Emergency heat on a thermostat is the backup heat source for a heat pump, and you use it when the heat pump fails or cannot keep up. Most switches are a DIY change that takes 1 to 2 minutes, but the reason for using it should be checked first so you do not run the expensive backup heat by mistake.
Say your heat pump is running, the house sits at 63°F, and the outdoor unit is buried under ice after an overnight storm. In that case, emergency heat can keep the place livable while you sort out the fault.
Flip it on without a reason and you can strand the compressor, spike the bill, and miss a problem that needs repair. The switch is simple; the diagnosis is the part that takes a little thought.
What Is Emergency Heat on a Thermostat
In short
Emergency heat is the backup heat source for a heat pump, used when the heat pump cannot heat the home or has failed. Selecting EM Heat manually shuts off the heat pump compressor and runs the backup source instead.
The emergency heat setting on a thermostat means “use backup heat now.” Most heat pumps use electric resistance strips, which heat air much like toaster coils. Some systems use a different built-in backup source, such as another furnace, if the equipment was designed and wired for it.
Automatic auxiliary heat and manual emergency heat are different. Auxiliary heat can turn on by itself during a defrost cycle or when the heat pump needs help reaching the setpoint; the compressor may keep running while the backup heat assists it. Manual EM Heat tells the system to stop normal compressor operation and rely on the backup source.
Thermostat label
What it does
Who controls it
Heat
Runs the heat pump normally
Thermostat
Auxiliary heat
Adds backup heat when needed
Thermostat automatically
EM Heat
Stops the compressor and runs backup heat
You manually
Labels vary by thermostat, but the operating difference matters.
Read the label, not the color
Some thermostats show “Emergency,” some show “EM Heat,” and smart models may hide the option under system mode. The function is the same only if your thermostat is connected to a heat-pump system with backup heat; a conventional furnace thermostat won’t gain emergency heat from a menu setting.
When To Use Emergency Heat
The decision rule
Use EM Heat when the thermostat calls for heat but the heat pump has a real fault: the outdoor unit is dead, damaged, or heavily iced and not clearing itself, or the system cannot maintain the indoor temperature. If the outdoor unit is running normally and the house is warming, leave EM Heat off.
There is no exact outdoor temperature that triggers automatic EM Heat on every system. Many heat pumps bring on auxiliary heat below freezing, during defrost, or after the indoor temperature falls behind, but the trigger depends on the equipment, thermostat settings, and heating load. Around 25°F can be a rough point where some systems struggle; it isn’t a universal switch point.
1
Check the call for heat
Confirm the thermostat is set to Heat and the indoor temperature is below the setpoint. Give the system a moment to respond rather than switching modes at the first cool draft.
2
Look outside
If the outdoor fan and compressor are silent during a heat call, or the cabinet is damaged or buried in solid ice, EM Heat can provide short-term heat while you arrange repair.
3
Plan the repair
Use EM Heat only until the heat pump fault is corrected, then return the thermostat to normal Heat mode.
Power outage limit
EM Heat won’t work during an outage unless a generator or another powered source can supply the HVAC system. Never connect a generator to house wiring with a makeshift cord; a proper transfer setup is required.
What It Costs To Run
EM heat uses more electricity than normal heat-pump mode because resistance strips turn electricity directly into heat, while a heat pump moves heat indoors and usually needs less power for the same comfort. Emergency heat increases energy costs mainly when the strips run for long stretches or the thermostat is left on EM by mistake.
Worked example
A 1,500 W auxiliary heater uses 1.5 kW. Running it for 8 hours uses 1.5 × 8 = 12 kWh; at 17 cents per kWh, that’s about $2.04 for one day before taxes and fixed charges.
Heater1,500 W
Run time8 hours
Energy used12 kWh
Electric rate$0.17/kWh
Myth
Emergency heat always costs far more the moment you turn it on.
Fact
Short, necessary use is the right move when the heat pump is broken. The expensive part is leaving resistance heat on for days, especially after the outdoor unit could have been repaired or the thermostat was accidentally left in EM mode.
Best practice
Use EM heat only long enough to protect comfort while service is arranged. Keep the thermostat at a reasonable setpoint, and return it to normal Heat after the heat pump fault is fixed; limiting strip runtime limits the bill.
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How To Switch It On Safely
Manual EM heat is safe when your heat-pump system is designed for it—but check the outdoor unit first, then switch back to normal Heat after the fault is fixed. Thermostats may label the control EM Heat, Emergency Heat, Aux Heat, or place it inside a heat-pump changeover menu on a smart thermostat.
1
Check the outdoor unit
Look for ice that doesn’t clear, a fan that won’t run, repeated breaker trips, unusual noises, or no temperature rise after a long call for heat. Don’t open the electrical panel or reach into the unit.
2
Set EM heat
Select EM Heat or Emergency Heat on the thermostat. Manual activation normally disables the heat pump compressor, so the backup heating strips carry the call.
3
Wait for warm air
Give the system a few minutes, then check a supply register. Warm air should return and the room temperature should begin rising; airflow alone doesn’t prove the strips are working.
4
Call for service
Arrange heat-pump service if the air stays cool, the indoor fan won’t run, breakers trip again, or the temperature still falls. Leave EM heat on only as needed while the fault is addressed.
5
Restore normal Heat
After the outdoor unit operates normally and the repair is complete, select Heat rather than EM Heat. Leaving the override active is a common, costly mistake.
Before You Leave It Running
Outdoor unit is clearly failing or unable to heat
Thermostat is set to EM Heat, not a cooling mode
Warm air reaches the room after the change
A service call is planned if the fault remains
Useful for this step
R.W.FLAME Small Space Heaters for Indoor Use
If the thermostat’s labels confuse you or its heat-pump controls seem unreliable, look for heat-pump thermostats that clearly support emergency heat before replacing the unit.
EM heat usually doesn’t damage a heat pump by itself; the risk is using it while an easy airflow or outdoor-unit problem goes unfixed. To prevent unnecessary activation, replace the filter on schedule, keep snow and leaves clear of the outdoor coil, and have the thermostat’s heat-pump setup and auxiliary-heat staging checked after service.
A dirty filter or blocked return can restrict airflow, while ice on the outdoor unit may point to a problem that needs attention. EM heat won’t repair low refrigerant, a failed compressor, or a faulty reversing valve. Repeatedly switching modes, allowing short cycling, or ignoring the outdoor unit can overwork the system and conceal the cause.
Mistake
Better practice
Leaving EM heat on after the fault is resolved
Return the thermostat to Heat and confirm normal operation.
Toggling modes repeatedly
Avoid repeated manual changes; watch for short cycling and call for service if it continues.
Ignoring the outdoor unit
Keep the coil clear; don’t try to fix persistent icing by opening the system.
Changing controls after service
Ask the technician to confirm heat-pump setup and auxiliary-heat staging.
These checks can prevent avoidable backup-heat calls and wasted energy.
Leave sealed parts and wiring alone
Don’t open refrigerant lines or field-wire controls yourself. Refrigerant work and control wiring can create safety hazards or damage equipment; leave diagnosis and repair to a qualified technician.
Use EM heat for the right reason
The useful rule is simple: correct an obvious filter or airflow issue, but don’t treat persistent icing or a system fault as a thermostat problem. Keep EM heat use to the situation your system needs it for, rather than switching it on just because the weather is cold.
When A Licensed Pro Is Required
Call a licensed HVAC pro when EM heat points to a system fault—or when the system won’t leave EM heat, blows cold air during a heat call, or trips a breaker. Changing a thermostat mode is a reasonable DIY task; repairing the heat pump is a different job.
Refrigerant work, compressor repairs, electrical service work, gas work, and venting changes are licensed jobs in most places. If your heat pump uses a gas furnace or another fuel-burning backup, have a licensed pro check its venting, combustion, and safety controls.
Permits may be required for equipment replacement, new circuits, or major HVAC changes, depending on local code. Check with your local building department or contractor before work begins; requirements vary by location.
Keep the boundary clear
You can select a thermostat setting, replace a filter, and keep the outdoor coil clear. Don’t try to re-engineer the heat pump, work on refrigerant lines, or bypass controls. If a breaker trips, don’t keep resetting it; arrange service.
Stop if the fault repeats
A system that stays in EM heat or repeatedly trips a breaker needs diagnosis, not more thermostat adjustments. Switch settings only as directed by the manufacturer or technician, and call for service.
If the repair involves the compressor, knowing what the heat pump compressor does can help you understand the technician’s diagnosis—but it won’t make compressor work a DIY repair.
More Questions, Answered
What does emergency heat on a thermostat mean?
Emergency heat tells a heat-pump system to use its backup heater immediately. In manual EM Heat, the compressor is usually locked out; automatic auxiliary heat can run alongside the heat pump when the thermostat needs extra capacity.
When should I use emergency heat on my heat pump?
Use it when the heat pump has failed, the outdoor unit won’t run, or the house temperature keeps falling despite a normal heat call. A brief defrost cycle doesn’t require manual EM Heat because the system handles that change itself.
Does emergency heat come on automatically in cold weather?
Often, but there is no universal outdoor-temperature trigger. The thermostat and heat-pump controls use their own outdoor sensor, setpoint gap, and defrost logic, so one system may call auxiliary heat sooner than another.
Can I leave emergency heat on all winter?
The system can usually run that way, but leaving EM Heat selected all winter may sharply raise electricity use and can add unnecessary wear. If the heat pump is unavailable for an extended repair, use EM Heat temporarily and have the fault diagnosed rather than treating it as the new normal.
What should I check before I turn on EM heat?
Check that the thermostat is calling for heat, the air filter isn’t packed with dust, and the outdoor unit has power and isn’t blocked by snow or debris. Resetting a tripped breaker once is reasonable; repeated trips, burning smells, heavy ice, or no airflow call for an HVAC professional instead.
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