EM heat (emergency heat) is a thermostat setting on heat pump systems that shuts off the outdoor unit and heats your home using only the backup heat source, usually electric heat strips inside the indoor air handler. It keeps you warm when the heat pump itself has failed or can't run. Because heat strips use two to three times more electricity than a working heat pump for the same amount of warmth, EM heat is a short-term fix until the heat pump is repaired, not a setting for cold days.
What Does EM Heat Mean on a Thermostat?
On most thermostats the setting shows up as "EM Heat," "EMER," "EMHT" or "Emergency Heat." You'll only see it on a heat pump system. A furnace and AC setup has no emergency mode because the furnace is already the heat source.
A heat pump system has two ways to make heat. The outdoor unit pulls heat out of the outside air, even cold air, and moves it indoors through refrigerant lines. The indoor unit holds a backup: in an all-electric home that's a bank of electric resistance heat strips, and in a dual-fuel home it's a gas furnace. Selecting EM heat tells the thermostat to stop calling the outdoor unit entirely and run only that backup.
How Emergency Heat Works
Electric heat strips work like the glowing coils in a toaster or a space heater. Current runs through a resistance wire, the wire gets hot, and the blower pushes air across it and into your ducts. Every unit of electricity becomes one unit of heat.
A heat pump in mild winter weather moves two to three units of heat for every unit of electricity it uses, because it's relocating heat that already exists outside instead of creating it. Strips are simple and dependable, which makes them a good backup, but they're expensive to lean on.
EM Heat vs. Heat: What's the Difference?
Regular Heat mode runs the heat pump as the main heat source and lets the thermostat bring in backup heat automatically when it's needed. EM Heat mode takes the heat pump out of the picture and runs on backup alone, every time the system cycles on.
In Heat mode, the system picks the cheapest way to keep you comfortable. In EM Heat mode, you've overridden that choice and told it to use the most expensive option. For a healthy heat pump, Heat is the right setting all winter long, including on nights that drop into the teens.
Aux Heat vs. Emergency Heat
Aux heat and emergency heat use the same heat strips. The difference is who turns them on and whether the heat pump keeps running.
Auxiliary heat vs. emergency heat uses the same heat strips. The difference is who turns them on and whether the heat pump keeps running.
Auxiliary heat is automatic. The thermostat adds the strips alongside the heat pump when the house is falling behind, when you raise the setpoint by more than a couple of degrees, or when the outdoor unit goes into defrost. The heat pump keeps working the entire time, and aux heat shuts off once the house catches up. Seeing "Aux" on the display during a cold snap is normal.
Emergency heat is manual. You choose it, it stays on until you switch it back, and the heat pump stays off the whole time.
If "Aux" is lit nearly all the time in moderate weather, the heat pump may be underperforming, and you're paying heat strip prices without realizing it.
When to Use Emergency Heat
Use the emergency heat setting when the heat pump can't do its job and you need heat until it's fixed.
The most common reason is an outdoor unit that isn't running at all. If the thermostat is calling for heat and the indoor blower is on, but the outdoor fan and compressor are silent, switch to EM heat to keep the house warm and schedule a repair.
Heavy ice is the second. Light frost on the outdoor coil is normal in winter, and the unit defrosts itself every so often (steam rising off the unit during defrost is normal too). A solid block of ice that stays through several defrost cycles means something is wrong, often a failed defrost control or low refrigerant. Running the heat pump in that condition can damage the coil and fan, so EM heat is the safer choice until it's checked.
Physical damage counts too, such as a fallen branch that bent the fan blade. And if a repair is waiting on parts, your technician may leave the system in EM heat until they return.
When Not to Use EM Heat
Don't switch to emergency heat just because it's cold outside. A working heat pump keeps pulling heat from outdoor air well below freezing, and when it can't keep up on its own, aux heat steps in automatically. Switching to EM heat on a cold night only shuts off the cheaper half of your system.
Don't use it to warm the house up faster, either. The vent air may feel hotter, but the house warms at about the same pace while your meter spins much faster.
How Much More Expensive Is Emergency Heat?
Emergency heat typically costs two to three times as much to run as a working heat pump in mild winter weather. The gap narrows in very cold weather, when the heat pump's efficiency drops, but in an all-electric home the strips never become the cheaper option.
Here's a quick example. Residential heat strips commonly range from about 5 to 20 kilowatts. A 10 kW strip running for one hour uses 10 kWh, so at 15 cents per kWh that's $1.50 for every hour it runs. A heat pump delivering similar warmth on a 40-degree day might use a third to half of that. Over a day or two, the difference is modest. Over a month, it can be your biggest power bill of the year.
The math changes in a dual-fuel home, where emergency heat runs a gas furnace instead of strips. Your cost then depends on local gas and electric rates, but the rule is the same: use emergency heat until the heat pump is repaired, then switch back.
How to Turn On Emergency Heat and Switch Back
Press the Mode or System button on your thermostat and cycle through the options until "Em Heat" or "Emergency" appears, then set your temperature as usual. Some smart thermostats tuck it into settings or only show it once they're configured for a heat pump.
When the repair is done, switch back to Heat. The system won't return to the heat pump on its own.
Accidentally Turned On Emergency Heat?
If you switched to the emergency heat setting by accident, set the thermostat back to Heat and you're done. Nothing is damaged. The only cost is the extra electricity used while it was on.
EM Heat Light On When You Didn't Select It
Some thermostats light an "Em" or "Aux" indicator on their own. If the mode is still set to Heat, the light usually means the system is in defrost or the strips are helping the heat pump catch up, and both are normal.
If the light stays on for hours, or the thermostat has switched itself into emergency mode, the heat pump may have locked itself out after a fault. Check whether the outdoor unit is running and whether its breaker, or the disconnect box mounted next to the unit, has tripped. A breaker that trips again after you reset it needs a technician, not another reset.
Emergency Heat Not Working
If EM heat blows cool air or nothing at all, the strips have lost power or a part has failed. Heat strips usually have their own breaker, separate from the outdoor unit's, so look in the panel for one labeled for the air handler or heat strips. Past that, the usual causes are a failed sequencer or relay, a tripped high-limit switch, a loose wire or a burned-out element. If the heat pump and the emergency heat are both down, you have no heat source at all, so treat it as an urgent repair.
Get Your Heat Pump Back Online
Emergency heat is built to carry you for a few days, not a whole season. The longer it runs, the more of your heating bill goes to resistance heat, and whatever stopped the heat pump rarely fixes itself.
MR. HVAC is a family-owned company with more than 25 years of experience in heat pump installation and repair. Our technicians find out why the outdoor unit stopped, explain the fix in plain terms and give you the price before any work starts. If your thermostat has been sitting on EM heat, schedule heat pump service and get back to the cheaper heat your system was built to deliver.
