What Is Auxiliary Heat on a Heat Pump? (And Why Your Bill Jumped)

What is auxiliary heat on a heat pump? It is the electric backup that switches on automatically when the heat pump cannot keep up on its own. It is normal in cold weather and expensive to run. Seeing AUX in mild weather is the warning sign, and usually means something needs a heat pump service call rather than a thermostat adjustment.

Key Takeaways

• AUX is automatic and normal. Your thermostat turns it on. You do not.
• EM HEAT is manual and different. It shuts the heat pump off entirely and should only be used if the heat pump has failed.
• It is resistance heat, the same principle as a toaster, and it costs far more per unit of heat than the heat pump itself.
• In this climate it should be rare. A properly sized heat pump in Auburn should lean on the strips only in a genuine cold snap.
• Aux in mild weather points to a dirty filter, a big thermostat setback, low refrigerant or a fault.

What Is Auxiliary Heat on a Heat Pump?

What is auxiliary heat on a heat pump in plain terms? Start here. A heat pump does not create heat. It moves heat from outdoor air into your house, which is why it is so efficient. As it gets colder outside there is less heat available to move, so the heat pump’s output falls just as your home’s demand rises.

At some point those two lines cross. That crossing point is called the balance point. Below it, the heat pump on its own cannot hold your setpoint, so the thermostat brings in a second heat source to make up the difference.

In most homes here that second source is a set of electric resistance heating elements in the air handler, usually called strips. They work exactly like a toaster element. Reliable, simple, and expensive. Some homes have a dual fuel setup where a gas furnace plays the backup role instead, which changes the cost picture considerably.

Auxiliary Heat vs Emergency Heat

If you are asking what is auxiliary heat on a heat pump, this is the pair that gets confused, and the difference matters to your bill.

The practical rule is simple. If you switched EM HEAT on yourself, book a repair, because you are paying the most expensive possible way to heat your home until someone fixes the heat pump.

What Auxiliary Heat Actually Costs You

Nobody puts numbers on this, so here is the arithmetic.

Electric heat strips are commonly rated between 5 and 20 kilowatts, with 10 kW being a typical size for an average home. A 10 kW strip running for one full hour consumes 10 kilowatt hours of electricity. Multiply that by your rate per kilowatt hour and you have the cost of one hour of backup heat.

Now compare. Resistance heat converts electricity to heat at roughly one to one. A heat pump moves several units of heat for each unit of electricity it uses, which is why it is the cheaper machine to run whenever conditions let it work. Department of Energy guidance for heat pump owners describes auxiliary strip heat plainly as costly and inefficient, and advises having frequent use looked at. You can read that DOE backup heat guidance in full.

If you are weighing a replacement, ENERGY STAR publishes certified heat pump efficiency data, and higher efficiency systems hold their output further down the temperature scale before the strips are needed at all.

This is why a heat pump that has quietly started leaning on its strips produces a winter bill that looks like a billing error. Nothing about your habits changed. The machine simply stopped using the cheap method.

When Aux Should Come On

There is no single outdoor temperature that applies to every home. Your balance point depends on how well insulated the house is, how big the heat pump is, and how efficient it is. A modern variable capacity system holds its output far further down the scale than an older single stage unit.

As a rough guide, older single stage systems often start calling for backup somewhere near the mid thirties, while a well sized modern system in a reasonably insulated home may rarely need it at all in a typical Puget Sound winter. Manufacturers deliberately avoid publishing one universal number, and they are right to.

Aux should also come on briefly during defrost. When frost builds on the outdoor coil, the system temporarily reverses to melt it, which means it is pulling heat out of your house for a few minutes. The strips run to stop cold air coming from the vents. A defrost cycle normally completes within about fifteen minutes.

Five Reasons Aux Runs When It Should Not

1. A dirty filter. Restricted airflow means the system cannot deliver its heat, so the thermostat calls for help. This is the most common cause and the cheapest fix.
2. A large thermostat jump. Asking for more than two or three degrees at once will usually bring the strips in. Regional research from the Northwest Energy Efficiency Alliance found that recovery from a deep setback can leave the compressor and the backup heat running together. Their Northwest setback research recommends no setback at all for absences under six hours.
3. Low refrigerant. A leak reduces capacity, so the heat pump falls short earlier and the strips cover the gap. Topping up without repairing the leak just delays the same problem.
4. A stuck defrost or failed reversing valve. If the system cannot complete a defrost, it can sit in a cycle where the strips run far longer than they should.
5. Undersized or oversized equipment. A heat pump that never matched the house will lean on backup heat every winter of its life.

How Much Aux Runtime Is Normal Around Auburn

Our winters sit in a narrow band. It is damp and grey for months, and genuinely cold snaps are short. That combination is close to ideal for a heat pump, and it is why they suit this region.

A reasonable expectation is this. During a hard freeze, seeing AUX is normal and nothing to worry about. Brief appearances during defrost are normal. What is not normal is AUX showing when it is in the forties outside, or staying on for an hour or more once the house has reached temperature. That is the point to investigate.

If you have a smart thermostat, most models will show you auxiliary runtime in their monthly energy report. That report is the fastest way to tell whether your instinct about the bill is right.

What to Check Before Calling

1. Change the filter. Do this first, every time. Then watch for a day.
2. Flatten your thermostat schedule. Remove any setback larger than two or three degrees and see whether AUX stops appearing.
3. Confirm EM HEAT is off. Check nobody switched it on during a cold night and forgot.
4. Look at the outdoor unit. Clear snow, leaves and vegetation, and make sure the fan is turning when the system is running.
5. Check the runtime report on your thermostat if you have one.
6. If AUX still runs in mild weather, book service. At that point it is refrigerant, controls or airflow, and none of those are homeowner fixes.

Still seeing AUX when it is mild out? Call Air Pro Solutions at (253) 397-7220. We will check charge, airflow and defrost operation with full system diagnostics rather than guessing.

Auxiliary Heat Questions We Get Asked

Is it bad if auxiliary heat comes on?

Not by itself. In cold weather and during defrost it is normal, designed behaviour. It becomes a problem when it runs in mild weather, stays on long after the house reaches temperature, or appears far more often than in previous winters. Those patterns point to a fault worth checking.

How do I stop my auxiliary heat from coming on?

Change the filter, remove deep thermostat setbacks, and raise the temperature in small steps rather than large jumps. If it still engages in mild weather after those three, the cause is mechanical. Common culprits are low refrigerant, restricted airflow or a defrost control fault.

Does auxiliary heat run up your electric bill?

Yes, significantly. Resistance strips convert electricity to heat one for one, while the heat pump moves several units of heat per unit of electricity. Running the strips regularly through a winter is the single most common reason a heat pump home gets an unexpectedly high bill.

At what temperature should a heat pump switch to auxiliary heat?

There is no universal figure. It depends on your home’s insulation, the system’s capacity and its efficiency. Older single stage systems often call for backup in the mid thirties. Modern variable capacity systems in well insulated homes may rarely need it at all in a typical winter here.

Is it okay to let my heat pump and electric aux heat run simultaneously?

Yes. That is exactly what auxiliary heat is designed to do, and it is the efficient arrangement of the two. Running them together is normal. Running the strips alone, which is what emergency heat does, is the expensive scenario you want to avoid.

Getting Your Winter Bill Back Under Control

What is auxiliary heat on a heat pump, in one line? A safety net, not a fault. The problem is when the safety net becomes the main heating system without anyone noticing, and the first sign is usually a bill rather than a breakdown.

Call Air Pro Solutions Heating, Cooling & Electrical at (253) 397-7220 and ask us to check why your auxiliary heat is running. You can also schedule a system check online. If the answer turns out to be a system that never fitted the house, we will talk honestly about replacing an older heat pump rather than patching it. We serve Auburn and the surrounding area, including as a Covington HVAC contractor.