Switch to emergency heat when your heat pump stops heating your home, even though the outdoor unit is running

Emergency heat (also called auxiliary heat or backup heat) is a second heating system built into most heat pumps—usually electric resistance coils or a gas furnace. You switch to it when your heat pump cannot keep up with demand or stops working altogether. The most common reason is outdoor temperatures below your heat pump's effective range, typically around 25°F to 35°F depending on your model, though modern cold-climate heat pumps work down to 0°F or lower.

Emergency heat is not meant to run all winter. It costs more to operate than a working heat pump and puts strain on your electrical system. But on the coldest days, or when your heat pump malfunctions, it keeps your home warm while you arrange a repair.

Key Takeaways

  • Switch to emergency heat when outdoor temperature drops below your heat pump's rated minimum or when the heat pump stops producing warm air despite running.
  • Emergency heat costs significantly more per BTU than a functioning heat pump, so use it only when necessary, not as a default in winter.
  • If you switch to emergency heat and your home still does not warm up, the problem is likely a failed compressor or refrigerant leak, not cold weather.
  • Most heat pumps have a manual switch or thermostat setting for emergency heat; check your owner's manual to locate it on your specific model.
  • Leaving emergency heat on continuously can overload your electrical panel and trigger breaker trips, especially in older homes with 100-amp service.

How to recognize when your heat pump has stopped working

A working heat pump pulls heat from outside air and moves it indoors, even on cold days. You will hear the outdoor unit running and feel warm air from your vents. If the outdoor unit is running but no warm air comes from your registers after 5 to 10 minutes, your heat pump is not transferring heat.

Check your thermostat display. Most modern thermostats show whether the system is in heating mode and whether auxiliary heat has kicked in. If the display shows "heating" or a flame icon but your home is not warming, the compressor inside the outdoor unit has likely failed or refrigerant has leaked out. This is a repair situation, not a cold-weather situation.

Do not confuse the defrost cycle with a failure. Heat pumps periodically reverse to melt ice off the outdoor coil. During defrost, the outdoor unit stops pulling heat for 5 to 15 minutes, and your thermostat may automatically switch to emergency heat to keep your home warm. This is normal. Once defrost ends, the system returns to heat pump mode.

Temperature thresholds where emergency heat becomes necessary

Most standard heat pumps lose efficiency below 35°F and stop heating effectively around 25°F. At that point, the heat pump still runs but cannot pull enough warmth from outside air to meet your thermostat's demand. Your system automatically switches to emergency heat to close the gap. This is called balance point—the outdoor temperature where your heat pump alone cannot keep up.

Cold-climate heat pumps, which are becoming more common, have lower balance points—some work down to 0°F or below without emergency heat. Check your equipment manual or the nameplate on your outdoor unit to find your model's rated minimum. If you live in a climate where temperatures regularly drop below that point, you will use emergency heat several times each winter, and that is normal.

If your thermostat has a setting called "auxiliary heat" or "emergency heat," do not turn it on manually unless your heat pump has failed. Leaving it on forces your system to use the expensive backup instead of the efficient heat pump whenever outdoor temperature drops, even slightly.

What happens inside your system when you switch to emergency heat

When you flip the emergency heat switch or your thermostat automatically triggers it, a relay inside your furnace or air handler closes. This energizes electric resistance heating elements (like a giant toaster) or ignites a gas burner. At the same time, the heat pump compressor shuts down. Your backup heater then warms air and sends it through your ducts.

This process is straightforward but expensive. Electric resistance heat costs roughly three times more per BTU than a heat pump running at 35°F, and five to seven times more than a heat pump running at 50°F. Gas backup is cheaper than electric but still costs more than the heat pump. That is why emergency heat is a temporary solution, not a seasonal strategy.

Some newer systems use a hybrid approach: the heat pump runs alongside emergency heat on very cold days, with the backup filling in only the gap the heat pump cannot cover. This is more efficient than emergency heat alone but still more expensive than the heat pump by itself. Your thermostat settings control whether this happens automatically or requires manual intervention.

Manual switch location and how to set up it

Most thermostats have an "emergency heat" or "auxiliary heat" button or toggle, usually on the main screen or in a settings menu. Look for a label that says "EM," "AUX," or "Emergency." Flip or tap it, and the system switches when ready. On older mechanical thermostats, it is a physical lever or dial.

If your thermostat does not have a visible switch, check the owner's manual—some models require you to enter a menu using arrow buttons. Write down the exact steps so you can find it again if needed. If you cannot locate the manual, search online for your thermostat model number (usually printed on the unit itself) plus "emergency heat."

Some systems do not have a manual override. Instead, the thermostat automatically switches to emergency heat when outdoor temperature drops below the balance point or when the heat pump fails to reach your set temperature within a certain time. In that case, you cannot force emergency heat on; the system decides when to use it.

When NOT to use emergency heat

Do not switch to emergency heat just because outdoor temperature is cold. If your heat pump is running and your home is warming, leave it alone. The heat pump is doing its job and costing you less money than the backup.

Do not leave emergency heat on for days or weeks. If you switched it on because your heat pump failed, call a technician the same day. Running emergency heat continuously will spike your heating bill and can overload your electrical panel, especially in older homes with 100-amp service. A 5 kW electric resistance heater draws about 20 amps—a significant load on an already-taxed system.

Do not assume emergency heat means your heat pump is broken. On very cold days, it is normal for emergency heat to run part of the time, especially if your home is large or poorly insulated. Check whether your outdoor unit is running. If it is, the system is working as designed.

Signs you need to call a technician, not just switch to emergency heat

If outdoor temperature is above 25°F, your heat pump is running, but no warm air comes from your vents, the problem is not cold weather. Call a technician. The compressor may have failed, refrigerant may have leaked, or an indoor blower may be stuck. Emergency heat will keep you warm temporarily, but the underlying problem will not fix itself.

If emergency heat is on and your home still does not reach your set temperature, the backup heater itself may be faulty, or your system may be undersized for your home. This also requires a technician visit.

If your breaker trips when you switch to emergency heat, your electrical panel may not have enough capacity for the backup heater. This is a safety issue. Do not keep resetting the breaker. Call an electrician or your HVAC technician to discuss options, which may include upgrading your service or installing a smaller backup heater.

Frequently Asked Questions

Can I run emergency heat all winter to save money?

No. Emergency heat costs significantly more than a working heat pump. Running it continuously will roughly double or triple your heating bill compared to letting the heat pump do its job. Use emergency heat only when the heat pump cannot keep up or has failed.

Why does my thermostat show emergency heat running on a 40-degree day?

Your heat pump may be in defrost cycle, temporarily stopping heat production to melt ice off the outdoor coil. This is normal and lasts 5 to 15 minutes. If emergency heat stays on for more than 30 minutes on a mild day, call a technician—your heat pump may not be working correctly.

What is the difference between emergency heat and auxiliary heat?

They are the same thing. "Auxiliary" and "emergency" are interchangeable terms for the backup heating system. Some manufacturers use one term, some use the other. Check your manual to see which label your system uses.

Should I switch to emergency heat before a cold snap arrives?

No. Let your thermostat manage it automatically. If you switch manually before the cold arrives, you will waste money running expensive backup heat when the heat pump is still efficient. Your system will automatically switch to emergency heat if outdoor temperature drops below your balance point.

Does emergency heat damage my heat pump?

No. Switching to emergency heat does not harm the heat pump itself. The compressor straightforward shuts down while the backup heater runs. However, running emergency heat continuously for weeks can strain your electrical system and inflate your bill, so use it only when necessary.