The EM Heat setting on a thermostat stands for emergency heat, and it does not work the way many homeowners assume. It does not boost the normal heating output. Instead, it switches the system to a separate backup heating source that runs when the primary heat cannot keep the house warm. Understanding the setting matters because misusing it raises utility bills and masks problems in the main system. Finding the right thermostat settings for every area of your home keeps the primary system in charge for most of the winter.
What EM Heat Means on a Thermostat
EM Heat controls the emergency backup heating system, which is separate from the main heating source. On a heat pump, the backup is usually electric resistance strips inside the air handler. On dual-fuel systems, it can be a gas or oil furnace. The thermostat routes control to that backup circuit only when you select the EM setting manually.
Emergency Heat vs. Auxiliary Heat
Auxiliary heat, sometimes labeled AUX, engages automatically when the heat pump cannot keep up, such as during a defrost cycle or an extreme cold snap. Emergency heat, by contrast, is a manual override. When you switch to EM Heat, the thermostat locks out the compressor and runs the backup source exclusively. That distinction matters: AUX protects the system automatically, while EM is reserved for actual failures.
How the Heat Pump Fits In
Heat pumps move heat from outside air into the house using refrigerant. Their efficiency drops as outdoor temperatures fall, and below a certain balance point they produce little useful warmth. The backup system covers that gap. Keeping the heat pump working within its range saves energy, which is why the optimal winter thermostat settings stay close to 68 degrees during the day and drop only modestly at night.
Where the EM Label Comes From
Manufacturers print EM Heat on the thermostat face because the backup circuit was originally called the emergency heat strip. The name stuck even though most modern heat pumps use the same strip as automatic auxiliary heat. When you see the label, remember it refers to the same resistance elements or backup furnace that AUX uses, with one difference: you are forcing it on instead of letting the system decide.
How and When to Use Emergency Heat
Emergency heat exists for real failures, not for ordinary cold weather. Turning it on just to warm the house faster reduces the overall efficiency of the system and increases utility costs. Reserve the setting for situations where the primary heat genuinely cannot do the job.
When EM Heat Is Justified
- The heat pump compressor fails and stops producing warm air
- The outdoor unit is iced over and cannot complete a defrost cycle
- The system is under repair and the technician instructs you to run backup heat
- Outdoor temperatures fall far below the heat pump balance point for an extended period
Reduce the Load First
Before reaching for the EM setting, reduce the demand on the system with simple measures. Sealing drafts, closing blinds at night, and letting sunlight in during the day all cut heat loss. How to heat your home without touching the thermostat covers passive strategies that keep rooms comfortable while the primary system handles the load.
Warning Signs That Something Is Wrong
If the house stays cold even though the thermostat calls for heat, check the outdoor unit before assuming the backup is needed. Ice on the coil, a fan that never spins, and a compressor that hums without starting all point to a service issue. Running EM Heat with a broken compressor keeps the house warm temporarily, but it does not fix the underlying failure. A service visit that replaces a capacitor or clears a blocked coil costs far less than a season of resistance heat.
What to Do After Switching to EM
Once EM Heat is on, limit how long it runs. Set the thermostat to a modest temperature such as 65 degrees and let the backup bring the house to that point, then monitor the system. Follow this sequence to get back to normal heat as quickly as possible:
- Confirm the outdoor unit is not iced over or blocked by snow.
- Switch the thermostat to EM Heat and set it to a modest temperature.
- Let the backup run for an hour, then check the temperature at the registers.
- If the compressor restarts and holds the set point, switch back to normal heat the same day.
- If the house only stays warm because EM is running, schedule the repair and keep the setting only until the technician arrives.
How Emergency Heat Works
Emergency heat on a heat pump system draws from electric resistance elements mounted in the air handler. Air blows across the heated coils and moves through the ductwork. The system delivers heat quickly, but it consumes far more electricity per unit of heat than the heat pump does in normal operation.
Resistance Heating Elements
Resistance elements convert electricity directly into heat, much like a space heater. They have a coefficient of performance near 1.0, meaning one unit of electricity produces about one unit of heat. A heat pump, by comparison, can move two to four units of heat for every unit of electricity it draws. That ratio explains why running EM Heat for days shows up so clearly on the bill.
Control and Wiring
The thermostat controls the changeover between compressor heat and backup heat. A miswired thermostat defeats the staging logic and can run both stages at once, which wastes energy and stresses the equipment. Following the correct programmable thermostat installation steps, including wiring, mounting, and scheduling, keeps the emergency circuit isolated and ready for actual failures.
What the Thermostat Display Tells You
Most thermostats show a small indicator next to the EM or AUX label when backup heat is active. Learn what each light means on your specific model. A flashing EM indicator during normal operation can signal a wiring problem or a failing compressor, so treat persistent indicators as diagnostic clues rather than normal behavior.
Emergency Heat Cost and Efficiency
The cost gap between normal heat and emergency heat comes down to physics. The table below compares typical performance for a residential heat pump system.
| Heating mode | Heat per unit of electricity | Typical use |
|---|---|---|
| Heat pump, normal | 2.0 to 4.0 units | Most of the heating season |
| Auxiliary heat | About 1.0 unit | Defrost cycles and cold snaps |
| Emergency heat | About 1.0 unit | Compressor failure and repairs |
Why the Bill Climbs
Running resistance heat for a full day draws several times the electricity of a heat pump on the same day. A homeowner who leaves EM Heat on for a week can see the heating portion of the bill double or triple. The setting is a safety net, and treating it like a normal mode costs money without adding comfort.
Settings That Avoid the Backup
Most households never need EM Heat if the thermostat is set sensibly and the system is maintained. The best thermostat setting for staying warm balances comfort against energy use, usually 68 degrees while occupied and a 7 to 10 degree setback at night. Large setbacks force the backup stage to run during recovery, so keep changes modest. On two-stage thermostats, the second stage engages automatically when the first stage runs too long without reaching the set point, which is normal staging behavior rather than a malfunction.
Thermostat Settings for Winter and Empty Houses
Seasonal settings protect the system and the pipes without pushing the backup stage into service. Planning for both occupied and vacant periods avoids surprises on the coldest nights.
Setbacks and Schedules
Program the thermostat to hold a steady daytime temperature and a modest nighttime setback. Avoid dropping the set point more than 10 degrees at a time, because the recovery load can trigger auxiliary heat and erase the savings. A consistent schedule also keeps the heat pump cycling less and extends compressor life.
When the House Is Empty
A vacant house still needs enough heat to protect plumbing, but it does not need normal occupied temperatures. The best thermostat temperature for a house left empty in winter sits low enough to save energy and high enough to prevent frozen pipes, typically around 55 degrees. Leave the system in normal heat mode rather than EM, since backup heat wastes money when nobody is there to notice. Open cabinet doors under sinks and keep the heat registers clear so warm air reaches the pipes that run through exterior walls.
EM Heat is a tool for emergencies, and a properly maintained system rarely needs it. When it does run, use it as a bridge to a repair rather than a permanent setting. The thermostats and temperature control library at build-construct.com explains the modes, wiring, and scheduling behind modern HVAC so homeowners can diagnose problems before they call for service.