Heat Pump Emergency Heat: What That Light Means

The two lights, and why people mix them up

The first winter with a heat pump brings a question almost everyone asks: a small word lights up on the thermostat, “aux” or “EM heat,” and nobody told you what it means. The two look alike and share the same hardware, which is why they get confused. They are not the same thing.

Both refer to the backup heater, usually a set of electric resistance coils inside the indoor unit, which exists because a heat pump pulls heat out of outdoor air and has less to pull from as the air gets colder. The difference is who turns the backup on.

Auxiliary heat is automatic. When the heat pump alone cannot keep up with the thermostat setting, the system adds the backup heater, and on most systems the heat pump keeps running alongside it.¹ Emergency heat is manual. You select it, the heat pump shuts off, and the backup heater runs by itself.¹ Lennox puts it plainly: emergency heat “is when you use your supplemental heat by itself, without the use of your heat pump.”¹

So the aux light is your system working. The EM heat light is you, or someone in the house, telling the system to stop using its most efficient part.

When the aux light is normal

Expect to see auxiliary heat in three situations.

A cold snap. Auxiliary heat generally kicks in when outdoor temperatures fall below a threshold where the heat pump by itself cannot hold the house at the set temperature.³ On the coldest nights of the year, an aux light that comes and goes is the system doing its job.

A big catch-up. If the house has dropped several degrees below the setting, because you turned the thermostat down overnight or came home to a cold house, the system brings on the backup to recover quickly.³ This one you can control, and we cover it below.

A defrost cycle. In damp cold, frost builds on the outdoor coil. The heat pump pauses, warms the outdoor coil to melt it, and you may see steam rising off the unit and water on the ground. Many systems run the backup heater for a few minutes during defrost so the vent air does not feel cold. Efficiency Maine’s advice to homeowners is not to worry about ice on the unit, because “heat pumps automatically defrost.”⁵

When to use emergency heat

Emergency heat has one job: keep you warm while the heat pump is out of action.

Switch to EM heat when the outdoor unit has stopped running, when it is frozen into a block that a defrost cycle does not clear, when a storm or a fallen branch has damaged it, or when a technician tells you to use it until a part arrives.² Then call for service, and switch back to normal heat the day it is fixed.

Do not switch to emergency heat just because it is cold outside. A modern heat pump keeps working in the cold. ENERGY STAR cold-climate models are tested at 5°F and continue to run below that, with the backup source there for the lowest temperatures.⁴ The system will add auxiliary heat on its own if it needs help. Flipping to EM heat throws away the efficient half of the system for nothing.

A quiet trap: on some thermostats EM heat can be selected by accident, or a visiting grandchild or a well-meaning helper flips it on during a cold spell and nobody flips it back. If your bill jumps, check the mode first. It takes five seconds and it is the most common cause of a shocking winter bill.

If aux heat keeps running, check these before you call

Frequent auxiliary heat in ordinary weather is a symptom, and the cheap causes come first.

The thermostat is set to Auto. In Auto, the system can flip between heating and cooling on a sunny winter afternoon and then heat hard again at night. Both ENERGY STAR and Efficiency Maine say to set the mode to Heat in winter, not Auto.⁴⁵

A deep nighttime setback. Dropping the setting six or eight degrees overnight means a big catch-up every morning, and the system uses the backup heater to do it. More on this in the next section.

A dirty filter or a blocked outdoor unit. A clogged filter starves the indoor unit of air, and leaves, shrubs, or a snowdrift against the outdoor unit choke it. Efficiency Maine’s tip sheet says to keep shrubs away, remove stuck leaves, and clear snow drifts away from the unit, while being careful not to bend the fins.⁵ Filters are a job most people can do; anything on the outdoor unit beyond brushing snow off with a broom is better left to a pro or a family member.

Something is actually wrong. Lennox lists the problems that frequent emergency or auxiliary heat can mask: refrigerant leaks that cut heating capacity, faulty sensors or thermostats, compressor trouble, blocked or frozen outdoor coils, and a heat pump that is undersized for the house.² If the simple fixes do not change the pattern, that is the call to make.

The thermostat habit that saves the most

Here is the single biggest money lesson for a first-winter heat pump owner, and it runs against what we were all taught with a furnace.

Do not turn it down at night. Heat pumps run most efficiently when they hold a steady temperature. ENERGY STAR’s guidance is direct: unlike a furnace or boiler, heat pumps “do not save energy by turning it down when you’re away or asleep,” so “pick a comfortable temperature and leave it there.”⁴ Efficiency Maine says the same, with the reason: heat pumps use more energy to raise the temperature than to maintain it, so adjusting it for short periods, such as overnight, will cost you.⁵

The mechanism is the aux light. A deep setback means a cold house at 6 a.m., and the system reaches for the backup heater to recover. That morning burst can erase the overnight savings and then some.

If you want to trim the bill, do it the other way: set the house a degree or two cooler for the whole season and wear the cardigan, rather than swinging the setting every day. Pair that with sealing the drafts so the heat pump has less to make up. For a reader who is home all day and feels the cold more with age, holding one comfortable setting steady is both the cheaper and the more comfortable choice.

What to do when the heat pump quits in a cold snap

A short plan for the night it happens.

1. Switch the thermostat to emergency heat. The house will warm, more slowly and at a higher cost, but it will warm.
2. Look at the outdoor unit from a window. If it is buried in snow or encased in ice, note that for the technician. Do not chip at ice with tools; you can puncture the coil.
3. Call for service in the morning, and tell them you are on EM heat. If you are 65 or older or have a health condition, say so. Many companies prioritize those calls in a freeze.
4. Switch back to Heat the day it is repaired. Put a note on the thermostat if someone else in the house might forget.

If you have a second heat source, such as an old furnace or a wood stove, Efficiency Maine’s advice is to heat with the heat pump as your main system and reserve the furnace or boiler for emergency backup.⁵ A cold snap with a failed heat pump is exactly the night that backup is for.

If the bill still hurts

A long cold month on a fixed income can produce an electric bill that is hard to pay, especially in a first winter while you are still learning the system.

The Low Income Home Energy Assistance Program (LIHEAP) is a federal program, run through each state, that helps households stay safe and warm in winter by assisting with home heating bills, preventing energy shutoffs, reconnecting service, making homes more energy efficient, and repairing or replacing heating equipment.⁶ Eligibility is based on income and varies by state, and many states give priority to households with an older adult. Your state LIHEAP office or your Area Agency on Aging can tell you in one phone call whether you qualify. Our guide to paying for home modifications lists the other programs that help with energy work.

If the heat pump itself is struggling in your climate rather than the bill being a one-month spike, read our guide to cold-climate heat pumps before deciding anything. Many older systems were never built for the coldest nights, and the right fix is sometimes a better-matched unit rather than more backup heat.

Key takeaways

• Aux heat is automatic and normal in a cold snap, during a morning catch-up, and briefly during defrost.
• Emergency heat is manual and expensive. Use it only when the heat pump has failed or is iced solid, then switch back after the repair.
• Hold a steady temperature. Nighttime setbacks make the backup heater do the morning catch-up and cost more than they save.
• Set Heat, not Auto, keep the filter clean and the outdoor unit clear, and call a technician if aux heat runs in mild weather.
• LIHEAP can help with a bill you cannot pay and, in some cases, with equipment repair.

What’s next

• Make the heat pump’s job easier with sealing air leaks.
• A smart thermostat can show aux heat use and warn family when the system falls behind.
• If winter nights are a struggle every year, see cold-climate heat pumps.
• Keep the whole house safe in a freeze with our winterizing guide for seniors.

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Reviewed by John Smith, CAPS, Certified Aging-in-Place Specialist. This guide is general information about how heat pump thermostats work. Systems differ by brand and model, so follow your owner’s manual and your installer’s advice for your specific equipment, and call a licensed HVAC technician for any repair.