Heat Pumps in a Nutshell

By Frank G. Ross

Although it doesn’t happen often, on occasion I inspect a home that has a forced air system with a heat pump.  I have often answered the question about how this type of system works. The best way to explain it is to start by explaining how an air-conditioner (or a heat pump in cooling mode) works.

Forced air systems that have central air-conditioning are two-part units, the part inside the home and the part outside the home. Inside the home forced air units consist of ducts which distribute the conditioned air throughout the house with the help of a fan. Attached to the duct system is some type of furnace or air-handler, where the blower fan is located. Within the ductwork near the fan is an evaporator coil that all the air blows through before it is distributed to the house.  Leading from the coil are two small refrigerant lines which go outside to the compressor.

The compressor is inside the air-conditioner unit. Surrounding the compressor is a condenser coil (it looks like a radiator). Also in this unit is a fan that pulls air in through the condenser coils to extract the heat it produces. Because an air conditioner is used to cool a home, it appears that an air conditioner cools the home’s air, but it actually 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 process 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 difference is in the heating cycle. A heat pump has a reversing valve, which reverses the process, so in the heating mode the unit collects heat from the outdoor air and transfers it inside your home. In that case the coil on the inside of the house would be hot, and the coil on the outside of the house would be cool. Even when the air outside feels cold, it still contains some heat. The heat pump extracts the heat from the outdoor air and sends it inside to warm your home. 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 strip in the indoor unit helps to make up the difference to warm your house.   And that’s the process in a nutshell.

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