Emergency heat activates when your heat pump cannot keep up with the cold
Emergency heat (also called auxiliary heat or backup heat) is a second heating system built into most heat pumps. It turns on automatically when outdoor temperatures drop so low that your heat pump works harder than it can manage, or when the system is in defrost mode and your home is losing warmth. You should understand when this happens because emergency heat costs significantly more to run than your heat pump alone — typically two to three times the operating cost per hour.
Your thermostat usually switches to emergency heat without you doing anything. However, some thermostats have a manual emergency heat setting that you can turn on yourself. Knowing the difference between automatic and manual set up, and understanding what triggers each one, helps you avoid accidentally running expensive backup heat when you do not need it.
Key Takeaways
- Emergency heat activates automatically when your heat pump cannot meet your home's heating demand, usually below 35°F outdoor temperature.
- Defrost cycles cause your heat pump to pause heating briefly; emergency heat keeps your home warm during this 5- to 15-minute window.
- Manual emergency heat mode should be used only during heat pump repairs or when the system malfunctions, not as a cost-saving choice.
- Leaving manual emergency heat on continuously costs two to three times more than letting your heat pump run normally.
- If emergency heat runs constantly, your heat pump may need servicing or your thermostat may be set incorrectly.
How automatic emergency heat works
Your thermostat monitors the difference between your set temperature and your home's actual temperature. When your heat pump cannot close that gap — usually because outdoor air is too cold for the system to extract enough heat — the thermostat automatically switches on emergency heat. This typically happens when the outdoor temperature falls below 35°F, though the exact trigger point depends on your system's design and your thermostat settings.
The automatic switch happens in stages. First, your heat pump runs at full capacity. If the indoor temperature still drops below your set point after a few minutes, the thermostat activates emergency heat as a backup. This two-stage approach means emergency heat only runs when necessary, not constantly throughout winter.
You will see "emergency heat" or "aux heat" light up on your thermostat display when this happens. Some systems also show it on a separate indicator light on the outdoor unit or indoor air handler. This is normal operation on very cold days — it does not mean something is broken.
Emergency heat during defrost cycles
Heat pumps reverse their operation periodically to melt ice buildup on the outdoor coil. During a defrost cycle, which lasts 5 to 15 minutes, your heat pump stops heating your home temporarily. Your indoor temperature would drop noticeably during this time if emergency heat did not set up automatically.
Defrost cycles happen more often in humid, cold climates where frost accumulates quickly. In dry cold climates, they may happen only a few times per week. Either way, the automatic emergency heat set up during defrost is working as designed — you should not manually override it or try to prevent it.
When to manually switch to emergency heat mode
Manual emergency heat mode exists for specific situations: when your heat pump is broken and you need heating while waiting for a repair, or when a technician asks you to switch to it during diagnosis. Outside these scenarios, manually turning on emergency heat costs you money without benefit.
If your heat pump stops working entirely, switching to manual emergency heat keeps your home warm until a technician arrives. However, do not leave it on longer than necessary. Call for service when ready rather than running expensive backup heat for days or weeks.
Some older thermostats have a three-position switch: Heat, Emergency Heat, and Cool. If your thermostat has this design, make sure the switch is on Heat (not Emergency Heat) during normal operation. Leaving it on Emergency Heat continuously will run your backup system instead of your heat pump, even on mild days when your heat pump works efficiently.
Signs your emergency heat is running too often
If your emergency heat indicator stays on most of the time during winter, or if it activates on days when the outdoor temperature is above 40°F, something is wrong. This usually means either your heat pump needs service or your thermostat is miscalibrated.
A heat pump that cannot reach your set temperature may have low refrigerant, a dirty outdoor coil, or a failing compressor. A thermostat that triggers emergency heat too early may have an incorrect temperature offset setting or a faulty sensor. Both problems require a technician to diagnose and fix.
Check your utility bills as a rough indicator. If your heating costs spike suddenly in mid-winter, emergency heat may be running more than it should. This is worth investigating because the cost difference between heat pump operation and emergency heat operation is substantial over a full heating season.
The cost difference between heat pump and emergency heat
A heat pump uses electricity to move heat from outside air into your home, which is efficient even in cold weather. Emergency heat (usually electric resistance heating) converts electricity directly into heat, which is much less efficient. Running emergency heat for one hour costs roughly the same as running your heat pump for two to three hours.
Over a winter season, unnecessary emergency heat use can add hundreds of dollars to your heating bill. This is why automatic set up is designed to use emergency heat only when the heat pump genuinely cannot keep up, and why manual emergency heat should be a temporary measure, not a permanent setting.
If you are tempted to switch to manual emergency heat because you think it will heat your home faster, understand that it will not. Your thermostat controls how much heat enters your home regardless of the heat source. Switching to emergency heat just makes that heat more expensive to produce.
What to do if emergency heat stays on
First, check your thermostat. If it has a manual switch, make sure it is set to Heat, not Emergency Heat. If the switch is correct and emergency heat still activates constantly, write down the outdoor temperature when it happens and call a technician with that information.
Do not try to disable emergency heat or prevent it from activating. It is a safety feature that keeps your home warm when your primary heating system cannot. Disabling it could leave you without adequate heat on very cold days.
If you are waiting for a service appointment, you can reduce costs slightly by lowering your thermostat by a few degrees. This reduces the temperature gap your heat pump has to close and may reduce emergency heat set up. However, this is a temporary measure — have the system serviced as soon as possible.
Frequently Asked Questions
Is it bad if my heat pump uses emergency heat on cold days?
No. Using emergency heat automatically on very cold days is normal operation. Your heat pump is designed to work with backup heat in winter. The system is working correctly if emergency heat activates only when outdoor temperatures are very low or during defrost cycles.
Should I turn on manual emergency heat to heat my home faster?
No. Your thermostat controls heating speed regardless of whether the heat pump or emergency heat is running. Switching to manual emergency heat will not warm your home any faster — it will only make heating more expensive. Leave your thermostat on the Heat setting and let the system manage itself.
Why does my emergency heat run during defrost?
Defrost cycles pause your heat pump's heating to melt ice off the outdoor coil. Emergency heat activates automatically during this 5- to 15-minute window so your indoor temperature does not drop. This is normal and necessary — do not try to prevent it.
Can I save money by using emergency heat instead of my heat pump?
No. Emergency heat costs two to three times more per hour than your heat pump. Always keep your thermostat on Heat mode during winter and let the system switch between heat pump and emergency heat automatically based on outdoor temperature and heating demand.
What should I do if emergency heat runs constantly?
Check that your thermostat is set to Heat mode, not Emergency Heat mode. If it is set correctly and emergency heat still runs constantly, contact a technician. Your heat pump may need service, or your thermostat may be miscalibrated. Do not run manual emergency heat as a long-term solution.