What Is Emergency Heat on Your Thermostat?

Emergency heat, usually labeled EM HEAT or EMER on the thermostat, is a manual setting that shuts down your heat pump's outdoor unit and heats your home entirely with the backup heat source, most often electric resistance strips inside the air handler. It exists for one situation: the heat pump itself has failed, and you need to keep the house warm while you arrange a repair. Flipping the switch is safe and simple, but the mode is meant to be a bridge to a fix, not a way to heat your home all winter.

The setting confuses homeowners because thermostats also show an auxiliary heat indicator, usually AUX, that looks similar but means something different. Auxiliary heat is backup heat turning on automatically when the heat pump needs help during cold weather or defrost. Emergency heat tells many systems to stop relying on the outdoor heat-pump stage and use backup heat instead. Electric resistance heat makes heat directly while a heat pump moves heat, so extended emergency-heat operation can use substantially more electricity. The actual cost difference depends on outdoor temperature, equipment performance, thermostat controls, electric rates, and whether the backup source is electric or fuel-fired.

Emergency Heat vs. Auxiliary Heat: Not the Same Thing

Auxiliary heat is automatic. A heat pump warms your home by pulling heat out of the outdoor air, and as the temperature falls there is less heat to pull. Somewhere around 25 to 40 degrees, a threshold called the balance point that depends on your equipment and how well the home holds heat, the heat pump alone can no longer keep up, and the thermostat quietly energizes the backup strips to help. Both run together: the heat pump keeps contributing whatever it can, and the strips make up the difference. You will also see the AUX indicator during defrost cycles, when the outdoor unit briefly runs in reverse to melt frost off its coil, and after a big thermostat adjustment, since most thermostats call for aux whenever the room is more than about two degrees below the setpoint. Short bursts of auxiliary heat in cold weather are normal behavior, not a malfunction.

Emergency heat is manual. When you switch the thermostat to EM HEAT, the outdoor unit is locked out entirely and the backup source carries the whole heating load by itself. In most all-electric systems that means the resistance strips; in a dual-fuel system it means a gas or oil furnace, which changes the cost picture considerably. The mode exists so a home with a dead or damaged heat pump still has heat, and so a failing compressor is not forced to keep running and turn a repairable problem into a bigger one. What emergency heat is not: a boost mode for very cold nights. A healthy heat pump plus automatic aux heat will keep the house warm through freezing weather on its own, and running EM heat when nothing is wrong simply replaces inexpensive heat with expensive heat.

When to Switch to Emergency Heat, and What to Do Next

Why Emergency Heat Can Cost More to Run

Electric resistance strips make heat directly, while a working heat pump moves heat from outdoors. U. S. Department of Energy guidance says modern heat pumps can reduce electricity use for heating by as much as 75 percent compared with electric resistance heating under applicable conditions. That does not produce one universal bill multiplier: the difference depends on outdoor temperature, heat-pump capacity and efficiency, strip size, run time, thermostat controls, electric rates, and the home's heat loss. A dual-fuel system may use a gas or oil furnace rather than electric strips, which changes the comparison again. Use your utility rate and the equipment input listed on the nameplate or service documentation for a home-specific estimate.

If Aux Heat Runs Constantly, Your Heat Pump Is Asking for Help

• Low refrigerant charge from a leak: the heat pump runs but moves too little heat, so the strips quietly cover the shortfall. Finding and fixing the leak requires a technician certified under EPA Section 608.
• A failing compressor or reversing valve: heating output drops, or the system gets stuck behaving like it is in cooling mode, and aux picks up the slack.
• Airflow problems: a clogged filter or dirty coils choke the system's capacity. Check the filter first; it is the one cause you can fix yourself in five minutes.
• Defrost faults: if the defrost control fails, the outdoor coil ices over and loses capacity until the strips are carrying most of the load.
• Large thermostat setbacks: a quick setpoint increase can call for auxiliary heat on some controls. Check the thermostat and equipment instructions; adaptive-recovery settings may reduce unnecessary backup-heat use without sacrificing comfort.
• An aging or undersized system: if aux heat has crept on a little more each winter, the heat pump may simply be losing capacity with age.

Short periods of auxiliary heat can be normal during recovery from a setback, defrost, or weather below the system's balance point. Long or unexplained operation in milder weather, an indicator that rarely turns off, or a sharp increase in electricity use deserves attention. Start with safe checks: inspect the filter, keep supply and return vents open, and avoid a large thermostat setback. If auxiliary heat continues, a technician can test controls, heat-pump output, defrost, airflow, refrigerant circuit, and backup stages. For a major failure on older equipment, compare documented repair and replacement scopes before deciding.