Quick answer
A furnace that won't turn on after a power outage usually has a tripped breaker, a thermostat that lost its batteries or settings, a control board locked out by low voltage, or surge damage: a blown 3 A or 5 A board fuse, a failed transformer or a dead board. Switch the furnace off for 30 seconds once. If it stays dead, call for service.
When a furnace won't turn on after a power outage, the cause is nearly always electrical and usually on the inexpensive side. Power does not come back cleanly. It flickers, sags and spikes as the utility re-energizes the line, and a furnace control board is a small computer that dislikes all three. Work from the simple checks toward surge damage before assuming the worst.
Check these first once the power is back
- Confirm the power is fully back. Dim lights, or half the house dead, mean a partial outage, and motors should stay off until your utility (Burlington Hydro, Oakville Hydro, Milton Hydro, Alectra or Hydro One) has fixed it.
- The furnace breaker. A surge, or everything in the house restarting at once, can trip it. Push it fully Off, then On, one time.
- The thermostat. Battery models can come back blank or with the clock and schedule wiped, and some return to Off. Fit fresh batteries, select Heat and raise the setpoint a few degrees above room temperature.
- The GFCI receptacle feeding the condensate pump. If it tripped, the pump reservoir fills and its safety switch stops the furnace. Press Reset on the receptacle and listen for the pump.
- The furnace service switch and the blower door. Someone checking the furnace by flashlight during the outage may have left one off or ajar.
A board locked out by low voltage needs one power cycle
A brown-out, where voltage sags without dropping out, is harder on a furnace than a clean outage. The igniter does not get hot enough, the gas valve chatters, the flame signal drops, and after a few failed attempts the board locks itself out. When full power returns, the board can still be sitting in that lockout with its light flashing.
Turn the furnace service switch off, wait 30 seconds and turn it back on. Then give the furnace a few minutes to run through its start-up sequence. Do this once. A furnace that locks out again on good power has a real fault, and the flash code on the board says which one.
Surge damage: board fuse, transformer and control board
If the furnace is silent while the thermostat is calling, look through the sight glass in the lower door. No light at all on the control board, with the breaker and switch both on, points to surge damage.
- Blown board fuse. A small automotive-style blade fuse, 3 A or 5 A, protects the 24-volt circuit. A surge can take it out and leave everything else intact. A fuse that blows a second time has a short behind it, covered in why a furnace keeps blowing its control-board fuse.
- Failed transformer. It steps 120 volts down to 24 for the thermostat and the controls. When its winding burns open, the furnace has line power and no control power, so nothing responds.
- Failed control board. Scorched traces or a dead relay stop the sequence before it begins. The electronic module on a variable-speed blower motor is also surge-sensitive and can fail in the same event.
All three are tested with a meter in a few minutes. The fuse sits inside the cabinet beside live 120-volt terminals, so leave the door on and have it checked.
After an ice storm: clear the vents and check the condensate drain
A furnace that sat idle through a long winter outage can have two extra problems, and neither is electrical. High-efficiency furnaces breathe through two plastic pipes on a side wall, and freezing rain can glaze the ends shut. Clear ice and snow from both terminations by hand before restarting, and never run the furnace with a blocked exhaust; what to do when a furnace vent is blocked by snow has the details.
The second is water. Condensate left in the trap and drain line can freeze once the house drops toward 0 °C, especially where the line crosses a garage, a crawlspace or an outside wall. The furnace will try to start, fail on its pressure switch and lock out. The thaw procedure is in what to do if a furnace condensate line is frozen.
When to call, and what a post-outage repair costs
- Call now if the breaker trips a second time, if you smell burning at the furnace, or if the indoor temperature is falling toward 10 °C with no heat.
- Call the same day if the board shows no light, or the lockout returns after one power cycle.
- Diagnostic visit: $90–$150, counted toward the repair if you proceed.
- Most furnace repairs, including a fuse, a transformer or a wiring fault: $150–$800.
- Control board: $500–$1,200.
On rural lots in Caledon and Flamborough where the furnace burns propane or oil, the electrical checks on this page are the same; only the fuel side differs. ZK Mechanical runs a 24/7 emergency line with no overtime, weekend or holiday surcharges, which matters after a storm when half the street has the same problem. For furnace repair in Burlington, Oakville, Milton, Hamilton or the west GTA, call (647) 801-1252 or book online. The heating answers hub also covers running a furnace during an outage and on a generator.