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 a second heating system built into most heat pumps—usually electric resistance coils or a gas furnace. You should turn it on only when your heat pump fails to warm your home to the temperature you set, or when outdoor temperatures drop so low that the heat pump becomes inefficient. Running emergency heat constantly wastes energy and money because it costs far more to operate than the heat pump itself.
Many heat pumps switch to emergency heat automatically when they detect a problem or when the outdoor temperature falls below a certain threshold—often around 35°F to 40°F, depending on your system. However, some systems require you to switch manually. The key is understanding the difference between letting your system work as designed and forcing it to use the expensive backup when it is not needed.
Key Takeaways
- Emergency heat should only run when your heat pump cannot keep up with heating demand or stops working entirely.
- Most modern heat pumps set up emergency heat automatically, but you should know how to switch it on manually if your system requires it.
- Running emergency heat continuously costs two to three times more than running the heat pump alone, even in cold weather.
- If emergency heat runs constantly, your heat pump may need service—do not assume the system is working correctly.
- Check your thermostat manual to find the emergency heat button or setting, as the location and label vary by manufacturer.
How to identify the emergency heat switch on your thermostat
The emergency heat button or setting is usually labeled "EM Heat," "Aux Heat," "Emergency," or "E Heat" on your thermostat. On older dial thermostats, it may be a separate switch. On digital thermostats, it is often a button you press or a mode you select from a menu. Some thermostats hide it in a submenu under "Settings" or "System Mode."
The fastest way to find it is to check the manual that came with your thermostat—search for the model number on the back or bottom of the unit, then look up the manual online if you no longer have the paper copy. If you cannot locate the manual, look for a small button or switch near the mode selector (where you choose between Heat, Cool, and Auto). Press or toggle it once and listen for a click or watch for a light to turn on. Do not leave it on; this is just to confirm you found the right control.
When your heat pump stops heating altogether
If your home is not warming up at all and the outdoor temperature is above freezing, switch emergency heat on when ready. This tells your system to use the backup heating while the heat pump is offline. Your home will warm up, though more slowly and at higher cost than normal operation.
Once emergency heat is on, call a heat pump technician to diagnose the problem. Common causes include a refrigerant leak, a failed compressor, a broken reversing valve, or a tripped circuit breaker. Do not run emergency heat for more than a few hours without professional help—extended use will show up as a spike in your electric bill, and the underlying problem will not fix itself. If the outdoor temperature is below 35°F, your heat pump may have frozen up; in that case, switch to emergency heat and wait 30 minutes to an hour for the system to thaw before calling for service.
When outdoor temperatures drop below your heat pump's effective range
Heat pumps pull warmth from outdoor air, even in cold weather. However, as the temperature drops, the heat pump has to work harder and harder to extract that warmth. Below a certain point—usually 35°F to 40°F for most residential units—the heat pump becomes so inefficient that it cannot keep up with your heating demand, and your home temperature will start to drop.
If you notice your thermostat is set to 70°F but your home is only reaching 65°F or 66°F, and the outdoor temperature is in the 30s, your heat pump is struggling. At this point, switching on emergency heat will bring your home to the set temperature faster. However, if your system has an automatic changeover feature, it should already be doing this without your input. Check your thermostat settings to see if "Auto" mode is selected—if it is, the system will switch to emergency heat on its own when needed.
Recognizing when emergency heat is running unnecessarily
If your emergency heat light is on constantly, even when outdoor temperatures are mild (above 40°F) and your home is reaching the set temperature, something is wrong. This could mean the heat pump is stuck in defrost mode, the outdoor sensor is broken, or the thermostat is misconfigured. Running emergency heat in these conditions will double or triple your heating costs for no benefit.
Turn off emergency heat and check that your thermostat is set to "Heat" or "Auto," not "Emergency Heat." If the light goes off and your home stays warm, the problem was just a setting. If the light stays on or comes back on within a few minutes, call a technician. Do not ignore this—unnecessary emergency heat use can add $50 to $100 or more to a single month's heating bill.
Understanding automatic versus manual emergency heat set up
Newer heat pumps (built in the last 10 to 15 years) almost always have automatic emergency heat set up. The thermostat monitors the outdoor temperature and the indoor temperature, and when the heat pump cannot keep up, it automatically switches on the backup heating without any action from you. You will see an "Aux Heat" or "EM Heat" indicator light on your thermostat when this happens.
Older systems or those with manual-only thermostats require you to flip the switch yourself. If you have an older system, get in the habit of checking your thermostat during winter cold snaps. If your home is not warming up and the outdoor temperature is very cold, switch to emergency heat and call for service. Once the heat pump is repaired or the weather warms up, switch it back off.
What to do if emergency heat is your only option
If your heat pump has failed completely and you are waiting for a technician, emergency heat is your lifeline. It will keep your home warm, though at a higher cost. To minimize that cost while the system is down, lower your thermostat by 2°F to 3°F from where you normally keep it, close off rooms you are not using, and use space heaters in the rooms where you spend the most time (if you have them available).
If the outdoor temperature is expected to drop below 0°F and your heat pump is not working, emergency heat alone may not be enough to keep your home above 60°F—the minimum temperature needed to prevent pipe freezing. In that case, contact your technician about expedited service, or consider staying with family or friends until the system is repaired. Do not attempt to repair a heat pump yourself; the system contains refrigerant under high pressure and requires specialized tools and training.
Frequently Asked Questions
Will turning on emergency heat damage my heat pump?
No. Emergency heat is a separate system that does not interact with the heat pump itself. However, running emergency heat while the heat pump is also trying to operate can cause the compressor to work harder than necessary. If your system has automatic switching, it will turn off the heat pump when emergency heat is on, so there is no conflict.
Why does my electric bill spike when emergency heat runs?
Emergency heat uses electric resistance coils or a gas furnace, both of which are far less efficient than a heat pump. A heat pump moves heat from outside to inside; emergency heat generates heat directly, which requires much more energy. Running emergency heat for a full month instead of the heat pump can increase your heating costs by 50% to 100% or more.
Should I turn on emergency heat before a cold snap?
No. Turn it on only if your heat pump stops working or cannot keep your home warm. Turning it on early wastes energy and money. If your system has automatic set up, it will switch on when the temperature drops low enough that the heat pump cannot keep up.
Can I use emergency heat in summer if my air conditioning breaks?
No. Emergency heat only produces warmth; it has no cooling function. If your air conditioning fails, you will need to have the heat pump or AC unit repaired. In the meantime, use fans, close blinds during the day, and stay in shaded areas of your home.
What if my thermostat does not have an emergency heat button?
Some very old thermostats or straightforward models do not have a separate emergency heat control. In that case, your backup heating (if you have one) may be controlled by a separate switch on the furnace or air handler itself, usually located in a basement or utility closet. Check your heat pump manual or call a technician to locate it.