Quick answer
First check the simple causes: thermostat batteries and settings, the furnace power switch, the breaker, the filter, and snow over the exterior vent pipes. If the furnace still won't run, call a 24/7 emergency HVAC service — on a -15°C night, supply pipes in outside walls can freeze before morning. While waiting, keep taps trickling, close off unused rooms, and use electric space heaters safely.
When your heat stops working at night in Toronto, the order is: quick checks, then the phone, then protect the plumbing. A furnace that quits in a January cold snap is a genuine emergency, but plenty of overnight calls end with a technician flipping a switch the homeowner could have flipped. Run the list below first — it takes five minutes and costs nothing.
The 5-minute checklist before calling
- Thermostat — fresh batteries, set to HEAT, set point above room temperature. Smart thermostats: check it hasn't lost power or Wi-Fi schedules haven't changed it.
- Furnace switch — the light-switch-style toggle on or near the furnace is easy to knock off. Confirm it's ON.
- Breaker panel — reset a tripped furnace breaker once. If it trips again, leave it off and call.
- Filter — a fully clogged filter can overheat the furnace and lock it out on its high-limit switch. Replace it and let the unit cool 30 minutes.
- Vents — confirm the exterior intake/exhaust pipes (white PVC, usually on a side wall) aren't blocked by snow or ice. This is a top cause of GTA shutdowns during storms.
- Gas — if other gas appliances are also dead, the issue may be supply; contact Enbridge. And if you smell gas at any point, stop troubleshooting, get everyone outside, and call Enbridge's emergency line at 1-866-763-5427 or 911 from outside.
Read the furnace's own diagnosis before you call
Take the lower panel off the furnace and watch one full start attempt. On most furnaces built since the late 1990s, a small LED on the control board flashes a fault code when the unit locks out, and the legend is printed inside the door — count the flashes and write them down. Then listen to the sequence: the inducer motor hums first, the hot surface igniter glows orange, and the burners light with a soft whump. Where it stops points at the part — a glow with no flame suggests the gas valve or flame sensor; no inducer at all suggests the pressure switch circuit or the motor. What a flashing furnace light means lists the common codes, and telling the dispatcher the pattern gets the right part on the truck.
If it still won't run: call, don't wait until morning
Below -10°C, the interior of a Toronto semi or bungalow cools steadily once the burner stops, and the plumbing in exterior walls and the rim joist is already colder than the hallway thermostat suggests — a burst supply line turns a $400 repair into a water-damage claim. A genuine 24/7 emergency HVAC service dispatches overnight rather than at 8 a.m.; ZK Mechanical answers around the clock at (647) 801-1252 with no overtime surcharges. No honest contractor promises an arrival time on the night half the city's furnaces quit, but calling right away puts you ahead of everyone who waited.
Protect the house while you wait
- Open taps to a pencil-thin trickle, especially on exterior walls — moving water resists freezing
- Open cabinet doors under sinks to let room heat reach the plumbing
- Close doors to unused rooms and gather the household in one or two rooms
- Use electric space heaters on the floor, a metre from anything flammable — never an oven, BBQ, or patio heater indoors (carbon monoxide risk)
- Let sunlight in during the day; close curtains at night for insulation
- If the outage will run past a day, the full plumbing routine is in how to keep pipes from freezing when the furnace fails
What most overnight no-heat calls turn out to be
- Flame sensor coated in oxide — burners light for a few seconds, then drop out. Cleaning or replacement runs $150–$300.
- Hot surface igniter cracked — no orange glow, no flame. $200–$450.
- Pressure switch or capacitor — the inducer runs and nothing follows, or a motor hums and stalls. $250–$500.
- Draft inducer motor seized — the thermostat calls and nothing happens at all. $400–$900.
- Blocked condensate trap on a high-efficiency furnace — water backs up and the pressure switch opens. $150–$350 to clear or replace.
- Control board — random lockouts or a dead LED. $500–$1,200.
Add a diagnostic of $80–$150, which ZK credits toward the repair — full pricing is in our guide to furnace repair costs in Toronto. A terminal diagnosis, such as a cracked heat exchanger on a 15-year-old unit, means the furnace stays off; the contractor's job then is safe temporary heat and a replacement scheduled as a priority, not a workaround.
Questions we hear on overnight no-heat calls
- Should I keep hitting reset? Once. Repeated resets on a furnace that locks out each time can leave unburned gas in the burner box — after the second failure, leave it off and call.
- Can I use the gas oven for heat? No. An open oven in a closed house produces carbon monoxide, and it is the shortcut behind many winter CO calls.
- Is it safe to leave the furnace switched on while I wait? Yes, if there is no gas smell and no burning odour. The board keeps retrying on its own schedule, and a furnace sometimes recovers once a vent clears itself.
- The furnace runs but blows cold air — same emergency? Usually less urgent: a limit switch, a clogged filter or a stuck gas valve. A running blower means power and control are intact.
- Will insurance cover frozen pipes? Often, if the house was heated and occupied — leaving a home unheated is the standard exclusion, so read the policy before travelling in winter.
Prevention beats 2 a.m. phone calls: nearly every winter breakdown shows warning signs at a fall tune-up, and maintenance-plan members get priority dispatch. When the house is already cooling, ZK Mechanical runs 24/7 emergency heating service in Toronto and across the GTA and Hamilton — call (647) 801-1252 or reach us online.