The house is getting colder, the thermostat is asking for heat and nothing is happening in the basement. It is an unsettling feeling, especially at night, but most furnaces that will not start have a short list of ordinary causes. Some you can check yourself in a few minutes. Others belong to a gas technician. This guide sorts one from the other.
Is it safe to check anything yourself?
Yes, as long as you stay on the outside of the furnace. Thermostats, switches, breakers, filters and outdoor vent pipes are all fine for a homeowner to look at. Anything behind the burner panel, the gas valve, the wiring and the safety switches is not.
One rule comes before everything else. If you smell gas, do not touch switches or try to restart anything. Leave the house and call your gas utility from outside.
It is also a good moment to check your carbon monoxide alarms. 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. If an alarm is sounding, get everyone outside and call for help before thinking about the furnace.
Why is my furnace not kicking on for heat?
Start with the simple things, in this order:
- The thermostat. Make sure it is set to heat, the set temperature is above the room temperature and no schedule or hold is overriding it. If it runs on batteries, a blank or dim screen usually means they need replacing.
- The furnace power switch. It usually looks like a light switch on the wall or ceiling near the furnace, and it is easy to flip off by mistake when someone is looking for a basement light.
- The breaker. Check the panel for a tripped breaker marked furnace. Reset it once. If it trips again, leave it off and call.
- The front panel. Most furnaces have a small door switch that cuts power if the lower panel is not seated properly. If someone changed the filter recently, make sure the panel is pushed fully home.
- The filter. A badly clogged filter can make the furnace overheat and shut itself down. Replacing a dirty filter is always safe.
If all of that looks right and the furnace still sits silent, or it starts, clicks and stops, the cause is more likely inside the unit.
Does the age and type of house change the likely cause?
Often, yes. Technicians tend to find different problems in different parts of Stoney Creek.
In the newer houses in Fruitland and Winona, the furnace is usually a high-efficiency condensing unit that vents through two plastic pipes in an outside wall. After a snowfall, a drift over those pipes is one of the most common reasons the furnace shuts itself off. Clearing the snow by hand is safe. These furnaces also make water, and a blocked or frozen condensate drain will stop them from running. If you see water pooling or a drain line that looks iced, that is worth mentioning when you call.
On the mountain, in the Felker, East Felker and Heritage Green subdivisions, many houses still have the forced-air gas furnace they were built with in the 1990s or 2000s. Those original units are reaching an age where inducer motors wear, control boards fail and pressure switches stop closing. You may hear the small inducer fan start, then nothing, or see a flashing light code through the little window on the front panel. Write down the pattern of flashes before you do anything else, because it helps a technician narrow things down.
In the older houses of Olde Towne, along King Street and Lake Avenue, the heat may come from a boiler and radiators rather than a furnace and ducts, often added long after the house was built. The thermostat, power switch and breaker checks still apply, but the internals are different, and an ageing boiler is best left to a technician.
If your property is rural, especially above the escarpment, it may not be on a natural gas main and may run on propane or oil. An empty tank is a simple explanation that is easy to overlook, so check the gauge, and tell whoever you call which fuel you have.
What is the most common furnace failure?
Once the outside checks are done, a few parts account for most no-heat calls:
- A dirty flame sensor. The furnace lights, runs for a few seconds, then shuts off because it cannot confirm the flame. This is very common and usually a modest repair.
- A failed hot-surface igniter. The igniter is a small element that glows to light the gas. They are fragile and wear out with use.
- A pressure switch or inducer motor problem. The furnace checks that the exhaust is moving before it lights. A blocked vent, a blocked condensate drain or a tired inducer motor will stop it at this step.
- An overheating limit switch. Usually a symptom of poor airflow from a clogged filter, closed vents or a struggling blower.
- A control board fault. Less common, but more likely in the original units now aging out on the mountain.
The first three are the ones technicians see most. A larger repair, such as a control board or a blower motor, costs noticeably more, and on an older furnace it is fair to weigh that against the age of the unit.
How do I reset a furnace?
Most modern furnaces do not have a single reset button. When they fail to light several times in a row, they go into a lockout and wait. To clear it:
- Turn the thermostat down so it is not calling for heat.
- Turn the furnace power switch off and leave it off for a minute or so.
- Turn the power back on, then raise the thermostat.
Some older furnaces have a red reset button on the blower motor or a limit switch. Pressing it once is reasonable. Pressing it again and again is not.
When is resetting not safe?
A reset clears the lockout. It does not fix whatever caused it. If the furnace locks out again after one reset, stop there. Repeated resets can push unburned gas into the burner area or keep running a furnace with an exhaust or heat exchanger problem.
Leave it off and call if:
- It locks out again after a single reset.
- You smell gas, burning or something electrical.
- You see scorch marks, soot or water inside or around the cabinet.
- A breaker trips more than once.
- Your carbon monoxide alarm has sounded at any point.
Do not remove the burner panel, jumper a safety switch or adjust the gas valve. Those switches are there to protect the house.
What happens when a technician comes?
In Ontario, gas appliances must be serviced and repaired by a registered contractor using qualified gas technicians, and gas work is done to the CSA B149.1 Natural Gas and Propane Installation Code. Work is performed by TSSA-certified gas technicians. Tell them what you checked, what the thermostat showed and any light codes you saw. That saves time and points straight at the likely part. You should see the price in writing before anything is replaced.
If the checks above have not brought the heat back, our furnace repair page explains how a repair visit works.