If you’ve spotted an “emergency heat” or “EM heat” setting on your thermostat and wondered whether you should ever use it, you’re not alone. This backup heating mode exists on many heat pump systems, but activating it at the wrong time can send your energy bill soaring without actually solving the problem.
Understanding how emergency heat works, when it’s truly necessary, and what happens when you turn it on will help you keep your home comfortable while avoiding expensive mistakes during cold weather.
What Is Emergency Heat on a Thermostat?
Emergency heat is a backup heating mode that bypasses your heat pump and relies entirely on electric resistance heating to warm your home. When you switch to this setting, your system stops using the efficient outdoor heat pump unit and instead uses only the auxiliary heating elements inside your air handler, which function like a large space heater and consume significantly more electricity.
How Heat Pumps Normally Work
To understand emergency heat, you first need to know how your heat pump operates under normal conditions. A heat pump doesn’t generate heat by burning fuel or using electric coils as its primary method.
Instead, it extracts heat from outside air and moves it indoors, even when outdoor temperatures feel cold to you. This process uses a refrigerant cycle similar to how your refrigerator works, but in reverse.
During mild winter weather, this method is remarkably efficient. Your heat pump can deliver three to four times more heat energy than the electricity it consumes because it’s moving existing heat rather than creating it from scratch.
When Auxiliary Heat Kicks In Automatically
Most heat pump systems include auxiliary or supplemental electric resistance heat that activates automatically when needed. This happens in two common situations.
First, auxiliary heat turns on when outdoor temperatures drop so low that the heat pump alone can’t keep up with your thermostat setting. The exact temperature depends on your system, but this often occurs below 25 to 35 degrees Fahrenheit.
Second, auxiliary heat runs during recovery periods when you’ve raised the thermostat setting significantly. If you’ve been away and bump the temperature up by five or more degrees, the system may use auxiliary heat to reach your target temperature faster.
This automatic auxiliary heat is normal and designed into your system. You’ll often see “AUX heat” displayed on your thermostat when it runs, and your energy use will increase moderately during these periods.
The Difference Between Auxiliary and Emergency Heat
Auxiliary heat and emergency heat use the same heating elements inside your system, but they operate differently. Understanding this distinction matters because one is automatic and efficient, while the other is manual and costly.
Auxiliary heat works alongside your heat pump. The outdoor unit continues to run and provide the majority of heating, while the electric resistance coils supplement when needed.
Emergency heat shuts down the heat pump entirely. When you manually switch to emergency heat mode, only the electric resistance heating operates, making your system work like a giant electric furnace.
This difference dramatically affects your energy consumption. Emergency heat can cost two to three times more to run per hour compared to your heat pump with automatic auxiliary support.
When You Should Actually Use Emergency Heat
The emergency heat setting exists for specific mechanical failures, not everyday cold weather. You should switch to this mode only when your heat pump has stopped working properly.
Heat Pump Malfunction
If your outdoor heat pump unit won’t run at all, emergency heat keeps you warm until a technician can repair the problem. You might notice the outdoor fan isn’t spinning, the compressor isn’t running, or the unit is making unusual grinding or clicking sounds.
Ice buildup that won’t clear is another sign. While a thin layer of frost is normal and your heat pump should defrost itself automatically every 30 to 90 minutes, thick ice that persists for hours indicates a defrost cycle failure.
Refrigerant Leaks
A heat pump losing refrigerant can’t transfer heat effectively. Signs include the outdoor unit running constantly without warming your home, ice forming on refrigerant lines, or hissing sounds near the equipment.
If you suspect a refrigerant leak, switch to emergency heat and contact an HVAC professional immediately. Refrigerant handling requires certification, specialized equipment, and proper disposal practices.
Mechanical or Electrical Component Failure
Reversing valve problems, compressor issues, or failed control boards can prevent your heat pump from heating while still allowing the backup elements to function. If your thermostat shows the system is calling for heat but no warm air comes from your vents after several minutes, your heat pump may have failed.
Emergency heat gives you a temporary solution while you arrange for repairs. Think of it as a spare tire that gets you to the mechanic, not a permanent fix.
What Happens When You Turn On Emergency Heat
When you switch your thermostat to emergency heat, several things change in how your system operates. Your outdoor unit stops running entirely, so you won’t hear the fan or compressor outside.
The air handler blower inside continues operating, but now it’s forcing air across electric resistance heating coils instead of a heat exchanger warmed by the heat pump. These coils heat up like toaster elements.
You’ll notice your home warms up, often with air coming from vents that feels slightly hotter than usual. However, your electric meter will spin much faster, and your system will cycle more frequently to maintain temperature.
Most thermostats display “EM heat” or “emergency heat” to remind you the system is in backup mode. Some models also show a warning or different color to indicate this isn’t normal operation.
The Cost of Running Emergency Heat
Emergency heat is expensive because electric resistance heating is one of the most costly ways to warm your home. While exact costs vary by your electricity rate and home size, the difference is substantial.
A typical heat pump might cost 50 to 75 dollars per month to run during moderate winter weather. Switching to emergency heat for the same period could increase that cost to 150 to 200 dollars or more, depending on outdoor temperatures and how long your system runs.
The colder it gets outside, the more emergency heat must work, and the higher your bill climbs. This is why you should return to normal heat pump mode as soon as your system is repaired.
Common Mistakes to Avoid
Many homeowners accidentally leave emergency heat on or switch to it for the wrong reasons. Avoiding these mistakes will save you money and prevent unnecessary wear on your equipment.
Using Emergency Heat During Normal Cold Weather
Some people mistakenly believe emergency heat is for extremely cold days. Your heat pump is designed to work in cold weather, and your system will automatically use auxiliary heat if it needs support.
Unless your heat pump has malfunctioned, leave your thermostat in normal heating mode and let the system manage itself. Switching to emergency heat won’t make your home warmer faster, but it will definitely cost you more.
Forgetting to Switch Back After Repairs
After an HVAC technician fixes your heat pump, remember to turn off emergency heat mode. Your thermostat won’t automatically switch back, and you could run on expensive electric heat for weeks without realizing it.
Set a reminder on your phone or place a note on your thermostat when you activate emergency heat. Check your thermostat display after any service call to confirm it shows normal heating mode.
Ignoring the Underlying Problem
Emergency heat is a temporary solution, not a permanent one. If you find yourself relying on this setting for more than a day or two, you’re paying premium electricity costs for a problem that needs professional repair.
Continuing to run emergency heat for weeks can also shorten the lifespan of your heating elements and increase the risk of system failure during the coldest weather. Schedule repairs promptly rather than treating emergency heat as a long-term option.
How to Switch to Emergency Heat
If you’ve determined your heat pump has failed and you need emergency heat, the process is straightforward. Locate the emergency heat setting on your thermostat, which may be a physical switch, a mode option in your digital display, or a setting in your smart thermostat app.
Some older thermostats have a small switch near the temperature dial labeled “EM heat” or “emergency.” Newer programmable and smart thermostats typically require you to access the system mode menu and select emergency heat from a list that includes heat, cool, auto, and off.
Once activated, verify that warm air is coming from your vents within a few minutes. Your thermostat display should confirm emergency heat is running.
Contact an HVAC technician to diagnose and repair your heat pump as soon as possible. Provide details about what you observed before switching modes, such as unusual sounds, ice buildup, or whether the outdoor unit stopped running entirely.
Preventing Heat Pump Problems
Regular maintenance reduces the likelihood you’ll ever need emergency heat. Most heat pump issues develop gradually and can be caught before they cause complete system failure.
Schedule professional maintenance at least once per year, ideally before heating season begins. A technician will check refrigerant levels, clean coils, test the defrost cycle, inspect electrical connections, and verify that all components function correctly.
Between professional visits, keep your outdoor unit clear of debris, leaves, snow, and ice. Airflow restrictions force your heat pump to work harder and can trigger failures.
Change your indoor air filter every one to three months depending on household conditions. A clogged filter reduces airflow and can cause your system to overheat or freeze up, potentially forcing you to use emergency heat.
Signs Your Heat Pump Needs Service
Catching problems early helps you avoid emergency situations. Watch for these warning signs that indicate your heat pump needs professional attention before it fails completely.
- The outdoor unit runs constantly but your home stays cold
- Strange noises such as grinding, squealing, or rattling from indoor or outdoor units
- Ice or frost that doesn’t melt after a defrost cycle completes
- Short cycling where the system turns on and off every few minutes
- Steadily increasing energy bills without explanation
- Weak airflow from vents throughout your home
- The system blows cold air when set to heating mode
If you notice any of these issues, call for service before your heat pump stops working entirely. Proactive repairs are usually less expensive than emergency calls during the coldest nights of winter.
Alternative Heating Options During Repairs
If your HVAC professional can’t arrive immediately and you’re concerned about emergency heat costs, you have other options to stay comfortable. Space heaters can warm individual rooms while you wait for service, though you should follow all safety guidelines.
Use only space heaters with automatic shut-off features, keep them away from curtains and furniture, never leave them unattended, and plug them directly into wall outlets rather than extension cords. Never use your oven or stovetop for heating, as this creates serious carbon monoxide risks.
Closing off unused rooms and focusing heat in occupied areas reduces how much your emergency heat system must run. Lowering your thermostat by a few degrees also decreases operating costs, though not as effectively as having your heat pump repaired.
Emergency heat serves an important purpose when your heat pump fails, but it’s designed for short-term use while you arrange professional repairs. Understanding the difference between normal auxiliary heat and true emergency operation helps you make informed decisions, avoid unnecessary costs, and keep your home comfortable even when your primary heating system needs attention.
Continue learning about home comfort systems with more helpful articles covering heating, cooling, and maintenance topics throughout our Home and Garden section.