How Your Heat Pump Works

I receive a lot of questions about forced air systems with heat pumps. The questions are generally, how heat pumps work for heat and how they work in the air-conditioner mode. The best way to answer most of theses questions is to start by explaining how an air-conditioner works.
With forced air systems that have central air-conditioning, or a heat pump, they are a two-part unit, the part inside and the part outside of the home. Forced air units consist of ducts that distribute the conditioned air through the house, and a fan to help this process. Attached to the duct system is some type of furnace or air-handler, where the blower fan is located. At the furnace or air-handler in the ductwork is an evaporator coil that all the air blows through before it is distributed to the house. All of this is part of the inside unit, now from the coil are two small refrigerant lines, theses 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 process the coil on the inside of the house is cold, and the coil on the outside of the house is hot.
Now 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. Now the difference comes in the heating cycle. 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 transferring it inside your home. So in the heating process the coil on the inside of the house is hot, and the coil on the outside of the house is cool. Even when the air outside feels cold, the air still contains some heat. The heat pump extracts the heat from this cold 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.
You may here different terms when it comes to rating cooling or heating. Most air conditioners have their capacity rated in BTU (British thermal units). Your central air-conditioner may be rated in “tons”, this rating is translated in heating and cooling terms to one "ton" equals 12,000 BTU.
In cooling systems you will here the term “SEER” (Seasonal Energy Efficiency Ratio). The higher the SEER rating, the less your unit will cost to operate. For example a 10 SEER unit is less efficient than a 15 SEER unit.
To rate the efficiency of the heating portion of a heat pump, the term “HSPF” (Heating Seasonal Performance Factor).
Tune in tomorrow for infomation on what a home inspector will look for when inspecting your HVAC system.