When the furnace stops, the worry is rarely the cold alone. It is the uncertainty: is this a small part, or the start of something large? Most of the time it is a small part. A few repairs are genuinely big, and it helps to know which ones they are before anyone opens the cabinet.
This guide sorts the common faults into small and large, explains roughly how long each kind takes to put right, and says what affects the cost, without figures, because the real number depends on your furnace and the part.
What are the small, common repairs?
The faults that stop most furnaces are inexpensive parts that wear out with use. In plain terms:
- A hot-surface igniter, the small element that glows to light the gas. It is fragile and wears out over time.
- A flame sensor, a thin rod that confirms the burner has lit. When it gets coated, the furnace lights and then shuts off a moment later.
- A blocked filter, which makes the furnace overheat and shut itself down.
- A cracked or disconnected hose to the pressure switch, or a blocked condensate line on a high-efficiency furnace.
- A thermostat that has lost its settings or needs new batteries.
These are usually the cheaper end of furnace repair. If you want to know what you can safely check yourself before calling, our guide on why a furnace won't turn on and whether it is safe to reset it goes through it step by step.
Which furnace repairs are the big ones?
A handful of parts make up most of the larger repair bills. They cost more because the part itself is expensive, because reaching it means taking much of the furnace apart, or both.
- **The heat exchanger.** This is usually the most expensive part to fix on a furnace. It is the metal chamber that keeps the burning gas separate from the air blown through your house. Replacing it means stripping the furnace down to its frame, so the labour is heavy even when the part itself is covered by the manufacturer.
- **The control board.** The furnace's electronic brain. It is a costly part, and a failing board can show up as odd, intermittent behaviour before it stops altogether.
- **The inducer motor.** The small fan that draws combustion gases out through the vent before the burner lights. A worn one often hums, rattles or squeals for a while before it fails.
- **The blower motor.** The large fan that moves warm air through the ducts. Variable-speed blower motors in particular are a significant part.
- **The gas valve.** Less common to fail, but a careful and moderately costly repair.
On a heat exchanger, the cost question often turns into a different one: whether repairing a furnace of that age makes sense at all. A heat exchanger replacement on an older furnace can approach a meaningful share of what a new furnace would cost, so it is worth seeing both options side by side before deciding. Our furnace installation and replacement page covers what a replacement involves.
Why does a cracked heat exchanger matter so much?
A crack or hole in the heat exchanger can let combustion gases, including carbon monoxide, mix with the air your furnace circulates. That is why a technician will not leave a furnace running once a cracked heat exchanger is confirmed. The Ontario Fire Code requires a working carbon monoxide alarm next to each sleeping area in a home with a fuel-burning appliance, a fireplace or an attached garage, and it is worth testing yours whenever you are thinking about the furnace. If an alarm sounds, leave the house and call for help from outside.
How long does it take to fix a furnace?
There is no single answer, but the pattern is fairly predictable.
- **Small parts** such as an igniter, flame sensor, pressure switch hose or condensate line are usually diagnosed and fixed in one visit, as long as the part is on hand.
- **Motors and control boards** are often fixed in one visit too, but if the exact part for your model has to be ordered, the repair becomes two visits: one to diagnose, one to fit the part.
- **A heat exchanger** is the longest repair. The part almost always has to be ordered for that specific furnace, and fitting it is a substantial job. In the meantime the furnace stays off, so it is sensible to think about space heaters and keeping pipes warm while you wait.
The diagnosis itself often takes longer than people expect. A good technician confirms the fault rather than guessing, because replacing the wrong part costs you money and time.
What usually fails depends on when your house was built
Stoney Creek's housing came in distinct waves, and each one tends to bring its own repairs.
- **Olde Towne, King Street and Lake Avenue.** Houses from the 1800s and early 1900s were first heated by wood or coal and converted later. Technicians here often find ageing boilers, odd or undersized ducts, and unlined chimneys left with no appliance after an upgrade. Repairs on these systems can take longer simply because nothing is standard.
- **Felker, East Felker and Heritage Green.** The mountain subdivisions built from the 1990s onward mostly have forced-air gas furnaces with central air, and many original units are now near the end of their life. Worn inducer motors, failing control boards and heat-exchanger wear are the big repairs most often found in these homes.
- **Fruitland and Winona.** Newer houses from the 2010s onward usually have high-efficiency condensing furnaces with plastic venting. The common problems here are smaller: frozen or blocked condensate drains and wall vents covered by snow. A furnace this young is also more likely to have manufacturer parts coverage, so keep the paperwork handy.
How can you lower the odds of a big repair?
You cannot stop a part from ageing, but you can stop it from ageing faster than it should.
- Change or clean the filter regularly. An overheating furnace wears its heat exchanger and blower harder.
- Keep the outside vents clear of snow and ice on high-efficiency furnaces.
- Listen for new noises. A hum, rattle or squeal from a motor is often a warning that comes well before a breakdown.
- Have the furnace maintained. A tune-up is where worn inducer motors and early heat-exchanger wear tend to be caught. Our guide on what furnace maintenance involves explains what is checked.
- Check your manufacturer's warranty terms. Many require regular professional maintenance to stay valid, and some parts, often including the heat exchanger, carry longer coverage than the rest of the furnace.
What is safe to do yourself, and what is not?
Checking the filter, the thermostat, the furnace power switch and the outside vents is safe for any homeowner. Opening the burner compartment, handling the gas valve or testing electrical parts is not. In Ontario, gas appliances must be serviced by a registered contractor using qualified gas technicians, and installed to the CSA B149.1 Natural Gas and Propane Installation Code under Ontario Regulation 212/01. Work is performed by TSSA-certified gas technicians. If your home is on a rural property and heats with propane or oil rather than natural gas, mention that when you book, since the parts and checks differ.
Whatever the fault turns out to be, ask for the price in writing before any part goes in, and if it is one of the big repairs, ask what a replacement would involve as well, so the decision is yours and not rushed.
If your furnace has stopped or is making a new noise, our furnace repair page explains how a visit works.