aux heat setting on Nest thermostat for heat pump

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?

What is auxiliary heat?

Auxiliary heat (aux heat) is a supplemental heating system that provides additional heat when the primary heat pump system cannot maintain or achieve the thermostat’s setting due to low outdoor temperatures. The auxiliary system works with the heat pump to warm the home’s air.

Different types of heat pumps are used in homes today. They all function uniquely, but every heat pump has one thing in common–they transfer heat. An air source heat pump, the most common type, transfers heat from the outside air to the air inside the home in heating mode. The refrigerant flowing through the heat pump’s outdoor coil absorbs the air’s heat and releases the heat at the indoor coil.

Heat pumps become less efficient at absorbing heat as outside temperatures approach freezing. An auxiliary system can supplement the heat pump’s warmth by generating additional heat. Electric resistance coils or heat strips in the air handler are the most common auxiliary system, but some heat pumps utilize gas furnaces and are known as dual-fuel heat pumps.

aux heat setting shown on Nest heat pump thermostat

How does auxiliary heat work?

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.

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Is it bad if auxiliary heat comes on?

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 vs Emergency Heat

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

Why does AUX heat stay on?

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.

Problem with Condenser Fan Motor

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.

Low Refrigerant

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.

Damaged Compressor

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.

Issues with Defrost Control Board

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.

Faulty Reversing Valve Problem

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.

How to stop auxiliary heat from coming on?

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.

person lowering the heat pump thermostat settings down

Lower the set heat temperature.

Auxiliary heat turns on when the thermostat is set higher than the heat pump can achieve. Setting the thermostat between 62 and 68°F can prevent auxiliary heat from turning on and saves energy. The U.S. Department of Energy estimates that adjusting your thermostat by seven to ten degrees for eight hours a day can reduce your yearly heating and cooling costs by 10%. A programmable thermostat makes it easy to implement these temperature adjustments while sleeping or away from home.

person's feet with warm cozy socks on in front of fireplace

Make the house warmer.

Take actions that make the house warmer without turning up the thermostat. Open curtains and blinds to allow the sun’s thermal energy to warm the rooms, add insulation to your home or use portable heaters and fireplaces.

person closing door of unused room in house

Close unused areas.

Shut the doors and close the vents in spare bedrooms and other areas of your home that aren’t frequently used so the warm air stays where you spend most of your time.

HVAC tech performing heat pump maintenance

Maintaining your HVAC system is essential.

Heat pump maintenance keeps the system operating efficiently. Regular air filter changes are imperative. How often you change the air filter varies, but filters are generally changed every 30 to 90 days. Schedule annual professional tune-ups to ensure the system works properly and rectify potential issues before they become serious problems.

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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.

Frequently Asked Questions

How long should auxiliary heat run?

Auxiliary heat will run until the indoor temperature reaches the thermostat’s setting or the defrost cycle ends. Both processes should take less than thirty minutes.

Can auxiliary heat run too long?

Yes, you should have a professional inspect your system if the auxiliary heat runs longer than thirty minutes or turns on every time your heat pump system is on.

Will auxiliary heat automatically turn on?

Yes, auxiliary heat automatically turns on when supplemental heating is needed and automatically turns off when the indoor temperature reaches the thermostat’s setting or the defrost cycle ends.

What temp should AUX heat come on?

Auxiliary heat typically turns on when the outdoor temperature is between 32 and 40° F or the thermostat setting is three degrees higher than the indoor temperature.

Is auxiliary heat more expensive?

Yes, auxiliary heat generally has higher operating costs than heat pumps. The one exception might be a gas furnace.

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