Heat pump auxiliary heat running constantly? Here is why

What is the difference between auxiliary heat and emergency heat?

They use the same backup heat, the electric resistance strips inside your air handler, but they turn on completely differently, and knowing which one you are looking at is the whole diagnosis. AUX, or auxiliary heat, is automatic. The thermostat senses the heat pump cannot quite keep up and switches the strips on to help, while the heat pump keeps running underneath. You do nothing, and the two work together.

Emergency heat is manual. It is a mode you select, and it locks the heat pump out entirely, running only the resistance strips for every call for heat. The two show differently on the thermostat: AUX heat reads "AUX" or "Aux Heat," while emergency heat reads "EM HEAT," "EMER," or shows up as a switch position next to Heat and Cool. The key point is that emergency heat is meant for one situation only, when the heat pump is actually broken and you are keeping the house warm until a tech comes. It is not a cold-weather setting, and it is not a way to heat faster.

Why is emergency heat making my bill so high?

Because resistance heat is the most expensive way to make heat, and emergency mode runs nothing but resistance heat. Here is the math in plain terms. A heat pump does not make heat, it moves heat from outside to inside, so for every unit of electricity it uses, it delivers two to four units of heat. That is an efficiency of 200 to 400 percent. Electric resistance strips simply turn electricity into heat at 100 percent, one unit in, one unit out. So the strips cost roughly two to five times as much to deliver the same warmth.

Run the house on emergency heat all winter and you are paying that premium on every degree, which is why a heat pump owner who accidentally bumped the thermostat to EM HEAT in November opens a shocking electric bill in December. The fix costs nothing: switch it back to HEAT and let the heat pump do the work, with the strips only stepping in when they are actually needed. To see what your system costs to run either way, our HVAC operating cost calculator puts numbers on it. Aux heat is one of several reasons a winter bill jumps, and our guide on why your electric bill is so high in winter covers the rest.

When is it normal for the auxiliary heat to run?

Auxiliary heat is supposed to run sometimes, so do not panic every time you see the light. There are three legitimate reasons it comes on. The first is genuinely cold weather. Every heat pump has a balance point, the outdoor temperature where its heat output exactly matches what your house is losing. Above that temperature the heat pump carries the whole load alone. Below it, usually somewhere around 30 to 40 degrees for a typical system, the heat pump makes less heat just as the house needs more, so the aux strips fill the gap. That is the system working as designed.

The second is the defrost cycle. In cold, damp weather the outdoor coil frosts over, and the heat pump periodically reverses for a few minutes to melt it off. During those few minutes it would blow cool air, so the aux strips kick on to keep the vents warm. A normal defrost is brief, about 5 to 15 minutes, and occasional. The third is recovery from a setback, when you ask the thermostat to climb several degrees at once and it calls the strips to get there faster. Brief aux during cold snaps and defrost is normal. Aux that stays on for hours in mild weather is not.

How do I know if the aux heat is running too much?

The simple rule: aux heat running when it is cold out is normal, and aux heat running when it is mild is a problem. If the AUX or EM HEAT indicator stays lit for long stretches while it is warmer than about 40 degrees outside, the system is leaning on the expensive backup when the heat pump should be handling things on its own. A quick defrost cycle here and there is fine. Hours of aux on a 45-degree day is not.

The other tell is the bill itself. If your winter electric bill is far higher than you expected or higher than last year for the same weather, and the aux light seems to be on a lot, those two facts together point at over-reliance on the backup heat. Once you have ruled out emergency mode being switched on, the causes fall into three groups: thermostat settings telling the aux to come on too eagerly, the way you run the thermostat, and the heat pump not being able to make enough heat on its own.

Why does a big thermostat setback make a heat pump use more aux heat?

This one surprises people, because it is the opposite of how a gas furnace works. With a furnace, turning the thermostat down a lot overnight saves money, because the furnace just makes less heat. With a heat pump, a deep overnight setback can backfire and cost you more. Here is why: a heat pump recovers temperature slowly, on purpose, because running gently is what makes it efficient. When you drop the setpoint 8 or 10 degrees overnight, the morning recovery asks the thermostat to climb that whole gap fast, and most thermostats respond by firing the expensive aux strips to get there quickly.

So the strips you were trying to avoid get switched on by the very setback meant to save money. The fix is a habit change, not a repair: keep a heat pump at a steady setpoint, or use only a small setback of a couple of degrees rather than a deep one. Let the heat pump hold the house at a constant temperature and it runs in its efficient range instead of repeatedly clawing back from cold with resistance heat.

Can a thermostat setting make the aux heat run too much?

Yes, and it is one of the more common fixable causes. Three settings control how eagerly the aux comes on, and a tech, or a comfortable homeowner with a smart thermostat, can adjust them. The first is the aux heat lockout, the outdoor temperature above which the thermostat is not allowed to use aux at all. If that is set too high, the strips fire in mild weather. A sensible range is usually somewhere around 30 to 40 degrees. The second is the staging or differential setting, which decides how big a gap between setpoint and room temperature triggers the aux. Set too aggressively, it brings the strips on for a small shortfall the heat pump could have closed on its own.

The third is the most basic and the most overlooked: the thermostat has to be configured as a heat pump with auxiliary heat. If a thermostat was installed or set up as a conventional gas or electric system, or if a non-heat-pump thermostat was put in by mistake, its staging logic is wrong and it leans on the backup heat far too much. This is a frequent problem after a do-it-yourself thermostat swap. If the aux behavior changed right after a new thermostat went in, the system-type setting is the first thing to check.

What faults make a heat pump run on backup heat?

When the settings are right and the aux still runs too much, the heat pump itself cannot make enough heat, so the strips compensate. The homeowner-fixable version is airflow: a dirty filter, blocked vents and returns, or a snow-and-ice-covered outdoor unit all choke the heat pump's output, and the thermostat calls aux to make up the difference. Change the filter, clear the vents, and keep the outdoor unit clear of snow and leaves. A frozen outdoor unit from a failed defrost cycle does the same thing, and that one is a tech fix.

The deeper faults are all tech territory. A low refrigerant charge, which means a leak, cripples how much heat the compressor can move, so the strips run constantly to cover the shortfall. A bad outdoor temperature sensor can misread the weather as colder than it is and trip the aux on in mild conditions. And a heat pump that is simply undersized for the house, or a house that leaks heat through drafts and poor insulation, will lean on aux no matter what, because the heat pump can never quite catch up. If the aux runs constantly even after the easy checks, our guide on a heat pump that will not heat covers the refrigerant and defrost faults, and our guide on heat pumps in cold climates covers whether yours is sized and specced for your winters.

What does it cost to fix a heat pump that runs too much aux heat?

Most of the common causes are free. Switching off emergency heat, raising and steadying the thermostat instead of deep setbacks, changing the filter, and clearing the outdoor unit all cost nothing and solve a large share of cases. Correcting the thermostat's system-type setting to heat-pump-with-aux is also free if you are comfortable in the settings, or a short tech visit if you are not.

From there the paid fixes climb. A new thermostat properly set up for a heat pump runs a few hundred dollars installed and is worth it if yours cannot stage the aux correctly. Adjusting the aux lockout and staging is usually folded into a service visit. The bigger-ticket items, a refrigerant leak repair, a defrost board or sensor, are real repairs in the hundreds to low thousands depending on the part. And if the heat pump is genuinely undersized or the house is losing heat faster than any heat pump could replace, the answer is insulation and air sealing or, eventually, a right-sized system. Start with the free checks, because for a bill that suddenly spiked, the cause is usually the emergency-heat switch or a deep setback, not a broken part.