What Is Auxiliary Heat? AUX Heat Meaning & When It Runs
If your thermostat says AUX HEAT or auxiliary heat, it usually means your heat pump is using a backup heat source in addition to normal heat-pump heating. That can be completely normal in some situations, especially during colder weather, thermostat recovery, or defrost.
The important questions are why AUX came on, how long it stays on, and whether the home is still maintaining the thermostat setpoint. Constant or unexpected auxiliary heat can indicate a control, airflow, sizing, backup-heat, or heat-pump performance issue.
What Does Auxiliary Heat Mean?
Auxiliary heat is supplemental heating that helps the heat pump when additional output is needed. On many systems, the thermostat or control board turns AUX on automatically rather than requiring the homeowner to select it manually.
The backup source depends on the system. An all-electric heat pump may use electric resistance heat strips. A dual-fuel system may use a gas or propane furnace as the backup source.
Why Does AUX Heat Come On?
- Cold outdoor weather: the building load may temporarily exceed what the heat pump is carrying by itself.
- Large thermostat changes: some thermostats call for backup heat when recovering several degrees quickly.
- Defrost: some systems use auxiliary heat to maintain indoor comfort while the outdoor coil is being defrosted.
- Dual-fuel control strategy: a furnace may be intentionally brought on below a selected switchover point.
- Heat-pump performance problem: excessive AUX use can also occur when the heat pump is not producing its expected output.
Is It Bad If Auxiliary Heat Comes On?
No. Auxiliary heat is part of the normal design of many heat-pump systems. Occasional AUX during very cold weather or defrost does not automatically indicate a failure.
It becomes more worth investigating when AUX runs for long periods in ordinary weather, appears much more often than it used to, the home is still getting colder, or energy use changes sharply.
Why AUX Heat Can Increase Electricity Use
When the backup source is electric resistance heat, it creates heat directly from electricity instead of moving heat the way a heat pump does. That can make extended AUX operation more expensive than normal compressor heating.
In a dual-fuel system, the cost comparison is different because the backup source may be natural gas or propane. Use actual local energy prices rather than assuming one mode is always cheaper.
Compare heat-pump and furnace operating costs →
Why Large Thermostat Setbacks Can Trigger AUX
Some heat-pump controls react to a large thermostat increase by bringing on backup heat so the home recovers faster. If your system uses electric resistance backup, repeatedly making large temperature jumps can cause more AUX operation than keeping a relatively steady setpoint.
AUX Heat During Defrost
Frost can form on the outdoor coil during cold, damp weather. The heat pump periodically enters a defrost cycle to remove that frost. During defrost, some systems temporarily use auxiliary heat to reduce the indoor temperature drop.
A brief change in sound, visible steam at the outdoor unit, or short AUX operation can be normal during defrost. Heavy ice that does not clear or repeated loss of heating performance should be inspected.
AUX Heat Running Constantly
Constant AUX heat can have several explanations. The home may have a high heating load, the thermostat may be calling for backup aggressively, the equipment may be undersized for current conditions, or the heat pump may have an airflow, defrost, control, electrical, or refrigeration problem.
- Check whether the thermostat is in normal HEAT mode.
- Avoid a large thermostat increase while testing.
- Check the accessible air filter.
- Make sure supply and return registers are not blocked.
- Keep loose snow, leaves, and debris clear of the outdoor unit.
- Note whether the home can maintain the setpoint.
Do not open electrical cabinets, bypass controls, or attempt refrigerant work yourself.
Auxiliary Heat vs. Emergency Heat
Auxiliary Heat and Emergency Heat are not the same thing. AUX is normally supplemental heat that the controls can call automatically. Emergency Heat is typically a manually selected backup-only mode that may stop normal heat-pump compressor heating and rely on the backup source instead.
See the separate Emergency Heat guide for when that mode is used and why it can cost more on systems with electric resistance backup.
Auxiliary Heat in a Dual-Fuel System
A dual-fuel or hybrid system combines a heat pump with a furnace. The heat pump can handle much of the heating load during milder conditions, while the furnace can take over according to the thermostat or control strategy when conditions change.
Exact switchover behavior depends on equipment capability, thermostat settings, outdoor temperature, fuel prices, and installer configuration.
Read the Dual Fuel Heat Pump guide →
When AUX Heat Needs HVAC Service
- AUX runs nearly continuously during relatively mild weather.
- The home cannot maintain the thermostat setpoint.
- The outdoor heat pump does not appear to operate normally.
- The outdoor unit is heavily iced and will not clear.
- A breaker trips repeatedly.
- The thermostat displays an error or abnormal warning.
- There is new grinding, screeching, banging, or repeated buzzing.
Auxiliary Heat FAQs
What does AUX heat mean on a thermostat?
It generally means the heat pump is using supplemental heating in addition to normal heat-pump operation.
Is it bad if auxiliary heat comes on?
No. Occasional AUX can be normal during cold weather, defrost, or thermostat recovery. Persistent AUX deserves a closer look if comfort or energy use changes.
Why is AUX heat on when it is not very cold?
Possible reasons include a large thermostat recovery, aggressive control settings, airflow problems, a performance fault, or a system design that brings on backup heat sooner than expected.
Does AUX heat cost more?
It can when the backup source is electric resistance heat. Dual-fuel systems use a furnace instead, so the cost comparison depends on local fuel and electricity prices.
Can I turn auxiliary heat off?
Do not disable a backup-heat function without understanding how the system was designed. The backup may be needed for capacity, defrost, or equipment protection. Consult the thermostat/equipment documentation or a qualified technician.
Sources
Need Heat Pump Help in Eldora, IA?
If AUX heat is running unexpectedly, include the outdoor temperature, thermostat message, whether the outdoor unit is running, and whether the home can maintain setpoint when you request service.