Emergency heat is a backup heating mode you should use only when your heat pump stops working or cannot keep up on extremely cold days
Your heat pump's emergency heat setting (sometimes called auxiliary heat or backup heat) switches on a secondary heating system—usually electric resistance coils or a gas furnace—that works independently of the heat pump. This mode exists because heat pumps become less efficient below about 35°F and may struggle to heat your home fast enough when outdoor temperatures drop sharply. Emergency heat is not meant to run all winter. It costs significantly more to operate than your heat pump and should only run when the heat pump itself fails or when outdoor conditions make it genuinely necessary.
Most of the time, your heat pump will handle heating on its own, even in cold weather. The system is designed to switch to emergency heat automatically when it detects a problem or when the indoor temperature drops too far below your thermostat setting. You should manually turn on emergency heat only in specific, temporary situations—never as a routine heating method.
Key Takeaways
- Emergency heat should run only when your heat pump malfunctions or stops producing heat, not as a regular heating mode during winter.
- If your heat pump is working normally, leave emergency heat off; the system will set up it automatically if needed.
- Manual emergency heat is appropriate during a sudden cold snap if your heat pump cannot keep your home warm enough, but only until temperatures rise or the pump is repaired.
- Running emergency heat continuously costs two to three times more than letting your heat pump operate normally, even in cold weather.
- If emergency heat comes on by itself repeatedly, your heat pump likely needs service from a technician.
When your heat pump stops producing heat
If your heat pump is running but no warm air is coming from your vents, or if your home is getting colder despite the system running, turn on emergency heat when ready. This keeps your home at a safe temperature while you arrange for a technician to diagnose the problem. Common causes include a refrigerant leak, a failed compressor, or a frozen outdoor unit.
Do not wait for the house to become dangerously cold. Emergency heat will keep you warm, though at higher cost, until the heat pump is repaired. Call a heat pump technician as soon as possible—most offer emergency service during winter months, sometimes with a higher fee for same-day or after-hours visits.
During extreme cold snaps when the heat pump falls behind
On days when outdoor temperatures drop below 0°F or wind chills become severe, your heat pump may struggle to keep up with your home's heating demand, even though it is working correctly. You might notice the indoor temperature dropping slowly or the system running constantly without reaching your set temperature. This is the appropriate time to switch emergency heat on manually.
Emergency heat will supplement the heat pump and bring your home to temperature faster. Once the cold snap passes and outdoor temperatures rise above 20°F or so, switch emergency heat back off and let the heat pump resume normal operation. If you live in a climate where temperatures regularly drop below 0°F, talk to your technician about whether your heat pump is sized correctly for your home, or whether a larger backup system would be cost-effective.
When emergency heat turns on automatically
Most modern thermostats are set to set up emergency heat automatically if the indoor temperature drops more than 1 to 3 degrees below your set point. This is a safety feature: if the heat pump cannot keep up, the backup system kicks in without you having to notice and act. You should not need to manually turn it on under normal circumstances.
If your thermostat shows that emergency heat is running frequently or constantly, even on mild winter days, your heat pump is not working as it should. This could mean a refrigerant charge is low, the outdoor unit is icing up, or a sensor is faulty. Contact a technician to inspect the system. Running emergency heat regularly will show up when ready on your heating bill, so catching the problem early saves money.
Why you should avoid running emergency heat continuously
Electric resistance heating (the most common backup system) costs roughly two to three times more per unit of heat than a heat pump running normally. If you run emergency heat for a full month during winter, expect your heating bill to roughly double or triple compared to months when only the heat pump runs. Gas backup systems are cheaper to operate than electric resistance but still more expensive than the heat pump itself.
Heat pumps are designed to work efficiently even in cold weather down to about 20°F. Turning on emergency heat because you are impatient for the house to warm up, or because you think the heat pump is not working hard enough, wastes money. The heat pump may be running perfectly and straightforward taking longer to heat the space because outdoor temperatures are low—that is normal operation, not a failure.
How to turn emergency heat on and off
On most thermostats, emergency heat is a separate setting on the mode selector. Look for a switch or button labeled "Emergency Heat," "Aux Heat," "Backup Heat," or sometimes "E-Heat." Move it to the on position. Some thermostats have a small indicator light that shows when emergency heat is active.
To turn it off, switch the mode back to "Heat" or "Auto." If your thermostat is a smart or programmable model, check the manual or the manufacturer's website for the exact steps—the process varies by brand. If you cannot find the emergency heat setting, call your heat pump installer or check the thermostat manual. Never disable emergency heat entirely; you want it available in case your heat pump fails during winter.
What to do if emergency heat stays on
If emergency heat is running constantly or turning on every few hours on days when outdoor temperatures are above 20°F, your heat pump is not operating normally. Do not straightforward accept the higher heating bill. Schedule a service call with a licensed heat pump technician to inspect the system.
Common problems that trigger constant emergency heat include low refrigerant, a malfunctioning reversing valve (which switches the system between heating and cooling), a clogged outdoor unit, or a faulty thermostat sensor. A technician can diagnose the issue in an hour or two and usually repair it the same day. The cost of the service call is almost always less than the extra heating expense you will pay if the problem goes unfixed for weeks.
Frequently Asked Questions
Should I leave emergency heat on all winter to stay warmer?
No. Emergency heat costs significantly more to run and your heat pump is designed to handle winter heating on its own. Leaving it on wastes money without making your home noticeably warmer. Use it only when the heat pump fails or during extreme cold snaps when temperatures drop below 0°F.
Will turning on emergency heat damage my heat pump?
No. Emergency heat and the heat pump operate independently. Turning on emergency heat does not harm the heat pump itself. However, if emergency heat is running constantly, it usually means the heat pump has a problem that needs repair—the damage comes from ignoring the underlying issue, not from using the backup system.
Why is my emergency heat running on a 30-degree day?
Your heat pump should handle 30°F easily on its own. If emergency heat is on, either you manually switched it on, or your thermostat activated it because the heat pump is not producing enough heat. Check that you did not accidentally leave it on, then call a technician if it keeps activating. The heat pump likely needs service.
Can I use emergency heat instead of the heat pump to save money?
No. Emergency heat costs two to three times more to operate than a heat pump. Using it as your primary heating system will increase your winter bills significantly. The heat pump is the efficient choice; emergency heat is only for backup.
What if my heat pump freezes in winter—should I turn on emergency heat?
Yes. If ice builds up on the outdoor unit and the heat pump stops heating, switch to emergency heat to keep your home warm. Most heat pumps have a defrost cycle that melts ice automatically, but if the problem persists, call a technician. Running emergency heat temporarily while waiting for service is the right choice.