What Is Aux Heat, and Does Your Thermostat Control It?

On this page (8 sections)

1. What is aux heat on a heat pump?
2. Does your thermostat control auxiliary heat?
3. When does aux heat turn on?
4. What should you check before choosing a thermostat?
5. Which thermostats should you consider for aux heat?
6. How should you set up and care for the control?
7. Why might aux heat run often?
8. How should understanding aux heat guide your final choice?

Aux heat is backup heat for a heat pump, often electric resistance heat that adds warmth when the pump needs help. Your thermostat can request it automatically, so seeing AUX does not by itself mean something is wrong.

Understanding what aux heat is helps you avoid 2 costly mistakes: switching to emergency heat unnecessarily and buying a thermostat that cannot control your backup equipment.

What is aux heat on a heat pump?

Aux heat is a secondary heat source that helps when the heat pump cannot supply enough warmth on its own. Many systems use electric heat strips inside the air handler.

A heat pump moves heat from outdoor air into your home. As outdoor conditions change, its available heating capacity may fall while your home's heat demand rises.

Electric resistance backup uses electricity to produce heat directly. It generally uses more electricity for the same heat output than a heat pump operating efficiently, so unnecessary backup use can increase energy costs.

Some homes pair a heat pump with a fuel-burning furnace instead. These dual-fuel systems need controls suited to that arrangement because the furnace and compressor may need to operate separately.

What does aux heat mean on the display?

An AUX indicator usually means backup heat has been requested. It does not measure how much heat reaches your rooms or prove that every component is working.

Brief use during cold weather can be normal. A new pattern of long AUX calls with falling room temperature deserves attention.

Does your thermostat control auxiliary heat?

A compatible thermostat can call for auxiliary heat, but the equipment's controls also affect when and how backup heat runs. Replacing the thermostat does not change the heat pump's heating capacity.

Some systems use terminals marked AUX or W2, while emergency heat may use an E terminal. Labels vary, and a letter alone does not establish what a wire controls.

Installer settings matter as much as terminal connections. A thermostat configured for conventional heating may operate a heat pump incorrectly even when the wires appear to fit.

For the full system-matching process, see our guide to smart thermostats for heat pumps.

Is aux heat the same as emergency heat?

Aux heat usually works automatically; emergency heat is a separate operating mode. With electric backup, auxiliary heat can assist the compressor, while emergency heat generally relies on backup equipment without normal outdoor heat pump operation.

Follow the equipment manual if the outdoor unit fails or needs service. Don't select emergency heat merely because the weather turns cold or AUX appears.

When does aux heat turn on?

Aux heat can turn on during high heating demand, recovery from a setback, or defrost, depending on the system. There is no single outdoor temperature at which every heat pump needs backup.

During defrost, some systems use backup heat to limit cool air from the vents. A brief call during that cycle can be part of normal operation.

A large setpoint increase can also trigger backup heat on some controls. Turning the thermostat much higher does not make the heat pump itself produce unlimited extra heat.

Use modest schedule changes and watch whether your home recovers comfortably. Avoid assuming a deep overnight setback will save energy if the morning recovery repeatedly brings on electric backup.

What should you check before choosing a thermostat?

Match the equipment type, heating stages, backup fuel, and wiring before comparing app features. A thermostat marked “heat pump compatible” still needs to fit your particular system.

Use this order:

1. Find the indoor and outdoor equipment model numbers and installation instructions.
2. Identify whether backup heat is electric resistance or a furnace.
3. Confirm the compressor stages and backup control requirements.
4. Check the exact thermostat model's wiring diagrams and compatibility tool.
5. Confirm its power requirements and any needed accessories.

A 2-heat/1-cool designation describes supported stages, but it does not promise compatibility with every multistage compressor or dual-fuel arrangement. Separate auxiliary and emergency connections also need checking.

See what a C-wire does for the common power connection. Our guide to smart thermostats without a C-wire explains power options to investigate before ordering.

Once compatibility is clear, use our best smart thermostats guide to compare the features you'll actually use.

Which thermostats should you consider for aux heat?

These 4 heat pump thermostats suit different control preferences, from Wi-Fi access to flexible local scheduling. Choose among them only after confirming the exact backup heat arrangement.

Honeywell Lyric T6 Pro Wi-Fi Thermostat

Choose the Lyric T6 Pro if you want Wi-Fi and programmable operation for a compatible heat pump with up to 2 heat and 1 cool stage. It also supports conventional systems with up to 2 heat and 2 cool stages.

This model suits buyers who want connected control without choosing a thermostat mainly for extra room accessories. Skip it if your heat pump needs more stages than it supports.

Check the TH6220WF2006 installation diagram against your AUX and E connections. That check should come before removing the old control.

ecobee Smart Thermostat Essential

Choose ecobee Essential if you want app access, a color touchscreen, and optional room sensing. It supports compatible 24 VAC heat pump equipment with 2 heat and 1 cool stage.

It works with Apple HomeKit, Google Assistant, and Alexa. SmartSensors are sold separately, making this worth considering if temperature readings from another room would help guide comfort settings.

Use ecobee's thermostat compatibility checker before buying. The Power Extender Kit is also sold separately for compatible homes without a C-wire.

Skip it if you want only local schedule control or your system fails the compatibility check. A remote sensor cannot correct poor airflow or add heating capacity.

Honeywell T6 Pro Programmable Thermostat (TH6210)

Choose the TH6210 if you prefer push-button programming and a large backlit display. It supports 2 heat and 1 cool heat pump systems, or 1 heat and 1 cool conventional systems.

It suits a household that wants to set a routine at the wall and make occasional manual changes. Start with small temperature setbacks rather than a large morning recovery.

Skip it if phone access is central to your plans, and choose a model with clearly established app functions. Have the installer confirm backup heat operation before leaving the thermostat on its normal schedule.

Honeywell X2P Programmable Thermostat (RTH22B)

Choose the X2P if you want several scheduling formats and filter reminders for a compatible heat pump with electric backup. It supports up to 2 heat and 1 cool heat pump stages.

Choose 7-day, 5-1-1-day, 5-2-day, or whole-week programming to match your routine. A non-programmable option suits homes where someone is usually present and prefers one steady setting.

It needs 2 AA batteries, sold separately. Skip it for 120 to 240 V electric baseboard heat, and don't assume its electric-backup support establishes compatibility with a dual-fuel furnace.

How should you set up and care for the control?

Document the existing wiring and follow the exact model's instructions before making changes. Turn off power to the HVAC equipment before removing the thermostat from its base.

Photograph the connections and label wires by their terminal letters. Never use wire color alone or copy terminal positions from a different thermostat.

Heat pump setup may include reversing-valve behavior, compressor stages, and backup heat type. Guessing at those settings can cause incorrect operation, so use professional help when the equipment requirements are unclear.

After installation, confirm normal heating and cooling operation as directed by the manufacturer. Have a technician verify auxiliary and emergency heat functions if that requires service settings you do not understand.

Keep thermostat vents free of dust and follow battery replacement instructions where applicable. Check the HVAC filter on the equipment maker's schedule, since a reminder cannot tell you whether the filter is actually dirty.

Our guide to smart thermostat installation costs helps you decide when to budget for professional setup.

Why might aux heat run often?

Frequent aux heat can reflect cold weather, aggressive schedule changes, restricted airflow, or an equipment problem. Compare the pattern with indoor comfort rather than judging the display alone.

Check that the thermostat is in normal Heat mode and that the schedule is not repeatedly demanding large increases. Inspect accessible filters and keep supply vents and returns unobstructed.

For schedule planning, see programmable thermostats for heat pumps with aux heat. Change one setting at a time so you can understand its effect.

Arrange service if AUX use rises sharply in similar weather, the home cannot reach its setpoint, or the outdoor unit appears inactive during normal heating. Don't disable backup heat or force unfamiliar lockout settings to hide the symptom.

How should understanding aux heat guide your final choice?

Knowing what aux heat is should lead you to a wiring-first decision. If your current thermostat controls the system correctly, an AUX message alone is not a reason to replace it.

Pick the Lyric T6 Pro for compatible Wi-Fi control, ecobee Essential for app and optional sensor features, the TH6210 for wall-based programming, or the X2P for flexible schedules with electric backup.

If compatibility remains uncertain, resolve the equipment details before ordering. Use the heat pump thermostat guide to narrow the choices around your actual system.

Frequently asked questions

What is aux heat on a heat pump?

Aux heat is a secondary heat source that helps a heat pump meet your home's heating demand, often through electric resistance strips. It may run during cold weather, recovery from a temperature setback, or a defrost cycle, depending on how the system is designed and configured.

Does the thermostat control aux heat?

A compatible thermostat can request auxiliary heat through the correct wiring and installer settings, while equipment controls determine how the system responds. Before replacing it, match the exact thermostat model to your heat pump, backup heat type, heating stages, and any separate emergency heat connections.

Is aux heat the same as emergency heat?

Aux heat normally adds backup heating automatically when needed, while emergency heat generally uses backup equipment without normal outdoor heat pump operation. Follow your equipment manual before selecting emergency heat, since the wiring and controls determine its behavior and an AUX message alone is not a reason to select it.

Should I turn off aux heat?

Don't disable aux heat simply to reduce energy use, because your system may need it to maintain indoor temperature in cold weather. Start with modest schedule changes and routine airflow care, then arrange service if backup heat runs unusually often or your home cannot reach its setpoint.