Your heat pump switches to emergency heat when outdoor temperatures drop below its heating capacity, usually between 30°F and 50°F depending on your system
Most heat pumps stop pulling heat efficiently from outside air once the temperature falls to a certain point—typically somewhere between 30°F and 50°F. When that happens, your system automatically switches to emergency heat (also called auxiliary heat or backup heat), which is usually an electric resistance heating element built into your furnace or air handler. This switch happens without you doing anything. You might notice it only when your heating bill jumps, because resistance heat costs more to run than a heat pump does.
The exact temperature at which the switch happens depends on your heat pump model, your thermostat settings, and how your installer configured the system. Some systems are set to switch at 35°F, others at 40°F or 50°F. You can usually see which mode is running by looking at your thermostat display—it will show "heat pump" or "auxiliary heat" or sometimes just "aux" or "em heat."
Key Takeaways
- Emergency heat turns on automatically when outdoor temperature drops below your heat pump's efficient operating range, typically between 30°F and 50°F.
- Emergency heat is electric resistance heating, not a heat pump function, and costs significantly more to operate than your heat pump alone.
- You can check which heating mode is active by looking at your thermostat display or checking your system's control panel.
- Manually switching to emergency heat should only happen if your heat pump stops working, because it will raise your heating costs.
- If emergency heat stays on during mild weather or if your heat pump never switches back, contact a technician to check the refrigerant level and outdoor unit.
Why heat pumps need backup heat in cold weather
A heat pump works by moving heat from outside air into your home, even when it is cold outside. But as outdoor temperature drops, there is less heat available to move, and the system has to work harder and harder to extract it. At some point—your system's balance point—the heat pump cannot pull in enough heat to keep your home warm, even running at full capacity.
Rather than let your home get cold while the heat pump struggles, your system automatically switches to emergency heat. This resistance heating element produces heat directly, like an electric space heater, and fills the gap. It is less efficient than a heat pump (it converts electricity directly to heat at roughly 100% efficiency, but costs more per unit of heat), but it works fast and keeps your home at the temperature you set.
How to tell which heating mode is running
Most thermostats display which mode your system is in. Look at your thermostat screen—it should show "heat pump," "auxiliary heat," "aux," "emergency heat," or "em heat" somewhere on the display. Some older or simpler thermostats do not show this, but your system is still tracking it internally.
Another way to check is to look at your utility bill or your system's control panel if you have access to it. If emergency heat has been running, your heating costs will be noticeably higher than when the heat pump alone is running. A spike in heating costs during a cold spell is often the first sign that emergency heat has kicked in.
If you want to see the switch happen in real time, you can set your thermostat to a lower temperature on a cold day and watch the display. As the outdoor temperature drops, you should see the mode change from "heat pump" to "auxiliary heat" or similar.
The cost difference between heat pump and emergency heat
Running on emergency heat costs roughly two to three times more than running your heat pump alone, depending on your local electricity rates and how efficient your heat pump is. A heat pump moves heat from outside; an electric resistance element creates heat from electricity. Moving heat is cheaper than creating it.
If outdoor temperatures stay below your system's balance point for days or weeks—common in northern climates during winter—your heating bill will reflect that. This is normal and expected. However, if emergency heat is running during mild weather (above 40°F or 50°F), something is wrong with your heat pump, and you should have it checked.
When to manually switch to emergency heat
You should almost never manually switch your system to emergency heat mode. Your thermostat is designed to make that decision automatically based on outdoor temperature and how hard your heat pump is working. Manually switching to emergency heat means you are paying the higher cost for no benefit.
The only reason to manually switch is if your heat pump stops working entirely and you need heat right away while waiting for a technician. In that case, switching to emergency heat will keep your home warm, but call a repair service when ready. Do not leave the system in emergency heat mode once the heat pump is fixed.
What to do if emergency heat stays on too long
If your thermostat shows emergency heat running during mild weather (above 45°F), or if it stays on even after temperatures warm up, your heat pump may have a problem. The most common issue is low refrigerant, which prevents the heat pump from working efficiently and forces the system to rely on emergency heat sooner than it should.
Other possible problems include a faulty outdoor unit, a broken reversing valve (which switches the system between heating and cooling), or a thermostat that is not reading outdoor temperature correctly. All of these require a technician to diagnose and fix. Do not try to adjust settings yourself—call a may have access to HVAC technician and describe what you are seeing on the thermostat display.
While you wait for service, you can reduce costs by lowering your thermostat a few degrees if you can tolerate it. Every degree lower reduces heating costs, even when running on emergency heat.
Preventing unnecessary emergency heat use
Keep your outdoor heat pump unit clear of snow, ice, and debris. A blocked outdoor unit has to work harder and may trigger emergency heat sooner than necessary. In winter, check the unit after heavy snow and gently brush away accumulation from the fins and intake areas.
Have your heat pump serviced annually, ideally before winter starts. A technician will check refrigerant levels, clean the outdoor coil, and make sure the reversing valve is working properly. A well-maintained system will operate efficiently and use emergency heat only when truly necessary.
Set your thermostat to a consistent temperature rather than adjusting it frequently. Large temperature swings can cause the system to switch to emergency heat more often than needed. A programmable or smart thermostat can help by maintaining steady temperatures without manual adjustment.
Frequently Asked Questions
Is emergency heat the same as my heat pump?
No. Your heat pump moves heat from outside air into your home. Emergency heat is an electric resistance heating element that creates heat directly, like a space heater. They are separate systems, and emergency heat only turns on when the heat pump cannot keep up with demand.
Should I turn off emergency heat in spring?
Your thermostat handles this automatically. Once outdoor temperatures stay above your system's balance point (usually 40°F to 50°F), the heat pump will take over and emergency heat will not run. You do not need to manually switch anything.
Why is my heating bill so high this month?
If you have a heat pump and your bill jumped, emergency heat was probably running because outdoor temperatures dropped below your system's efficient range. This is normal during cold spells. If temperatures have been mild and the bill is still high, contact a technician to check whether your heat pump is working correctly.
Can I adjust the temperature at which emergency heat turns on?
Some thermostats allow you to change the balance point setting, but this should only be done by a technician. Changing it yourself can cause your system to use emergency heat too often or not often enough. If you think your balance point is set incorrectly, contact your HVAC service provider.
What does "aux heat" mean on my thermostat?
"Aux" is short for auxiliary, which is another name for emergency heat. If your thermostat shows "aux" or "aux heat," your system is running the backup electric resistance heating element because outdoor temperatures are too cold for the heat pump to work efficiently.