode, the coil on the inside of the house is hot, and the coil on the outside of the house is cool. In below freezing conditions, there may not be enough heat in the outside air to meet the demand of the thermostat setting, so an electric heater backup coil in the indoor unit helps to make up the difference to warm your house. Additionally, in very cold climates, the exterior unit should have a mechanism to defrost the coils periodically.

Central air conditioning and forced air furnaces are much more common in our area than heat pumps.  I see heat pumps in only about 20% of the homes I inspect.  As in most United States climates the outdoor-to-indoor temperature differential in Southwest Utah is greater in the winter than in the summer, and it is more difficult for the heat pump to move the heat. Hence the heat pump is less efficient in the winter.  However, since a heat pump is only moving heat instead of making new heat, it is much more efficient than direct heating (like electric baseboard heaters), easily able to transfer two to three times as much heat as can be directly generated with the same amount of electricity. Though heat pumps are always more efficient than direct heating, they are only more cost-effective in homes that are well insulated, as the capital cost will not be recovered if the heat leaks out too quickly. From a financial standpoint, heat pumps make the most sense where gas and electricity prices are comparable, because a heat pump typically runs on electricity.

Regardless of the type of system installed in a home, regular maintenance is a key to extending the life of the appliance.  Have it looked at by a professional at least every other year.  Additionally, the unit will work more efficiently if the filter is changed (or washed for the permanent type) once a month. The outside AC condensing unit needs a good air flow to work properly. Keeping leaves and debris out of the unit as well as pruning back bushes that are in close contact with the unit will also allow the unit to work more efficiently.

Frank Ross is a Certified Utah Home Inspector for Pillar to Post Professional Home Inspection and a full member of the National Association of Certified Home Inspectors and a member in good standing with the Utah Better Business Bureau.  Readers may contact Mr. Ross at (435) 867-6400 or frank.ross@pillartopost.com or www.pillarposthomeinspection.com