Heat Pumps


The air conditioner can pump heat only one way.  The heat pump is a refrigeration system that can pump heat two ways.  Since it has the ability to pump heat into as well as out of a structure, the heat pump system can provide both heating and cooling.

In a heat pump system, the four-way valve is used to switch the unit between the heating and cooling modes of operation.  By changing the mode of operation of a heat pump system, the functions of the indoor and outdoor coils change as well.  In the cooling mode, the indoor coil acts as the evaporator and the outdoor coil functions as the condenser.  In the heating mode, the indoor coil functions as the condenser and the outdoor coil operates as the evaporator.

There is heat in any substance until it is cooled down to -460° F.

The typical heat pump removes heat from the outside air in the winter and deposits it in the conditioned space to heat the house.  (Actually, about 85% of the usable heat is still in the air at 0° F.)  Hence, it is called an air-to-air heat pump.  In summer the heat pump acts like a conventional air conditioner and removes heat from the house and deposits it outside.  From the outside an air-to-air heat pump looks like a central cooling air conditioner.  The term “air to air” indicates the source of heat while the system is operating in the heating mode and the medium that is ultimately being treated.


The air-to-air heat pump resembles the central air-conditioning system.  It has indoor and outdoor system components.  When discussing typical air-conditioning systems, these components are often called the

evaporator (indoor unit) and the condenser (outdoor unit).  This terminology will work for air conditioning but not for a heat pump.

The system’s coils have new names when applied to a heat pump.  The coil that serves the inside of the house is called the indoor coil.  The unit outside the house is called the outdoor unit and contains the outdoor coil.  The reason is that the indoor coil is a condenser in the heating mode and an evaporator in the cooling mode.  The outdoor coil is a condenser in the cooling mode and an evaporator in the heating mode.


Air-to-air systems are normally installed in milder climates- in the “heat pump belt,” which is basically those parts of the United States where winter temperatures can be as low as 10°F.

The reason for this geographical line is the characteristic of the air-to-air heat pump.  It absorbs heat from the outside air; as the outside air temperature drops, it is more difficult to absorb heat from it.  The compressor is a fixed-size pump.  It will have more capacity on a 30°F day than on a 10°.  The heat pump loses capacity as the capacity need of the structure increases.  On a 10°F day the structure needs more capacity than on a 30°F day, but the heat pump’s capacity is less.  Thus, the heat pump must have help.


In an air-to-air heat pump system, the help that the heat pump gets is called auxiliary heat.  The heat pump itself is the primary heat, and the auxiliary heat may be electric, oil, or gas.  Electric auxiliary heat is the most popular

because it is easier to adapt a heat pump to an electric system.


The balance point occurs when the heat pump can pump in exactly as much heat as the structure is leaking out.  At this point the heat pump will completely heat the structure by running continuously.  Above this point the heat pump will cycle off and on.  Below this point the heat pump will run continuously but will not be able to maintain the desired temperature.  Although the auxiliary heat portion of a heat pump system will be

operational at some temperature above the balance point, it is impossible for a heat pump system to satisfy the heating requirements of a structure without auxiliary heat if the temperature falls below the balance point.

While operating in the heating mode, the outdoor coil functions as the evaporator.  When the outside ambient temperature is warm-above about 50°F- the evaporator saturation temperature will be above freezing and the coil will not freeze.  When the outside ambient temperature drops below 50°F, the outdoor coil can begin to freeze, since the evaporator coil operates at a temperature that is about 20°F to 25°F below the outside ambient temperature.  If the frost on the coil is allowed to accumulate, system performance will be reduced.  The defrost cycle is used to defrost the ice from the outdoor coil during winter heating operation.

Remember that the outdoor coil must be colder that the outside air if the coil is going to absorb heat from the outside air.  The need for defrost varies with the outdoor air conditions, which include temperature and relative humidity, and the running time of the heat pump.