Electronic Programmable Thermostat

DEAR TIM: We moved to a house that has a heat pump. There is a selector button on the thermostat that says auxiliary. When temps are below 40 degrees, should we move the selector button to auxiliary? Our heat pump runs constantly at temps below 40 degrees. Paula C., Somerset, KY

DEAR PAULA: The selector button that says auxiliary is a switch that allows you to bypass the heat pump. If you switch the heat pump into this mode, the heat pump creates heat strictly by using the electric resistance coils inside the air handler unit portion of the system. The air handler is often in your basement and sometimes an attic. The duct work connects to this tall rectangular metal box that is about the size of a water heater.

Creating heat in the auxiliary mode is the most expensive way to heat your home. The switch is there in case the heat pump malfunctions and doesn't work well. It allows you to create heat until such time as a service technician can come to your home and fix the heat pump.

Your heat pump is operating constantly below 40F because you are very close to the balance point of a heat pump. Just below 40F, a heat pump is extracting every last ounce of heat that it can get from the outdoor air. It runs constantly to get the small amount of heat that is outdoors. As the temperature drops into the twenties, teens and below there is less and less heat in the outdoor air. This means the electric resistance coils inside the air handler get hotter and hotter to generate the heat your home needs. Think of these coils as a giant electric toaster.

Heat pumps are very efficient machines when the outdoor temperature is in the 50 - 59F range. But in the dead of winter in cold locations, a heat pump just doesn't get much heat from the outdoor air. If you lived further south such as southern Tennessee or northern Georgia, the heat pump would really help keep your energy costs down as the climate is more moderate the further south you go.

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