Why does AUX HEAT (auxiliary heat) light keep coming on at my thermostat, even when it's not that cold outside?
Wednesday, June 13, 2018
Compressor To Aux Heat Delta
Most heat pump thermostats have a setting called “Compressor to Aux Heat Delta,” which is defined as the number of degrees in difference between the set point (desired temperature setting) at the thermostat and the actual indoor temperature. It is a minimum at which the system will turn on the auxiliary heat to run simultaneously with the heat pump as a supplemental “surge” for additional heating capacity. The default number is usually 3º F, but can be adjusted at digital programmable thermostats.
When you turn the heat setting up to 70º while the indoor temperature is 65º, for example, then the system will add the electric resistance heat coil to hasten the temperature increase. If, however, the indoor temp is 65º and you set the thermostat to 67º, the AUX HEAT will not come on as a supplement. In day-to-day ordinary usage, the system would restart before the temperature differential reaches 3º, so the “Compressor to Aux Heat Delta” setting would only kick in when you do a big temperature setting increase, such as after arriving back from vacation on a cold day.
When AUX Heat Activates
The only two other times that the AUX HEAT should come on are when the outdoor temperature is below approximately 35º—which is when the heat pump can’t produce enough heat by itself—or if the condenser (outdoor unit) is in defrost mode. Otherwise, the heat pump system is malfunctioning and it’s time to call for professional repair.
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