What Is Emergency Heat on a Heat Pump, and When Should You Use It?
Emergency heat is a thermostat setting that shuts off your heat pump completely and heats the house with the backup source alone. Lennox explains that the backup is usually electric resistance coils, and in some dual-fuel systems it is a gas or oil furnace. You should switch to it only when the heat pump has failed, the outdoor unit is frozen solid or damaged, or a technician tells you to while you wait for a repair.
Cold nights are when questions about this setting come up, and Somerset County gets far more of them than Pittsburgh. NOAA's 1991 to 2020 climate normals show the Meyersdale station in southern Somerset County averaging 8.2 nights a year at or below 0°F and 26.2 nights at or below 10°F, compared with 2.1 and 12.3 nights at Pittsburgh International Airport and 1.4 and 8.4 nights in Morgantown, WV. On those nights, a heat pump leans on its backup automatically, which is normal and is not the same as emergency heat. This guide explains the difference, what each one costs, and what to do when either light comes on.
What Is the Difference Between Aux Heat and Emergency Heat?
Both use the same backup heat source. The difference is whether the heat pump keeps running.
Aux heat, short for auxiliary heat, works alongside the heat pump. Lennox describes it as a boost the system adds on very cold days, when the heat pump alone cannot keep up with the thermostat. Your thermostat turns it on and off without any input from you.
Emergency heat turns the outdoor heat pump off and runs only the backup. Lennox calls it the system's safety net for times when the heat pump is damaged or cannot run. Depending on the thermostat, it can come on by itself or be switched on manually.
Here is how the two compare:
Aux heat | Emergency heat | |
Heat pump running? | Yes | No |
Who turns it on? | The thermostat, automatically | You, a technician, or the system in a fault |
Normal on cold days? | Yes | No |
Cost | Higher than heat pump alone | Highest |
How Much More Does Emergency Heat Cost to Run?
Electric resistance heat turns each kilowatt-hour of electricity into heat one for one. The U.S. Energy Information Administration lists one kilowatt-hour as equal to 3,412 Btu, and that is all the heat a resistance coil can produce from it.
A heat pump does better because it moves heat from the outside air rather than making it. The U.S. Department of Energy states that a heat pump can reduce electricity use for heating by up to 75% compared with electric resistance heating. When emergency heat runs for days, you give up that advantage, and Lennox warns that energy bills can climb quickly as a result.
Price per kilowatt-hour matters too. EIA's Winter Fuels Outlook, published October 6, 2026, forecasts Northeast residential electricity at an average of 26.19 cents per kilowatt-hour this winter, up 4% from last winter. Every hour on emergency heat is billed at that rate with none of the heat pump's help.
When Should You Switch to Emergency Heat?
Lennox lists three situations where turning on emergency heat makes sense:
Your HVAC technician tells you to use it while you wait for a repair appointment or parts.
The heat pump has completely failed and you need temporary heat until it is fixed or replaced.
The outdoor unit is frozen solid or damaged, for example by storm debris or ice, and cannot run safely until it is inspected.
Outside those cases, leave the thermostat on Heat or Auto. A cold night alone is not a reason to switch, because the system already brings in aux heat when it needs help.
If your home has a dual-fuel system with a furnace as the backup, emergency heat runs the furnace instead of electric coils. The guide to dual fuel systems in cold Pennsylvania weather explains how those systems decide when to switch fuels.
Why Does Aux Heat or Emergency Heat Come On by Itself?
Some backup heat is built into normal operation. Lennox notes that emergency or backup heat can activate while the outdoor unit runs a defrost cycle, and the guide to heat pump defrost cycles covers what happens during those few minutes.
Frequent backup heat on mild days is different. Lennox lists these possible causes when a system keeps defaulting to emergency heat:
A refrigerant leak that reduces heating capacity
Faulty sensors or a faulty thermostat
Compressor problems
Blocked or frozen outdoor coils
An undersized heat pump
A heat pump that runs nonstop and still falls behind can point to the same problems, and the article on a heat pump running constantly explains when that is normal and when it is not.
Why Do Some Heat Pumps Lock Out Backup Heat Above 40°F?
Federal construction standards for manufactured homes put a number on how backup heat should behave. Under 24 CFR 3280.714, a heat pump with supplemental electric resistance heat in a manufactured home must be sized to supply at least 60 percent of the home's calculated annual heating by compression, and a control must prevent the resistance heat from running at outdoor temperatures above 40°F, except during defrost.
That rule applies to manufactured homes, but the idea behind it is useful for any heat pump owner. If your backup heat runs often on 45°F or 50°F days, it is worth having the system checked, since Lennox lists refrigerant leaks, sensor faults and frozen coils among the reasons backup heat runs more than it should.
What Should You Do If Your Thermostat Shows Emergency Heat?
Lennox recommends these steps when emergency heat appears and you did not turn it on:
Check the thermostat mode. If it is set to Emergency Heat or EM Heat, switch it back to Heat or Auto and see if the system returns to normal.
Note any unusual sounds, smells or error codes.
Give the system a few minutes, since it may need time to reset and restart the heat pump.
Look for error messages on the thermostat that can help a technician diagnose the problem faster.
Avoid switching the system on and off repeatedly, which Lennox says can cause more damage.
Call a professional if the issue does not clear.
The list of signs a heat pump needs urgent repair can help you describe what you are seeing when you call.
FAQ
Is it bad to run emergency heat?
Occasional use during a repair or an extreme cold snap will not harm the system, according to Lennox. The backup coils are not built to be the main heat source for long periods, and extended use can mean more wear, element burnout and higher bills.
Does aux heat mean something is wrong with my heat pump?
No. Aux heat on very cold days is normal operation. It becomes a concern only when it runs often in mild weather.
Should I turn on emergency heat when it drops below zero?
Not by default. The thermostat adds aux heat automatically when the heat pump needs help. Emergency heat is meant for a failed, frozen or damaged heat pump.
Can a gas furnace be my emergency heat?
Yes. In a dual-fuel system, Lennox notes that the backup can be a gas or oil furnace instead of electric coils.
How long can I run emergency heat?
As long as you need to until a repair is made. Just expect higher energy use, and schedule the repair promptly so the heat pump can take over again.
Why does my heat pump use backup heat during defrost?
Lennox notes that backup heat can activate while the outdoor unit runs a defrost cycle to clear frost from its coil. Once defrost ends, the heat pump takes over again.
Van Bremen's Heat & Air Repairs and Maintains Heat Pumps
Van Bremen's Heat & Air installs and services heat pumps and ductless systems from its Somerset office at 214 Stoystown Rd, with service across Somerset, Fayette, Cambria and Westmoreland Counties and in Morgantown, WV. If your system keeps falling back on backup heat, a technician can check the refrigerant charge, sensors and outdoor coil. Learn more on the heating services page and maintenance services page.
Visit Van Bremen's Heat & Air or call (814) 483-1735 today. Contact us for more information.
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