Auxiliary heat, often displayed as “AUX heat” on your thermostat, is a backup heating source that kicks in automatically when your heat pump can’t keep up with demand on its own. It’s typically a set of electric resistance heating strips installed inside your air handler, and your thermostat activates them without any input from you. Seeing “AUX” light up on a cold day is normal, but understanding when and why it runs can save you real money on your energy bills.
How Auxiliary Heat Works
A heat pump heats your home by pulling warmth from outdoor air and moving it inside. That process is remarkably efficient in mild weather, but as outdoor temperatures drop, there’s simply less heat energy available to extract. At a certain point, the heat pump can no longer produce enough warmth to match what your home is losing through walls, windows, and doors. That crossover point is called the balance point, and it varies by home and equipment. For many systems it falls somewhere around 25°F to 35°F, though well-insulated homes with newer heat pumps can push that lower.
Once outdoor temperatures dip below that balance point, auxiliary heat supplements the heat pump. The two run together: the heat pump continues extracting whatever heat it can from outside air, while the electric strips handle the gap. This is different from the heat pump shutting off entirely. Think of it as a teammate stepping in to help carry the load rather than a full substitution.
Three Common Triggers
Your thermostat decides when to fire up auxiliary heat based on a few specific conditions:
- Cold outdoor temperatures. When the outside air drops below roughly 40°F, or below a threshold you or your installer has programmed, the system recognizes the heat pump needs help.
- A large temperature jump. If you raise your thermostat setting by more than 3 to 4 degrees at once, the system knows the heat pump alone will take too long to close that gap. Auxiliary strips kick in to speed things up.
- Defrost mode. In winter, frost builds up on the heat pump’s outdoor coil. The system periodically reverses itself to melt that frost, which temporarily sends cool air through your ducts. Auxiliary heat switches on during this cycle to prevent a noticeable temperature drop inside. A typical defrost cycle lasts 5 to 15 minutes depending on weather and frost buildup.
Auxiliary Heat vs. Emergency Heat
These two terms confuse a lot of homeowners because they often use the same physical hardware, those electric resistance strips. The difference is in how and when they’re activated.
Auxiliary heat is automatic. Your thermostat turns it on and off as needed while the heat pump continues running alongside it. You don’t have to touch anything. Emergency heat, labeled “EM heat” on most thermostats, is a manual setting you switch on yourself. When you flip to emergency heat, the system bypasses the heat pump entirely and relies only on the electric strips. This is meant for situations where the heat pump has broken down completely and you need warmth while waiting for a repair. Running emergency heat as your sole source for days will produce a noticeably higher electric bill, since resistance heating uses far more electricity than a heat pump operating normally.
Why It Costs More to Run
A heat pump in normal operation is efficient because it moves heat rather than creating it. For every unit of electricity it consumes, it can deliver two to three units of heat energy into your home. Electric resistance strips, by contrast, convert electricity into heat at a 1:1 ratio. That means auxiliary heat can use two to three times more electricity per unit of warmth compared to the heat pump’s compressor cycle.
Short bursts of auxiliary heat on the coldest days or during defrost cycles are expected and won’t dramatically affect your bill. The concern is when auxiliary heat runs frequently or for long stretches. If you notice the “AUX” indicator staying on for hours when outdoor temperatures are above 40°F, something is likely wrong.
When “AUX” Stays On Too Long
Occasional auxiliary heat activation is perfectly normal. Persistent activation when temperatures aren’t extreme is a sign your heat pump is underperforming. Several common issues can cause this:
A dirty air filter restricts airflow, which reduces the heat pump’s ability to transfer warmth into your home. The system falls behind, and auxiliary heat fills the gap unnecessarily. Low refrigerant from a slow leak has a similar effect, reducing the heat pump’s heating capacity well before outdoor temperatures would normally require backup. A malfunctioning outdoor unit, whether from a failed defrost control, a faulty reversing valve, or iced-over coils that won’t clear, can also force the system to lean on auxiliary strips far more than it should.
Thermostat programming matters too. If your auxiliary heat threshold is set higher than necessary, say at 45°F instead of 35°F, the strips will engage during milder weather when the heat pump could handle the job alone. Some smart thermostats let you adjust this trigger point directly, while others require an HVAC technician to change the setting.
Keeping Auxiliary Heat Costs Down
The simplest way to reduce how often auxiliary heat runs is to avoid large thermostat swings. Instead of turning heat down dramatically at night and cranking it back up in the morning, keep your setback modest, no more than 2 to 3 degrees. This lets the heat pump recover gradually without triggering the electric strips.
Regular maintenance also helps. Changing your air filter every one to three months, keeping the outdoor unit clear of snow and debris, and scheduling an annual system checkup all keep the heat pump operating at full capacity longer into the cold season. A heat pump that’s clean and properly charged can often handle temperatures well below 30°F before it truly needs help. One homeowner with a well-maintained system in a moderately insulated home reported auxiliary heat not kicking in until outdoor temperatures hit 14°F to 18°F, well below the typical 25°F to 35°F balance point.
If you live in a climate where temperatures regularly drop into the teens or single digits, a dual-fuel system pairs a heat pump with a gas furnace instead of electric strips. The furnace takes over as the backup heat source, and since burning natural gas is generally cheaper than electric resistance heating, this setup can significantly lower winter energy costs compared to relying on electric auxiliary strips alone. Older dual-fuel systems typically switch over to the furnace at around 30°F to 35°F, while newer systems can be tuned to a more precise balance point.