What to expect from your heat pump when the temperature drops
Heat pumps work by extracting heat from the outside air. Even at very cold temperatures there is heat that can be removed from the air. Heat pumps also have a have backup system (usually electric heat banks) that supplement their heating ability when the outdoor temperature gets below the “balance point” of the house. The "balance point" is the temperature outside at which the unit cannot maintain the set temperature without using the auxiliary (electric) heat. When this happens the auxiliary heat will cycle on and off as needed to maintain the thermostat setting. There is a built in temperature differential of about 3 degrees so that the electric heat is not working when it is not needed. Therefore it is possible that on extremely cold days the temperature reading may be 3 degrees lower than the thermostat setting before the auxiliary heat comes on. When this happens it is not unusual for a heat pump to run non-stop with the electric heat cycling on and off until the outdoor temperature rises above the "balance point."
The temperature of the air blowing out of a heat pump will normally feel cool to the touch. The reason for this is that your body temperature is 98.6 degrees while the heat pump air temperature is around 84 to 86 degrees. This is normal. The only time the air blowing from the heat pump will feel warm to the touch is when the auxiliary heat is on, the unit is put in the emergency heat mode or it is in the defrost cycle which is programmed at the factory. If the unit goes into defrost while the thermostat is calling for heat the auxiliary heat will come on to meet the demand for heat in the house. You may see steam rising from the outside unit when this happens. This is because the unit is melting any frost or ice that may have built up on the outdoor unit as a result of frozen precipitation or typical frost accumulation. This steam is normal and will last only a short period of time but it will look like the unit is "on fire" until the defrost cycle is complete. While in the defrost cycle the outdoor unit will not be running. It will start again after the cycle is complete. This cycle takes about two minutes.
The emergency heat and auxiliary heat are from the same source. The only difference is that when the thermostat is set to the emergency heat mode it completely turns the outdoor unit off. The emergency heat should be used when the heat pump is not functioning in the normal mode. This will provide heat until a service tech can be scheduled to your home.
To recap:
  1. The air coming from the registers will feel cool to the touch.
  2. The auxiliary heat will come on when the temperature outside drops below the "balance point."
  3. The auxiliary heat may come on when the unit goes into the defrost cycle.
  4. The outdoor unit may look like it is "on fire" when it is in the defrost cycle.
  5. The outdoor unit will not be running when the unit is in the defrost cycle or if the thermostat is set to the emergency heat mode.