Turn on emergency heat when your heat pump stops heating your home, not as a regular backup
Emergency heat (also called auxiliary heat or backup heat) is the electric resistance heating built into most heat pump systems. It kicks in automatically when outdoor temperatures drop below your heat pump's effective range — usually around 35°F to 40°F — or when the heat pump malfunctions. You should manually switch to emergency heat only when your heat pump fails to heat and you need warmth when ready, or when a technician tells you to use it during a repair.
Emergency heat is expensive to run because it uses straight electric resistance instead of the heat pump's efficient refrigerant cycle. Running it continuously costs two to three times more than letting the heat pump work normally. Most homeowners never need to touch the emergency heat switch; the system handles the transition on its own.
Key Takeaways
- Emergency heat activates automatically when outdoor temperature drops below your heat pump's operating range, so you do not need to switch it on manually under normal winter conditions.
- Manual emergency heat mode uses electric resistance heating, which costs significantly more to run than the heat pump's normal cycle and should only be used when the heat pump fails.
- If your heat pump stops producing warm air, check the thermostat setting, look for ice buildup on the outdoor unit, and listen for the defrost cycle before assuming you need emergency heat.
- A heat pump that cannot keep up with demand in very cold weather is working correctly; emergency heat will not solve an undersized system, only mask the problem temporarily.
- Contact a technician if emergency heat is running constantly, if the heat pump never cycles off, or if you are using emergency heat more than a few days per heating season.
How emergency heat activates automatically in cold weather
Your heat pump has a built-in balance point — the outdoor temperature at which the heat pump alone cannot meet your home's heating demand. Below that temperature, the system automatically switches on the electric resistance heating strips inside your furnace or air handler. This happens without you touching anything. The balance point varies by system design but typically falls between 35°F and 40°F.
When the outdoor temperature drops below the balance point, the heat pump continues running and the resistance heat supplements it. Both work together. The thermostat controls this transition based on how much heat your home needs, not on a fixed outdoor temperature. If your home is well-insulated and you keep the thermostat at 68°F, the balance point might be 38°F. If you heat to 72°F or the home is poorly insulated, the balance point might be 32°F.
You do not need to do anything. The system is designed to manage this on its own. Switching the thermostat to "Emergency Heat" or "Aux Heat" manually overrides this automatic control and forces the resistance heaters to run alone, which wastes energy.
When to manually switch to emergency heat mode
Switch to emergency heat manually only in two situations: when your heat pump has failed and you need heat when ready, or when a technician instructs you to do so during a service call.
If your heat pump stops producing warm air on a cold day, first check that the thermostat is set to "Heat" and the temperature setting is higher than the current room temperature. Then go outside and look at the outdoor unit. If it is covered in ice or frost, the defrost cycle is running — this is normal and takes 10 to 15 minutes. The system will resume heating after defrost completes. If the outdoor unit is running but producing no warm air indoors after 20 minutes, or if you hear the outdoor fan but feel no heat from the indoor vents, the heat pump may have failed.
At that point, switch the thermostat to "Emergency Heat" to keep your home warm while you arrange a repair. This buys you time and prevents pipes from freezing. Once the technician fixes the heat pump, switch back to "Auto" or "Heat" mode when ready.
Why emergency heat costs so much to run
A heat pump moves heat from outdoors into your home using a refrigerant cycle and a compressor. For every unit of electricity it uses, it delivers two to four units of heat (the ratio is called the coefficient of performance). Emergency heat strips use electric resistance — they convert electricity directly into heat with a one-to-one ratio. No multiplication. This is why resistance heating costs roughly two to three times more per BTU than the heat pump cycle.
Running emergency heat for a full day in winter can add $15 to $40 to your heating bill, depending on your local electricity rate and how long it runs. Over a week, that becomes $100 to $280. Over a month, $400 to $1,200. The cost compounds if the heat pump is broken and emergency heat is your only source of warmth, but that is a temporary situation while you wait for repair.
If you find yourself using emergency heat regularly — more than a few days per heating season — the heat pump is either undersized for your home, the outdoor unit is dirty or blocked, or the system needs maintenance. Contact a technician to diagnose the problem. Switching to emergency heat does not fix the underlying issue; it only masks it while draining your budget.
What to check before assuming the heat pump has failed
Before you switch to emergency heat, verify that the heat pump is actually broken. Many homeowners mistake normal operation for failure.
First, confirm the thermostat is in "Heat" mode and set above the current room temperature. If it is in "Cool" or "Off", no heating will occur. Second, go outside and inspect the outdoor unit. During very cold weather, the heat pump runs a defrost cycle every 30 to 90 minutes. During defrost, the outdoor coil reverses to shed ice, and indoor heating stops for 10 to 15 minutes. This is normal. If you see frost or ice on the unit, wait 15 minutes and check again. The defrost cycle should clear it.
Third, listen to the outdoor unit. If the compressor is running and the fan is turning, the heat pump is operating. It may be producing heat even if you do not feel it when ready indoors — warm air from a heat pump is often cooler than warm air from a furnace, and it takes time to circulate through the home. If the outdoor unit is completely silent and the indoor vents are blowing cold air, the heat pump may have failed.
Fourth, check whether the resistance heat is already running. Many thermostats display which heating stage is active. If it shows "Stage 2" or "Aux Heat", the resistance heaters are already on because the outdoor temperature is below the balance point. This is correct operation, not a failure.
Heat pump undersizing and emergency heat
Some heat pumps are undersized for the home they serve. An undersized system cannot deliver enough heat to maintain the thermostat setting on the coldest days, even when running at full capacity. The resistance heat then runs continuously to make up the difference. This is not a failure — it is a design problem.
If your emergency heat (or auxiliary heat) runs for hours on end during moderately cold weather, the system is probably undersized. Switching to full emergency heat mode will keep the home warm, but at high cost and without solving the problem. The real fix is either to upgrade to a larger heat pump or to lower the thermostat setting on very cold days.
A technician can measure the heat output of your current system and compare it to your home's heating load to confirm whether undersizing is the issue. If it is, you have options: replace the heat pump with a larger model, add a supplemental heat source, or accept that emergency heat will run on the coldest 5 to 10 days per year.
Maintenance and emergency heat use
A heat pump that requires emergency heat more often than expected may have a maintenance problem. Dirty outdoor coils, blocked airflow, low refrigerant charge, or a faulty defrost sensor can all reduce heating output and force the resistance heaters to work harder.
If you notice emergency heat running earlier in the season than it did last year, or running on days when outdoor temperatures are not particularly cold, schedule a maintenance visit. A technician will clean the coils, check the refrigerant level, test the defrost cycle, and verify that the system is operating at design capacity. This often restores the heat pump's efficiency and reduces reliance on emergency heat.
Annual maintenance in the fall — before heating season begins — is the best way to prevent unexpected emergency heat use. Many heat pump owners skip fall maintenance and then wonder why their heating bills spike in winter.
Frequently Asked Questions
Should I leave my heat pump on emergency heat all winter to save money?
No. Emergency heat costs two to three times more to run than the heat pump's normal cycle. Leaving it on all winter will roughly double your heating bill. The heat pump is designed to handle winter heating on its own; emergency heat is a backup for when the heat pump fails or outdoor temperatures drop below its operating range.
Why does my heat pump blow cool air in winter?
The heat pump may be in defrost cycle, which reverses the refrigerant flow to melt ice off the outdoor coil. This stops indoor heating for 10 to 15 minutes. Alternatively, the air from a heat pump is naturally cooler than air from a furnace, even when it is heating. If cool air continues for more than 20 minutes and the outdoor unit is not iced over, contact a technician.
Can I use emergency heat if my heat pump is too small for my home?
Yes, but only temporarily. Emergency heat will keep you warm, but it costs much more than the heat pump and does not solve the undersizing problem. If your system is undersized, you will need to either upgrade the heat pump, lower your thermostat on very cold days, or accept high heating bills on the coldest days of winter.
What temperature should I set emergency heat to?
Set the thermostat to the same temperature you would normally use — usually 68°F to 72°F. Emergency heat will heat to that temperature, just at higher cost. Do not set it higher hoping to warm the home faster; the resistance heaters run at full capacity regardless of the setting, and a higher setting only keeps them running longer.
How do I know if my heat pump is broken or just in defrost cycle?
Defrost cycle lasts 10 to 15 minutes and happens every 30 to 90 minutes in cold, humid weather. If the indoor air is cold for longer than 20 minutes, or if the outdoor unit is not iced over, the heat pump may have failed. Listen for the outdoor compressor running and feel for warm air from the indoor vents after defrost ends. If neither occurs, contact a technician.