Emergency heat pump operation explained
When a heat pump's primary heating mode fails — usually because the outdoor unit has frozen over or the system has lost refrigerant — the emergency heat kicks in automatically. Emergency heat is a backup electric resistance heating element, similar to a toaster coil, that turns on inside your indoor unit to keep your home warm while the heat pump is repaired or thaws out. It is not a second heat pump; it is a straightforward electric heater that runs on its own circuit.
The emergency heat mode does one job: maintain indoor temperature without relying on the outdoor unit. Your thermostat switches to it either automatically (if your system detects a problem) or manually (if you flip the switch yourself). Once activated, the heat pump compressor shuts down, and only the electric resistance element heats your home. This keeps you from freezing while you wait for service, but it costs significantly more to run than normal heat pump operation.
Key Takeaways
- Emergency heat is an electric resistance heating element inside your indoor unit that activates when the heat pump cannot function.
- The system automatically switches to emergency heat if the outdoor unit freezes or loses refrigerant, or you can switch it manually on your thermostat.
- Emergency heat costs two to three times more per hour than heat pump heating because it uses pure electric resistance instead of moving heat from outside.
- Emergency heat is meant for temporary use during repairs or thaw cycles, not as a permanent heating solution.
- Most modern thermostats show when emergency heat is running so you know the system is in backup mode.
Why emergency heat activates on its own
Heat pumps rely on an outdoor unit to absorb heat from the air, even in cold weather. When outdoor temperatures drop below freezing and humidity is high, frost and ice build up on the outdoor coil. The system has a defrost cycle that reverses the refrigerant flow to melt the ice, but if this cycle fails or the ice accumulates faster than it can melt, the outdoor unit becomes blocked.
When the heat pump cannot pull heat from outside, indoor temperature begins to drop. Most systems have a sensor that detects this temperature drop and triggers emergency heat automatically. The same thing happens if the system loses refrigerant due to a leak — without refrigerant, the compressor cannot move heat, so the backup element turns on to prevent the home from getting dangerously cold. You do not have to do anything; the switch happens on its own.
Some older thermostats do not have automatic detection, so you may need to switch to emergency heat manually if you notice the heat pump is not warming your home. Check your thermostat display for a button or setting labeled "Emergency Heat," "Aux Heat," or "E-Heat." Flipping this switch tells the system to stop trying to use the heat pump and rely only on the electric element.
How the electric resistance element heats your home
The emergency heat element works like the heating coil in an electric oven or space heater. Electricity flows through a wire made of high-resistance metal, which heats up and radiates warmth into the air. Your furnace blower then pushes that warm air through your ductwork and into your rooms. There is no compressor, no refrigerant, and no outdoor unit involved — just electricity being converted directly into heat.
Because the element heats by resistance rather than by moving heat from outside, it requires much more electrical energy to produce the same amount of warmth. A heat pump in normal operation might produce three units of heat for every one unit of electricity it uses (this ratio is called the coefficient of performance). An electric resistance element produces only one unit of heat per unit of electricity. This is why your electric bill climbs steeply when emergency heat runs for more than a few hours.
The element is sized to heat your home at a moderate pace — usually enough to raise indoor temperature by one to two degrees per hour. It is not designed to heat quickly or to handle extreme cold. If outdoor temperatures are very low, the emergency element may not be able to keep up with heat loss through walls and windows, and your home will gradually cool.
Cost difference between emergency heat and normal operation
Running emergency heat costs roughly two to three times more per hour than running the heat pump in normal mode, depending on your local electricity rates and your system's efficiency. If your heat pump normally costs $2 per hour to heat your home, emergency heat might cost $5 to $6 per hour for the same temperature output. Over a full day, that difference adds up quickly.
The exact cost depends on three factors: your electricity rate (measured in cents per kilowatt-hour), the wattage of your emergency element (usually between 5,000 and 15,000 watts), and how long the element runs. A 10,000-watt element running continuously for eight hours at 15 cents per kilowatt-hour costs roughly $12. The same eight hours on a heat pump might cost $4 to $5.
This is why emergency heat is meant to be temporary. If your heat pump is broken and you are running emergency heat for days or weeks while waiting for a repair appointment, your heating costs will be substantially higher than normal. Some homeowners choose to lower their thermostat a few degrees while in emergency mode to reduce the electric bill, accepting a cooler home in exchange for lower costs.
When to switch to emergency heat manually
You should switch to emergency heat manually only if your heat pump is clearly not working and your home is getting cold. Signs that the heat pump has failed include: the outdoor unit is completely covered in ice and not cycling through a defrost, the indoor unit is blowing cold or lukewarm air even though the thermostat is set to heat, or the system is making unusual noises or not running at all.
Before you switch, check whether the outdoor unit is straightforward in a defrost cycle. During defrost, the system reverses to melt ice, and the indoor unit may blow cool air for five to ten minutes. This is normal and does not mean the system is broken. If the defrost cycle completes and the unit goes back to heating normally, leave the system alone.
If the outdoor unit is frozen solid and not cycling, or if the system has been running for an hour without raising indoor temperature, switching to emergency heat will keep you warm while you call for service. Once you switch, your home will heat on the electric element alone. Call a technician as soon as possible — emergency heat is not a permanent fix, and running it continuously will cost far more than repairing the heat pump.
How long emergency heat can run safely
Emergency heat can run indefinitely without damaging the system, but it should not run for more than a few hours without investigation. If emergency heat has been on for more than four hours, either the heat pump has a serious problem that needs repair, or the outdoor temperature is so cold that the heat pump cannot keep up even in normal mode.
In very cold weather (below 0°F), some heat pumps automatically switch to emergency heat to supplement the heat pump's output. This is normal and expected. The system will use both the heat pump and the electric element together to maintain temperature. You will see higher electric bills during these periods, but this is the system working as designed.
If emergency heat is running in mild weather (above 20°F) and the heat pump is not working, the system likely has a refrigerant leak, a failed compressor, or a control board problem. These are not quick fixes, and you will need a service call. Do not leave emergency heat on and ignore the problem — the longer you wait, the higher your electric bill becomes.
Preventing emergency heat set up
The most common cause of emergency heat set up is ice buildup on the outdoor unit. You can reduce this risk by keeping the area around the outdoor unit clear of snow and ice, and by making sure the unit has good airflow. Do not block the unit with a cover, fence, or landscaping.
Regular maintenance also helps. A technician should inspect and clean the outdoor coil once a year, usually in the fall before heating season. A dirty coil is more prone to frost buildup and less efficient at absorbing heat. If your system has a cracked or leaking refrigerant line, a technician can find and repair it before it causes a complete loss of refrigerant and forces emergency heat to run.
If you live in a very cold climate, ask your technician whether your system is sized correctly for your home. An undersized heat pump may struggle to keep up in extreme cold and trigger emergency heat more often. In these cases, a technician might recommend a higher-capacity unit or a backup heating system designed for very cold weather.
Frequently Asked Questions
Why is my thermostat showing emergency heat when the heat pump seems to be working?
Your system may have automatically switched to emergency heat because the outdoor unit was frozen or the heat pump was not keeping up with heat loss. Check the outdoor unit for ice or snow. If it looks clear and the system is heating normally, you can switch back to regular heat mode. If the heat pump is not warming your home adequately, leave emergency heat on and call for service.
Can I use emergency heat all winter instead of the heat pump?
You can, but it will cost two to three times more than running the heat pump. Emergency heat is an electric resistance element, not a heating system. It is designed as a backup for a few hours or days, not as a primary heating source. Using it all winter will result in a very high electric bill and puts unnecessary strain on the element.
Does emergency heat damage the heat pump?
No. Running emergency heat does not damage the heat pump itself. The compressor is straightforward shut off while the electric element runs. However, if the reason emergency heat is on is because the heat pump is broken, you still need to repair the underlying problem. Emergency heat only masks the issue.
What should I do if emergency heat is not keeping my home warm?
If your home is still getting cold even with emergency heat running, the outdoor temperature is likely extremely low, or the emergency element is not sized for your home's heat loss. Lower your thermostat by a few degrees to reduce the load, close off unused rooms, and call a technician. Do not assume the system is broken — very cold weather can overwhelm even a working emergency heat system.
How do I turn off emergency heat once the heat pump is fixed?
Once your heat pump is repaired and working normally, switch your thermostat back to regular heat mode. The system will return to using the heat pump as its primary heating source. If your thermostat switched to emergency heat automatically, it should switch back automatically once the heat pump is functioning again, but you can also switch it manually.