What Is Auxiliary Heat? Your Heat Pump Thermostat Settings

Does your thermostat indicate the auxiliary heat or aux heat is running even though no one turned it on? Auxiliary heat cannot be manually activated. If the thermostat shows the auxiliary heat is on, it activated automatically.

Some heat pump systems, particularly those in cold climates, include an auxiliary heating option. Auxiliary heating systems assist heat pumps to keep homes at comfortable temperatures through the winter months. Though auxiliary heat is beneficial, specific issues can cause it to turn on unnecessarily. Homeowners can take specific actions to prevent auxiliary heat from activating in this situation. Other matters might cause auxiliary heat to stay on, and faulty heat pump components are usually to blame.

But first, what is auxiliary heat, and why does it turn on?

Auxiliary heat automatically activates when the primary heat pump system can’t keep up. The auxiliary system works with the heat pump to warm the home and turns off automatically when the heat pump can maintain the indoor temperature independently.

Heat pumps operate differently than auxiliary heating systems. Whereas heat pumps transfer heat, auxiliary systems generate heat from electricity or fuel. Heat strips, electric resistance coils in the heat pump air handler, are most prevalent. The heat strips warm up like a toaster when electricity is applied to them, and the warmth transfers to the air flowing through the air handler. Even though heat pumps and heat strips are powered by electricity, heat strips are far less energy efficient but produce warmer heat. The differences between a heat pump vs. furnace are more significant. A gas furnace burns fuel and uses heat exchangers to transfer the flames’ warmth to the air inside the furnace. Gasses from fuel combustion are vented outside the home. Gas furnaces produce blazing heat and heat homes very efficiently.

No, auxiliary heat does not usually indicate a problem. It turns on when supplemental heat is necessary:

• The outdoor temperature drops below the threshold (typically 32 to 40° F)

• The indoor temperature is three degrees below the thermostat’s setting (programmed by the manufacturer)

• The heat pump defrost cycle begins

Auxiliary heat differs from emergency heat (EM heat). While auxiliary heat automatically activates and supplements the heat pump, emergency heat is manually turned on with the thermostat and shuts down the heat pump and compressor. The backup system provides all the home’s heating when emergency heat is activated and runs until it’s manually turned off.

As the name implies, emergency heat should be turned on during emergencies:

• The heat pump is inoperable or unable to provide sufficient heat

• The outside temperatures are below freezing or lower than the safe operating range of your heat pump

• An ice storm occurs, and you want to avoid damaging the heat pump’s coil

Emergency heat systems consume more energy and yield higher energy bills than heat pump systems. Heat strips, baseboard heaters, and other electric resistance heating systems consume approximately 50% more electricity than heat pumps, according to Energy.gov.

Auxiliary heat turns on and off automatically. Typically, the auxiliary heat runs only when the heat pump needs it to supplement heat and deactivates when the thermostat reaches the temperature setting. Some heat pump system issues can cause the auxiliary heat to stay on indefinitely.

The heat pump condenser or outdoor unit contains a fan that assists in the heat exchange process. The auxiliary heat might stay on if efficiency is reduced due to a fan motor problem.

The refrigerants flowing through heat pump systems absorb and transfer heat. Low refrigerant levels in the system reduce efficiency and might necessitate sustained auxiliary heating. Refrigerant levels don’t diminish over time unless there’s a freon leak.

The HVAC compressor is a crucial component in the refrigeration cycle. The compressor pressurizes the refrigerant to pump it through the system’s tubes and help it release the absorbed heat. A damaged compressor reduces the heat pump system’s ability to transfer heat.

Frost forms on the outdoor coil under certain weather conditions and inhibits its ability to absorb heat from the outside air. The defrost cycle runs the heat pump in reverse and transfers the home’s heat to the outdoor coil to melt the frost. The auxiliary heat system activates when the heat pump defrost cycle begins and provides the home’s heat during the ten-minute process. The auxiliary heat might run indefinitely if the defrost control board malfunctions or fails to defrost the coil.

The heat pump reversing valve reverses the refrigerant’s flow to switch between heating and cooling modes. A faulty reversing valve can keep the heat pump in cooling mode and cause cold air to blow from the vents. The air’s low temperature might trigger the auxiliary system to turn on to raise the home’s temperature.

Auxiliary heat can’t be disabled with the thermostat, though some Honeywell thermostats allow users to adjust the threshold that turns the auxiliary heat on. Instead, make the auxiliary heat less likely to turn on by taking specific measures.

Auxiliary heat is a supplemental heating system that works with the heat pump when outside temperatures are too low for the heat pump to keep up. The auxiliary system automatically turns on and generates heat until the indoor temperature reaches the thermostat’s setting. Typically, heat strips installed in the air handler provide auxiliary heat, but furnaces and other heating systems can be used as well.

Emergency heat differs from auxiliary heat. Emergency heat must be manually turned on with the thermostat. It shuts down the heat pump and compressor, and the backup system provides the home’s heating until it’s manually turned off.

Auxiliary heat does not usually indicate a problem. It turns on when supplemental heat is necessary because the outdoor temperature drops below the threshold, the indoor temperature is three degrees below the thermostat’s setting, or the heat pump’s defrost cycle is running.

A heat pump system issue is likely if the auxiliary heat doesn’t turn off and can indicate a problem with the condenser fan motor, low refrigerant, a damaged compressor, issues with the defrost control board, or a faulty reversing valve.

Although you can’t turn off auxiliary heat with the thermostat, you can make it less likely to turn on by lowering the thermostat, making the house warmer, closing off unused areas, and maintaining your HVAC system.