Aux heat, short for auxiliary heat, is a backup heating mode that kicks in automatically when your heat pump can’t keep up with demand on its own. If you’ve noticed “AUX” lighting up on your thermostat during cold weather, your system is working as designed. It’s supplementing your heat pump with a secondary heat source, usually electric heat strips installed inside your air handler, to help your home reach the temperature you’ve set.
How Auxiliary Heat Works
A heat pump heats your home by pulling warmth from outdoor air and moving it inside. This works remarkably well in mild and moderately cold weather, but as outdoor temperatures drop, there’s less warmth available to extract. At a certain point, the heat pump alone can’t keep up with the heat your home is losing through walls, windows, and the roof.
That’s where auxiliary heat comes in. Your thermostat monitors whether the heat pump is closing the gap between the current indoor temperature and your set temperature. When it detects the heat pump is falling behind, it automatically activates the backup heat source. You don’t need to do anything. The thermostat handles this on its own, choosing the most energy-efficient option available at any given moment. Once the heat pump can handle the load again, auxiliary heat shuts off.
The backup source is typically a set of electric resistance heat strips mounted inside your indoor unit. These come in common sizes of 3, 5, 7, and 10 kilowatts. A 10 kW strip, for example, produces roughly 34,000 BTUs of heat. You may notice warmer air blowing from your vents when aux heat is running compared to the heat pump alone.
When Aux Heat Normally Activates
There are a few common scenarios where you’ll see that “AUX” indicator:
- Cold outdoor temperatures. Most heat pumps hit their “balance point,” the temperature where they can no longer meet your home’s heating needs, somewhere between 32 and 38°F. Below that range, auxiliary heat picks up the slack.
- Large temperature jumps. If you raise your thermostat setting by several degrees at once, the heat pump may not be able to close that gap quickly. Aux heat helps bring the temperature up faster.
- Defrost cycles. In cold weather, ice can form on your outdoor unit. The system periodically reverses itself to melt that ice, which temporarily stops heating your home. Aux heat runs during these short defrost cycles so you don’t feel a blast of cold air.
Seeing “AUX” on your thermostat during any of these situations is completely normal.
Auxiliary Heat vs. Emergency Heat
These two terms confuse a lot of homeowners, but they’re quite different. Auxiliary heat is automatic. Your thermostat turns it on and off as needed while the heat pump continues running. The two systems work together.
Emergency heat is a manual setting you switch on yourself. It shuts down the heat pump entirely and relies only on the backup heat source. You should only use it when your heat pump has actually failed, such as a mechanical breakdown or a refrigerant leak, and you need heat to get through until a technician arrives. Running in emergency mode for extended periods will significantly increase your energy bills because you’re using only the least efficient part of your system.
Why Aux Heat Costs More
Heat pumps are efficient because they move heat rather than create it. For every unit of electricity they consume, they deliver two to three units of heat. Electric heat strips, by contrast, convert electricity directly into heat at a one-to-one ratio. A system combining a heat pump with backup strips is roughly 60% more efficient overall than running on electric strips alone. But every minute the strips are active, your electricity usage climbs.
This is why most modern thermostats include a feature called a lockout temperature. It prevents auxiliary heat from activating above a set outdoor temperature, ensuring the heat pump handles the load whenever it’s capable. Washington state’s energy code, for instance, requires this lockout to be set at 40°F or lower, with a target of 32°F at installation. Smart thermostats from brands like Nest and Ecobee take this further by tracking outdoor temperatures over roughly ten days and learning how early the heat pump needs to start warming your home so it can reach your set temperature without triggering the strips.
One limitation of these smart averaging systems: a sudden overnight temperature drop can catch them off guard. If the thermostat expected a mild morning based on recent trends, it may not start the heat pump early enough, causing aux heat to kick in during the morning warm-up even though the heat pump could have handled it with more lead time.
Dual Fuel Systems Use a Gas Furnace Instead
Not every system uses electric strips as the backup. In a dual fuel setup, the auxiliary heat source is a gas or propane furnace. The heat pump handles heating during milder weather because it’s more efficient. When temperatures drop below the balance point, the gas furnace takes over entirely. Furnaces convert fuel into heat quickly and can handle extreme cold without the efficiency penalty of electric strips. If you live in a region with harsh winters and affordable natural gas, a dual fuel system often makes more financial sense than relying on electric backup.
When Aux Heat Signals a Problem
Seeing “AUX” briefly on cold days or after a thermostat adjustment is fine. What’s not fine is aux heat running for hours on end, cycling on during mild weather above 40°F, or staying on constantly. These patterns suggest something is wrong, and every extra hour those heat strips run adds to your energy bill.
Common culprits include low refrigerant levels from a leak, which reduces the heat pump’s capacity. A malfunctioning outdoor unit, such as a failed compressor or a fan motor that isn’t spinning, forces the system to lean on backup heat. A thermostat with an incorrect lockout setting, or one that has lost communication with its outdoor temperature sensor, can trigger aux heat prematurely. Dirty air filters or a frozen outdoor coil that won’t defrost properly can also push the system past its limits.
If your aux heat is running when outdoor temperatures are well above freezing, or if your energy bills have spiked without an obvious explanation, it’s worth having an HVAC technician inspect the system. In many cases, the fix is straightforward: a refrigerant recharge, a sensor recalibration, or a thermostat setting adjustment.