Emergency heat will defrost your heat pump, but it uses far more electricity than normal operation
Emergency heat (also called auxiliary heat or backup heat) is a second heating system built into most heat pumps. When your heat pump gets too cold to work efficiently—usually below 35°F—it switches on automatically to keep your home warm while the heat pump defrosts itself. Emergency heat is electric resistance heating, similar to a space heater or toaster. It produces warmth by running electricity through a wire coil, which is why your electric bill rises noticeably when it runs.
The defrost cycle itself is what triggers emergency heat. Heat pumps pull warmth from outside air even in cold weather, but frost builds up on the outdoor coil as they do this. Every 30 to 90 minutes (depending on your model), the system reverses direction to melt that frost off. During those 5 to 15 minutes, your home would get cold if nothing else were heating it. Emergency heat fills that gap, keeping your indoor temperature steady while the outdoor unit clears itself.
You do not need to turn emergency heat on manually in most cases—it activates on its own when conditions call for it. However, some thermostats have an "Emergency Heat" or "Aux Heat" button you can press if your heat pump stops working entirely. Using emergency heat by choice should be temporary only, because it costs roughly two to three times more per hour than running the heat pump alone.
Key Takeaways
- Emergency heat is electric resistance heating that turns on automatically during defrost cycles to keep your home warm while frost melts off the outdoor coil.
- The defrost cycle runs every 30 to 90 minutes in cold weather and lasts 5 to 15 minutes, during which emergency heat prevents indoor temperature from dropping.
- Emergency heat costs significantly more to run than a heat pump, so it should only run during defrost cycles or when the heat pump has failed.
- If you manually switch to emergency heat mode on your thermostat, your heat pump will not run at all, and your heating costs will increase substantially.
- A properly functioning heat pump should handle defrost cycles without your intervention; if emergency heat runs constantly, the heat pump may need service.
When the defrost cycle triggers emergency heat automatically
Your heat pump's control board monitors outdoor temperature and humidity. When frost risk is high—typically when outdoor air is between 30°F and 50°F with moisture present—the system enters defrost mode automatically. The reversing valve switches the refrigerant flow, so the outdoor coil heats up and melts the frost. At the same time, the indoor coil would cool down, making your house uncomfortable. Emergency heat prevents this by turning on the moment defrost begins.
The whole cycle is brief. Defrost usually lasts 5 to 15 minutes, though some systems run longer if frost is heavy. Once the outdoor coil is clear, the heat pump switches back to normal heating mode and emergency heat shuts off. You may hear a slight hiss or see water dripping from the outdoor unit during defrost—that is the melted frost running off, and it is normal.
In very cold climates, defrost cycles happen more often because frost builds faster. A heat pump in Minnesota might defrost every 30 to 45 minutes on a cold, damp day, while one in a drier climate might go 90 minutes between cycles. Each time, emergency heat runs for a few minutes. Over a full winter day, this adds up, but it is still far cheaper than running emergency heat continuously.
Why emergency heat costs so much more
A heat pump moves heat from outside to inside using refrigerant and a compressor—it is efficient because it moves existing heat rather than creating it from scratch. Emergency heat creates heat directly by running electricity through a resistance coil. This is like the difference between moving water uphill with a pump versus boiling it to make steam. The resistance method works, but it demands much more energy.
The difference shows up when ready on your electric bill. If your heat pump normally costs $1.50 per hour to run in heating mode, emergency heat alone might cost $3.50 to $4.50 per hour. During a defrost cycle lasting 10 minutes, that is only 50 to 75 cents extra. But if emergency heat runs for hours because your heat pump is broken, or if you accidentally leave it on, the cost climbs fast.
Some newer heat pumps have a feature called smart defrost that reduces how often defrost cycles run by checking whether frost has actually built up, rather than running on a timer. These systems use emergency heat less frequently, which is why they tend to have lower winter heating bills.
What to do if emergency heat runs constantly
If your thermostat display shows "Aux Heat" or "Emergency Heat" running most of the time, or if your electric bill spikes unexpectedly during winter, your heat pump may not be working correctly. Constant emergency heat operation usually means one of three things: the heat pump has failed, the outdoor coil is severely iced over and not clearing, or the thermostat is stuck in emergency heat mode.
First, check your thermostat. Look for an "Emergency Heat" or "Aux Heat" button or switch. If it is set to "On" or "Emergency," switch it back to "Auto" or "Heat." If it is already on Auto, leave it there and move to the next step. Do not manually switch to emergency heat unless your heat pump is completely broken and you need heat when ready.
Next, look at the outdoor unit. If it is covered in thick ice (not just frost), the defrost cycle may not be working. Turn off the system and pour warm (not boiling) water on the outdoor coil to melt heavy ice, then turn the system back on. If ice builds up again within an hour, call a technician. If emergency heat continues running after you have checked the thermostat and the outdoor unit looks normal, the heat pump compressor or reversing valve likely needs repair.
Manually switching to emergency heat: when and why not to
Most thermostats have a way to switch to emergency heat mode manually, usually a button or setting labeled "Emergency Heat," "Aux Heat," or "Backup Heat." When you switch to this mode, the heat pump stops running entirely and only the electric resistance coil heats your home. This is useful only in one situation: if your heat pump has broken down completely and you need heat right away while waiting for a technician.
Do not use emergency heat mode for comfort or convenience. Running it for even one day will noticeably increase your electric bill. If your heat pump is working normally, leave the thermostat on "Auto" or "Heat" mode so the system can use the efficient heat pump first and only switch to emergency heat during defrost cycles.
If you are in a situation where your heat pump is broken and you must use emergency heat for several days, keep your thermostat set lower than usual—even 2 or 3 degrees makes a difference. Close off rooms you are not using. These steps reduce how long emergency heat runs and keep your bill from becoming unmanageable.
How to know if your defrost cycle is working normally
A working defrost cycle is nearly invisible. You may notice a slight temperature dip inside—maybe 1 or 2 degrees—for a few minutes, but your home should stay comfortable. You might hear the outdoor unit make a different sound (a slight hiss or click as the reversing valve switches), and you may see water or ice melt running off the outdoor coil. All of this is normal.
If you want to observe the defrost cycle, watch your outdoor unit on a cold, humid day. Every 30 to 90 minutes, you should see steam or mist rising from the coil as frost melts. The cycle lasts 5 to 15 minutes, then stops. If you see this happening regularly, your system is working as designed.
If the outdoor unit never shows signs of defrosting, or if you see thick ice building up and not melting, the defrost control board or sensor may have failed. This is a repair that requires a technician, because the control board is not something a homeowner can safely replace.
Comparing emergency heat to other backup heating options
Not all heat pumps have built-in emergency heat. Some older systems or ductless mini-split heat pumps rely on a separate backup system—a furnace, baseboard heater, or space heater—to provide warmth when the heat pump cannot. These backup systems are usually more efficient than electric resistance coils because they burn fuel (natural gas or propane) rather than converting electricity directly to heat.
If your home has a heat pump paired with a gas furnace, the furnace acts as backup. The heat pump runs first because it is more efficient, and the furnace takes over only when outdoor temperatures drop below the balance point (usually around 35°F) or when the heat pump fails. This setup is common in areas with cold winters and natural gas service.
If your heat pump has built-in emergency heat but you find yourself using it frequently because the heat pump is undersized for your climate, you may want to discuss a hybrid system with an HVAC technician. A hybrid system automatically switches between the heat pump and a furnace based on outdoor temperature, which can lower your winter heating costs compared to running emergency heat regularly.
Frequently Asked Questions
Will emergency heat damage my heat pump?
No. Emergency heat and the heat pump are separate systems that do not interfere with each other. Emergency heat runs only during defrost cycles or when you manually switch to it. The heat pump itself is not harmed by the defrost process or by emergency heat running alongside it.
Why does my electric bill spike in winter if my heat pump is efficient?
Heat pumps are efficient, but defrost cycles and very cold weather both trigger emergency heat use. On a cold, damp day, defrost cycles run frequently, and emergency heat runs for several minutes each time. Additionally, below a certain outdoor temperature (usually 35°F), many heat pumps become less efficient, and emergency heat runs more often. This is normal and expected in cold climates.
Can I turn off emergency heat to save money?
You should not turn off emergency heat entirely, because your home will become uncomfortably cold during defrost cycles. However, you can reduce emergency heat use by keeping your thermostat set lower, closing off unused rooms, and ensuring your heat pump is properly maintained so defrost cycles run as designed. If emergency heat is running constantly, have the heat pump inspected.
What is the difference between emergency heat and the defrost cycle?
The defrost cycle is when the heat pump reverses to melt frost off the outdoor coil. Emergency heat is the backup heating system that runs during defrost to keep your home warm while the heat pump is in reverse. They work together: defrost clears the coil, and emergency heat prevents indoor temperature from dropping.
Should I see emergency heat running on my thermostat display?
Most thermostats show "Aux Heat" or "Emergency Heat" on the display when it is running. If you see this for a few minutes every 30 to 90 minutes on a cold day, that is normal. If it runs constantly or for hours at a time, the heat pump may need service.