Quick answer
Use emergency heat only when the heat pump's outdoor unit cannot run: it has failed, iced over or been damaged. EM HEAT switches the compressor off and heats with the backup alone, electric strips or a gas furnace. It is not for ordinary cold weather, when auxiliary heat comes on automatically. Electric strips cost two to three times as much to run, so treat it as a stop-gap until service.
You should use emergency heat on your heat pump in one situation: the outdoor unit is out of action and you need the house warm until it is fixed. The EM HEAT setting on the thermostat is a manual override. It locks the compressor out completely and runs the backup heat alone. Cold weather is never the reason, however low the forecast goes.
AUX and EM HEAT are not the same thing
AUX is automatic and EM HEAT is manual. When the thermostat shows AUX, it has decided the heat pump needs help and has added backup heat for a while, with the compressor normally still running. You do nothing, and it drops out again when the house catches up. EM HEAT is a mode you select: the outdoor unit stays off no matter what, and the backup does all the work until you change it back.
The names vary by thermostat. Honeywell models list Em Heat as a system mode, ecobee calls the equivalent mode Aux, and Nest keeps Emergency Heat inside its settings menu. If AUX is appearing more often than you expect without you selecting anything, that is a different question, answered in why auxiliary heat is always on.
When switching to emergency heat is the right call
- The outdoor unit will not start, or its breaker trips, and the backup has not picked up automatically.
- The unit is encased in solid ice that a defrost cycle has not cleared, or the fan is striking ice.
- Something has damaged it: ice falling from the roof, a branch, a vehicle.
- It is making a metal-on-metal, grinding or banging noise.
- A technician has told you to leave the compressor off until a part arrives.
In any of these, also switch off the outdoor unit's breaker or disconnect if there is noise, a burning smell or visible damage. EM HEAT stops the thermostat from calling the unit; the breaker makes sure nothing else does.
When to leave the thermostat on HEAT
Leave it on HEAT whenever the outdoor unit is running and clearing its own frost, whatever the temperature. A cold-climate heat pump still delivers more heat per kilowatt-hour than strips do at -15 °C, and the thermostat adds aux automatically if the unit falls behind. Switching to EM HEAT during a cold snap does not warm the house faster. It turns off the cheaper heat source and leaves the expensive one running alone.
The same goes for steam rising from the outdoor unit, a pause in airflow, or a whoosh from outside every so often in damp weather. Those are a normal defrost cycle. Our answer on whether to turn a heat pump off in extreme cold covers the cold-weather side in full.
What emergency heat runs on, and what it costs at Ontario rates
What EM HEAT costs depends on the backup your system has. With an all-electric air handler it energizes resistance strips, typically 10 to 20 kW, at a COP of 1. A 15 kW bank running half of every hour through a cold day uses 180 kWh, where a working heat pump would need a half to a third of that for the same heat. On time-of-use rates the weekday on-peak hours hurt most, so a week on strips shows on the next bill.
On a hybrid system EM HEAT runs the gas furnace alone, which is an ordinary way to heat the house for a few days. A ductless mini-split with no backup has no emergency heat at all: the setting does not exist, and a failed unit means space heaters until the repair. If the strip bank in your air handler is small, 5 kW for example, it may not hold the house on the coldest nights without the compressor, which is worth knowing before you need it.
Emergency heat is a stop-gap: what to do next
- Switch the thermostat to EM HEAT and confirm warm air at the registers after a few minutes.
- If no heat arrives, check the air handler or furnace breaker; the backup has its own.
- Shut off the outdoor unit's breaker if the unit is noisy, damaged or smells of burning.
- Book the repair, and tell the technician what you saw and heard before you switched over.
- After the repair, set the thermostat back to HEAT. A system left in EM HEAT for the rest of the winter is a common cause of a shocking hydro bill.
The repair visit starts with a $90–$150 diagnostic that is put toward the fix. Common outdoor-unit failures on a conventional heat pump, a capacitor or a contactor, run $150–$350, and most repairs finish between $150 and $800.
ZK Mechanical answers the emergency line around the clock with no overtime, weekend or holiday surcharge. If your backup heat is holding the house, you have time to book a regular appointment instead. See heat pump repair, the full list of heat pump questions and answers, or reach us here or at (647) 801-1252.