Quick answer
No. Emergency heat (EM HEAT) only exists on heat pump systems, where it shuts off the heat pump and runs the backup heat alone. On a house with only a gas furnace the setting does nothing, and in a hybrid system the backup is the furnace, so it cannot rescue a dead one. Try one reset, check the filter and vents, then call for repair.
Whether you should switch to emergency heat if your furnace stops working depends on one thing: is there a heat pump outside? Emergency heat is a heat pump thermostat mode, not a furnace feature. If the only heating appliance in the house is a gas furnace, no thermostat setting can bring heat from anywhere else, and minutes spent on the thermostat are minutes not spent on the checks that do help.
What EM HEAT and AUX mean on a thermostat
EM HEAT tells the thermostat to lock out the heat pump's compressor and heat the house with the backup source alone. AUX is the same backup switching on automatically, alongside the heat pump or in its place, when the outdoor unit cannot keep up or is defrosting. In an all-electric system the backup is a bank of electric resistance strips in the air handler. In a hybrid (dual-fuel) system the backup is the gas furnace itself.
That last point matters when the furnace is what failed. Selecting emergency heat on a hybrid system sends the call straight to the appliance that is broken. Knowing when emergency heat is the right choice on a heat pump is a separate question with its own answer.
Which system do you have? Three cases
- Gas furnace only, with or without central air: there is no emergency heat. A correctly configured thermostat will not show the option at all.
- Heat pump with electric backup, common on rural lots in Caledon, Kilbride and Flamborough where gas mains do not reach: there is no gas furnace to fail. If the outdoor unit quits, EM HEAT is the right switch, and the strips will carry the house at a higher hydro cost until the heat pump is repaired.
- Hybrid system, a heat pump paired with a gas furnace: emergency heat calls the furnace. If the furnace's gas side has failed but its blower still runs, the opposite move can work. Set the thermostat to heat-pump-only heating, if the installer left that option available, and the outdoor unit can hold the house in milder weather.
In a hybrid system the heat pump relies on the furnace's blower to move air across the indoor coil. A dead control board or blower motor therefore takes both heat sources down together. A failed igniter or flame sensor does not.
A gas-only house that shows AUX or EM HEAT has a thermostat set up wrong
A furnace-only system that displays AUX HEAT, or offers an EM HEAT mode, has a thermostat configured as a heat pump. It usually follows a do-it-yourself smart thermostat install where the equipment type was answered wrong in the setup menu. The thermostat then treats the furnace as backup for a heat pump that does not exist.
The symptoms are specific. Heat arrives late, only after the thermostat gives up on its imaginary heat pump and brings on AUX, or the air conditioner starts outside when you ask for heat. The correction is in the installer settings: equipment type set to conventional furnace, with the heat wire on the W terminal. If you cannot tell which device is at fault, thermostat or furnace explains how to separate them.
What actually helps when a gas furnace is dead
- Read the flash code first. Look through the small sight glass on the lower furnace door and write down the blink pattern before you cut power, because switching off erases it on many boards.
- Reset once. Turn the furnace switch off for 30 seconds and back on; how to reset a furnace has the full sequence. A furnace that locks out again after one reset needs a diagnosis, not a third try.
- Check the filter. A plugged filter overheats the furnace until the limit switch shuts the burners down.
- Check the vents outside. On a high-efficiency furnace the two plastic pipes through the wall must be clear of snow, ice and nests.
- Check the basics: thermostat on Heat with fresh batteries, furnace switch on, breaker not tripped, and the burner door seated so the door switch is pressed.
For temporary heat, use electric space heaters, one to a circuit, plugged straight into a wall receptacle and kept a metre from bedding and curtains. Do not use the oven, a barbecue or a patio propane heater indoors: all three put carbon monoxide into the house. Running a space heater overnight covers the limits.
When a dead furnace is an emergency call
- Call now if the forecast is below freezing overnight and the house is already losing temperature.
- Call now if there is an infant, an older adult or someone unwell in the house.
- Call now, and leave the furnace off, if it quit with a gas smell, a sounding carbon monoxide alarm or scorch marks on the cabinet.
- It can wait until morning if the weather is mild, space heaters are holding the main rooms and nothing smells or sounds wrong.
A diagnostic visit costs $90–$150 and is folded into the repair if you go ahead, and most furnace repairs come to $150–$800. ZK Mechanical repairs gas furnaces, heat pumps and hybrid systems across Burlington, Oakville, Hamilton and the west GTA, and the 24/7 line carries no after-hours surcharge. Our heating services page lists what we work on; call (647) 801-1252 or book a visit online.