What does aux heat mean on a thermostat?
Aux heat means the auxiliary (backup) heat source has turned on to help your heat pump, and it is a label you only see on a heat pump system. A standard furnace-and-AC setup never shows it, because a furnace makes all its own heat and needs no backup. On a heat pump, though, the outdoor unit works by moving heat from outside air into your house, and the colder it gets outside, the less heat there is to move. When the heat pump alone cannot keep up with the thermostat's setpoint, the system energizes the electric resistance heat strips (or a gas furnace, in a dual-fuel system) to make up the difference, and the thermostat flags that with the Aux Heat indicator.
Because the Aux or Emergency Heat setting exists only on heat pumps, that little indicator is one of the surest signs of what you own; our guide on how to tell if you have a heat pump uses it as a primary check. The strips live in the air handler and are the same kind of heating element you would find in an electric furnace or a plug-in space heater: simple, reliable, and expensive to run, because they turn one unit of electricity into one unit of heat while the heat pump delivers two to four.
When does auxiliary heat turn on?
Auxiliary heat turns on automatically, and it does not wait for you to ask. Three situations trigger it. The first is cold weather: as the outdoor temperature drops toward the mid 30s Fahrenheit, a typical air-source heat pump loses capacity, and around its balance point (often 30 to 40 degrees F) it can no longer meet the heating load alone, so the strips come on. The second is a large thermostat jump: if you raise the setpoint more than about 2 to 3 degrees at once, most thermostats call for aux heat to close the gap quickly rather than wait for the slower heat pump. The third is the defrost cycle, when the heat pump briefly reverses to melt frost off the outdoor coil and the strips run for a few minutes so the vents do not blow cold air.
Two of those three are worth managing. You cannot do much about deep cold, but you can avoid the setback penalty by not cranking the thermostat up in big steps, and by keeping overnight setbacks small in winter. This is the backup-heat trap covered in the disadvantages of a heat pump: the money you think you save by turning the heat way down at night gets spent, and then some, when the strips fire to recover in the morning. A smart thermostat that understands heat pumps can ramp the setpoint gradually to keep the strips off; our best smart thermostats guide notes which models handle heat pump aux heat correctly.
What is the difference between aux heat and emergency heat?
Aux heat and emergency heat use the same backup heat strips, but they are not the same setting. Auxiliary heat runs automatically and alongside the heat pump: the thermostat brings the strips in to assist when the heat pump needs help, then drops them as soon as it can, with the heat pump still doing most of the work. Emergency heat (often labeled Em Heat) is a manual mode you select yourself, and it shuts the heat pump off entirely, running the house on the backup strips alone.
The practical rule is short: use emergency heat only when the heat pump is broken. If the outdoor unit is iced solid, making a bad noise, or not running at all, switching to Em Heat keeps your house warm on the strips until a technician can look at it. Turning it on at any other time just forces the most expensive heat source to do all the work while the efficient heat pump sits idle. If you ever find yourself reaching for Em Heat because the house will not warm up, that points to a heat pump problem worth diagnosing rather than a setting you should live on.
Does auxiliary heat run up your electric bill?
Yes. Auxiliary heat is the most expensive way your system makes heat, and heavy use of it is the single biggest reason a heat pump owner's winter electric bill spikes. The reason is efficiency: electric resistance strips run at a coefficient of performance of about 1.0, meaning they return one unit of heat for one unit of electricity. A heat pump in moderate weather runs at a COP of roughly 2 to 4, so it delivers two to four times the heat for the same power. Running the strips therefore costs about two to three times more per unit of heat than letting the heat pump do the work.
The strips themselves are big electrical loads, commonly 5 to 20 kilowatts depending on the system, which is why a house running on aux or emergency heat can pull as much power as several space heaters at once. A brief assist during a cold snap or a defrost cycle barely moves the bill. Aux heat that runs for hours every day, or in weather that is not truly cold, is what does the damage, and it usually signals a fixable problem rather than a normal winter.
How do I tell if aux heat is running, and is it normal?
You can tell aux heat is running because the thermostat shows it: most display Aux Heat or Auxiliary Heat on the screen while the strips are energized, and some also light an indicator when the second stage is active. If you are not sure, set the thermostat to hold a steady temperature and watch it on a cold morning; the aux label will usually appear when the heat pump falls behind.
Whether that is normal depends on the weather. Aux heat is normal when it is genuinely cold (below roughly the mid 30s), during a defrost cycle, or for a short stretch after a big temperature increase. It is not normal if it runs in mild weather above about 40 degrees F, or if it stays on constantly, because that means the heat pump is not carrying its share. The usual culprits are a dirty air filter or coil choking airflow, low refrigerant, or an oversized thermostat setback. Start with the cheap fix and confirm the filter is clean and changed on schedule; a clogged filter is the most common reason a heat pump underperforms and leans on the strips.
Why is my aux heat always on, and how do I stop it?
Aux heat that is always on means your heat pump cannot reach the setpoint by itself, so the strips never get a chance to shut off. In genuinely cold weather that can be expected, but the rest of the time it points to something you can address. Work through the common causes in order: a clogged filter or dirty outdoor coil cutting airflow, a heat pump low on refrigerant, an oversized overnight setback that forces a big morning recovery, or a thermostat set to hand off to aux at too warm a temperature. Smaller setbacks, a clean filter, and a correct changeover setting fix most cases.
You cannot really turn aux heat off, and you should not want to, since it is what keeps the house warm when the heat pump maxes out. What you can do is stop it from running unnecessarily with the steps above, and make sure the system is sized right in the first place, because an undersized heat pump leans on the strips far more than it should; size any replacement with the BTU calculator rather than guessing. One case needs a pro: aux heat showing on the thermostat but no warm air coming out ("aux on but no heat") usually means the heat pump has failed and even the strips are not keeping up, or a control wire has come loose. That is a service call, not a settings change.
Frequently asked questions
Does auxiliary heat run up your electric bill?
Yes, and it is the main reason a heat pump's winter electric bill jumps. Electric backup strips run at a COP near 1.0, while the heat pump runs at a COP of about 2 to 4, so aux heat costs roughly two to three times more per unit of heat. A short assist during a cold snap or a defrost cycle is cheap. Aux heat that runs for hours a day, or in mild weather, is what drives the bill up, and it usually signals a fixable problem.
How do I turn off auxiliary heat?
You cannot switch auxiliary heat off on its own, and you should not want to, because it is the backup that keeps your house warm when the heat pump cannot keep up in the cold. What you can do is keep it from running more than needed: change the filter, raise the thermostat in small steps instead of big jumps, use gentle overnight setbacks, and set the aux changeover temperature no higher than necessary. If aux heat runs constantly in mild weather, have the heat pump checked rather than trying to disable the strips.
Is it bad if auxiliary heat comes on?
No, not by itself. Aux heat coming on in cold weather, during a defrost cycle, or briefly after a big temperature increase is exactly how the system is supposed to work. It is only a warning sign when it runs in mild weather above about 40 degrees F, or stays on all the time, which means the heat pump is underperforming. The usual causes are a dirty filter, low refrigerant, or too aggressive a thermostat setback.
What does it mean when aux heat is on but there is no heat?
Aux heat showing on the thermostat with no warm air from the vents usually means the heat pump has failed and the backup strips cannot keep up on their own, or a control wire or relay has a problem. It can also happen if the strips themselves have burned out. This is not a settings issue you can fix from the thermostat; it is a service call for an HVAC technician to find why the system is not producing heat.
At what temperature does auxiliary heat come on?
Auxiliary heat typically starts helping when the outdoor temperature falls toward the mid 30s Fahrenheit, near the heat pump's balance point, which is often set between 30 and 40 degrees F. The exact point depends on your home's insulation, the heat pump's capacity, and how the thermostat is configured. Aux heat can also come on above that temperature for a few minutes during a defrost cycle, or when you raise the setpoint more than 2 to 3 degrees at once.