Auxiliary Heat

What auxiliary heat actually is

A heat pump does not create heat the way a furnace does. It moves heat from outside to inside, even in winter, because there is still usable warmth in outdoor air well below freezing. That process is efficient, which is why heat pumps are popular — but its output falls as the outdoor temperature drops, because there is less heat available to move.

Auxiliary heat is the backup that covers the gap. In the great majority of residential systems it is a set of electric resistance heating elements — heat strips — built into the air handler, essentially the same idea as a large toaster or space heater. When the heat pump alone cannot keep up with the thermostat's demand, the system energizes those strips to make up the difference.

The word to notice is backup. Auxiliary heat is not meant to be the primary source of heat. It is meant to assist the heat pump during conditions the heat pump cannot handle alone, and then switch back off when it is no longer needed.

When auxiliary heat is supposed to come on

There are three normal situations where a well-functioning system runs auxiliary heat, and none of them is a problem.

The first is genuinely cold weather. Below roughly the mid-30s Fahrenheit, and increasingly as it gets colder, a heat pump's output drops to the point where it needs help holding temperature. The exact threshold depends on the equipment and how the system was set up, but the principle is simple: colder outside means the heat pump does less of the work and the auxiliary heat does more.

The second is a large jump in the thermostat setting. If you raise the thermostat several degrees at once — say, turning it up after it was set back overnight — the system may bring on auxiliary heat to reach the new setting faster than the heat pump could alone. Many thermostats do this automatically.

The third is defrost. In cold, damp conditions a heat pump periodically reverses itself briefly to melt frost off the outdoor coil. During that defrost cycle the system would otherwise blow cool air indoors, so it runs auxiliary heat to offset it. That is why you might see auxiliary heat flick on for a few minutes and then off again, sometimes with a puff of steam from the outdoor unit.

When it is a problem

Auxiliary heat becoming a problem is less about whether it runs and more about when and for how long.

The clearest warning sign is auxiliary heat running in mild weather. If the outdoor temperature is comfortably above freezing — in the 50s, say — and the thermostat is still showing auxiliary or emergency heat, the heat pump is not doing work it should easily be able to do. That points to a mechanical problem: low refrigerant charge, a failing reversing valve, a defrost control that is stuck, an outdoor unit that is iced over or blocked, or airflow problems on the indoor side.

The second warning sign is auxiliary heat that runs constantly rather than intermittently, even in genuinely cold weather. A system leaning on the electric strips continuously, instead of using them to supplement the heat pump, is both expensive to run and usually a sign the heat pump is not carrying its share.

The reason this matters to a homeowner is cost. Electric resistance heat is one of the more expensive ways to heat a home. A heat pump running normally is efficient; a heat pump that has quietly stopped working and is running entirely on its backup strips can raise a winter electric bill dramatically, and the first sign is often the bill rather than any obvious loss of comfort — because the strips do keep the house warm, just expensively.

Auxiliary heat versus emergency heat

These two get confused constantly, and the difference is worth understanding because the thermostat gives you a switch for one of them.

Auxiliary heat is automatic. The system decides on its own when to bring the backup strips on to assist the heat pump, and you generally never touch it. It comes on when needed and goes off when it is not.

Emergency heat is a mode you select manually. Setting the thermostat to emergency heat, sometimes labeled EM HEAT, tells the system to stop using the heat pump entirely and run only on the backup heat. It exists for the situation where the heat pump itself has failed or been damaged — a compressor problem, a unit hit by ice, an outdoor coil that is not working — and you need to keep the house warm on the backup while you wait for a repair.

The practical takeaway: you do not switch to emergency heat just because it is cold. That is what auxiliary heat is already handling automatically. Emergency heat is for when the heat pump is actually broken. Running in emergency heat uses only the expensive resistance strips, so it is not a mode to leave the system in longer than necessary.

What to do if you are not sure

A short checklist covers most situations. If it is cold out and auxiliary heat is coming on and off, the system is very likely working exactly as intended, and you can leave it alone. If it is mild out and the thermostat is showing auxiliary or emergency heat, or if auxiliary heat seems to run all the time regardless of weather, that is worth having looked at — the heat pump may not be doing its job.

A couple of quick things are worth checking yourself first. Confirm the thermostat is not accidentally set to emergency heat, which would force the strips to run continuously. Look at the outdoor unit: if it is encased in ice or buried in leaves and debris, clearing an obvious blockage may help, though a unit that repeatedly ices over needs a technician. And notice whether the problem tracks with the weather or ignores it — a system that only uses backup heat when it is genuinely cold is behaving correctly, while one that uses it in mild weather is not.

Beyond that, diagnosing why a heat pump is leaning on its auxiliary heat — refrigerant charge, defrost controls, the reversing valve, airflow — is measurement work for a qualified technician, because the pressures and control behavior that tell the story cannot be read from the thermostat.