Ontario homes typically switch the heat on in September, once daytime highs and overnight lows settle into that gray zone where a house without a furnace or boiler running starts to feel damp and chilly. The season runs long, stretching all the way into June before it fully lets go. That’s the short answer. What decides exactly how much heat your house needs, month by month, comes down to a number most homeowners have never heard of.
When the heating season starts in Ontario
The number behind that September start date is 18 degrees Celsius. Environment and Climate Change Canada uses that temperature as the base for counting heating degree days, and at the Toronto Buttonville A station, the 30-year normal mean daily temperature drops below it starting in September (15.7°C), which is why heating demand begins that month. The season doesn’t let go until June, when the mean climbs back above the 18°C mark. That’s about nine months of the year where a house is asking its heating system to do something.
January carries the heaviest load, with 738 heating degree days on the 30-year normal. That’s the peak, and it’s a useful number to know if you’re sizing a system or just trying to understand why your January bill dwarfs your October one. But treat the whole thing as a season, not a switch. A monthly normal is a 30-year average, not a forecast for any single year, and the first real cold snap in a given fall almost always shows up before the calendar says it should.
Here’s the monthly breakdown, in heating degree days below 18°C, from the Canadian Climate Normals 1981-2010 at Toronto Buttonville A:
| Month | Heating degree days |
|---|---|
| January | 738 |
| February | 666 |
| March | 566 |
| April | 337 |
| May | 168 |
| June | 37 |
| July | 6 |
| August | 15 |
| September | 91 |
| October | 284 |
| November | 448 |
| December | 649 |
Notice that even the calendar’s warmest months carry a few degree days. A house in this part of the province is never entirely off the hook, even if the furnace spends July and August mostly resting. Data via Environment and Climate Change Canada, Canadian Climate Normals 1981-2010, station Toronto Buttonville A.
How hard the winter works here
Add up all twelve months in that table and you land at roughly 4,004 heating degree days a year at Toronto Buttonville A. Degree days are a rough stand-in for fuel use: a house that sees twice the degree days of another, similar house, burns roughly twice the fuel to stay comfortable over a season. It’s not an exact conversion, since insulation, windows, and how a family actually lives in a house all shift the number, but it’s close enough to matter when you’re thinking about a winter’s worth of gas, oil, or electricity.
What the annual figure hides is shape. Two places can rack up the same total degree days in very different ways, one with a long, mild slog and another with a short, brutal freeze. Ontario’s monthly table above shows a season that builds steadily from September, spikes hard through the deep-winter months, and tapers off through spring rather than dropping off a cliff. That gradual build matters for anyone deciding when to schedule maintenance or top off a fuel tank, because the real strain doesn’t land all at once.
This number is specific to one station and one 30-year period, so treat it as a regional snapshot rather than a promise for any single winter. It’s also not something to line up against U.S. heating degree day figures you might see elsewhere, since those are counted on a different temperature base and a different reference period, and the math simply doesn’t translate.
What most homes in Ontario actually heat with
Statistics Canada’s most recent survey of primary heating systems shows a province that leans hard on one type of equipment. Here’s the split:
| System | Share of homes |
|---|---|
| Forced-air furnace | 66% |
| Boiler with hot-water or steam radiators | 7% |
| Electric baseboard heaters | 7% |
| Heat pump | 4% |
| Heating stove | 1% |
Two out of three Ontario homes run on a forced-air furnace, and that share reflects what was cheap and standard to install when the housing stock in this province went up, not necessarily what suits any one house best today. A share of the market is not a recommendation, and a furnace that made sense for a builder in 1975 isn’t automatically the right call for a house being renovated now.
What a forced-air furnace is good at: heating a house fast and evenly, and pairing easily with central air conditioning since it already has the ductwork in place. Where it struggles is efficiency loss along the way, since the heat has to travel through a network of ducts before it ever reaches a room, and any gaps or disconnections along that path bleed warmth into the attic, the basement, or the space between walls. The one maintenance item that punishes neglect above all others is the filter. A clogged filter chokes airflow, forces the blower motor to work harder, and can eventually cause the whole system to overheat and shut itself down at the exact moment you need it most.
Run it once before you need it
A furnace or boiler that’s going to fail tends to fail on the first genuinely cold night of the year, which is also the night every heating technician in the region is already booked solid. The fix is simple: test the system in September, while the phone lines are still quiet and a small problem is still a small problem. Work through this in order, since the sequence itself is the advice.
- Turn the thermostat up a few degrees and run the system through one short cycle, just to hear it start, run, and shut off normally.
- Expect a faint burnt-dust smell on that first firing. It’s normal, caused by dust that settled on the heat exchanger over the summer burning off, and it should fade within a few minutes.
- Check the filter and replace it if it’s visibly gray or clogged. This is the cheapest thing on this list and the one most likely to prevent a breakdown.
- Clear furniture, boxes, or curtains away from vents, registers, radiators, or an outdoor heat pump unit so air or heat can move freely.
- Test the carbon monoxide alarm’s button and confirm the battery is fresh. A furnace that isn’t burning cleanly can push carbon monoxide into a house, and a working alarm is the one safeguard that actually gives you warning.
- Listen for anything off: rattling, a burning-electrical smell that doesn’t fade, short-cycling, or a pilot light that won’t stay lit. Any of those is the point to stop and call a licensed technician rather than keep testing it yourself.
Even a furnace in perfect working order can lose a real chunk of its output before that heat ever reaches a room. As the U.S. Department of Energy puts it, “In a typical house, about 20%-30% of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” That’s air escaping through the ductwork itself, not the house’s total heat loss, but it’s a good reason to have ducts checked if a particular room never seems to warm up no matter how hard the system runs.
What turning it down is actually worth
The Department of Energy is specific about what a setback thermostat actually buys you: “You can save as much as 10% a year on heating and cooling by simply turning your thermostat back 7-10°F for 8 hours a day from its normal setting.” That’s a yearly figure tied to a particular setback held for a particular stretch of hours, covering both heating and cooling together, and it doesn’t scale up or down by degree the way some shortcuts online suggest.
In a place where the heating season runs close to nine months, from September’s first chill through June’s last gasp, that 8-hour setback has a lot more hours to work with than it would somewhere the furnace only runs for a few weeks a year. A programmable or smart thermostat set to pull back overnight or during the workday, every day the furnace is running, is one of the few genuinely low-effort ways to chip away at a long Ontario winter’s fuel use without touching the equipment itself.