Most furnace calls come down to a short list of faults. Some you can check yourself. Others need a gas technician. This guide covers both, and what to look for in the kind of Caledonia house you live in.
The most common problem is a blocked filter
A dirty filter is behind more no-heat and poor-heat complaints than anything else. The furnace cannot move enough air across the heat exchanger. The heat exchanger runs hot, and a limit switch shuts the burners off to protect it. The blower may keep running and push cool air. Then the furnace cools down, relights and trips again. That on-off pattern is called short cycling.
Pull the filter and hold it up to a light. If you cannot see light through it, replace it. Match the size printed on the frame, and point the airflow arrow toward the furnace. A filter that is too restrictive for the system can cause the same trouble as a dirty one, so a thicker, denser filter is not always better.
Pets, renovation dust and a wood stove all load a filter faster. Check it monthly through the heating season.
The most common failure is in the ignition
When a part actually fails, it is usually somewhere in the start-up sequence. A modern furnace works through steps in order. The thermostat calls for heat, the inducer motor starts, a pressure switch confirms the venting is clear, the igniter heats up, the gas valve opens and a flame sensor confirms there is flame. A fault at any step stops the furnace.
- Hot-surface igniter: a brittle element that cracks with age. The furnace tries to start, nothing glows, and it locks out.
- Flame sensor: a thin metal rod that gets coated over time. The burners light, then shut off a few seconds later.
- Pressure switch: it will not close if the vent is blocked, the condensate has backed up or the inducer is weak. It is often the messenger, not the cause.
- Inducer motor: a hum, rattle or squeal before the burners light is the usual warning.
- Control board: it reads all of the above and flashes a code when something is wrong.
Most of these are smaller repairs. A cracked heat exchanger is different. It can let combustion gases, including carbon monoxide, into the air the furnace moves through the house, and it is the failure where the repair-or-replace question usually comes up.
What goes wrong depends on the house
Caledonia has three broad kinds of housing, and each tends to have its own heating setup. If you are not sure which you have, the guide to the three common heating and cooling systems walks through them.
The oldest houses around Edinburgh Square and Caithness Street were built for wood or coal and converted later. Some now have a boiler and radiators. Others have forced air added after the fact, with cramped ductwork squeezed in where it would fit. Common complaints are worn boilers, weak airflow to the far rooms and an old chimney that no longer suits the equipment. Old wrap on pipes or ducts in these basements may contain asbestos. Leave it alone and mention it when you call.
North of the Grand River, the mid-century houses around Shetland Street were usually built with standard-efficiency gas furnaces vented into a chimney. Many of those units are tired now: worn inducers, cracked igniters, rust in the cabinet. Where the furnace has already been changed to a high-efficiency model that vents through the side wall, the gas water heater can be left alone in a chimney sized for both. It may not draft properly, and condensation can form inside. Rust flakes or water at the base of the chimney are signs to watch for.
The newer builds in Avalon have condensing furnaces with two plastic pipes through the side wall, inside a tightly sealed house with mechanical ventilation. The usual faults are a blocked or frozen condensate drain, snow or frost over the intake and exhaust pipes, and ventilator filters nobody has cleaned. Gurgling in the furnace, water on the floor beside it, or a flashing code pointing at the pressure switch all suggest a drain or venting problem.
What to check before you call
These checks are safe for a homeowner and often find the problem.
- Thermostat: set to heat, with the setpoint a few degrees above room temperature. If the screen is blank, change the batteries.
- Power: find the furnace switch, which looks like a light switch on the wall or a joist near the furnace, and make sure it is on. Check the breaker too.
- Furnace door: the blower door must be fully seated. A safety switch behind it keeps the furnace off if the door is loose.
- Filter: check it as described above.
- Outside pipes: on a high-efficiency furnace, clear snow and frost from both pipes by hand. Do not chip at ice hard enough to crack the plastic.
- Condensate: look at the drain line and trap for water backing up. If there is a small condensate pump, check that it is not full or stalled.
- Gas supply: see whether the stove or water heater is working. The gas valve handle at the furnace should sit in line with the pipe. On propane, check the tank gauge.
- Error code: count the flashes on the control board light through the small window, and write the number down.
- Registers and returns: make sure supply registers are open and return grilles are not blocked by furniture or boxes.
If everything looks right, turn the furnace switch off, wait a moment, and turn it back on. That is one reset. If the furnace locks out again, stop there. Repeated resets do not fix the cause and can put extra wear on parts.
When to skip the checklist
Some symptoms are not a homeowner job at all.
- A smell of gas: do not touch light switches or the thermostat. Leave the house and call your gas utility's emergency line from outside.
- A carbon monoxide alarm sounding: get everyone out and call 911 from outside.
- A burning smell that goes beyond the dusty odour of the first run of the season, scorch marks or soot around the furnace.
- A loud bang when the burners light, or a flame that looks yellow and lazy instead of blue and steady.
- Water pooling near the wiring or the control board.
The Ontario Fire Code requires a working carbon monoxide alarm next to each sleeping area of a home with a fuel-burning appliance, a fireplace or an attached garage. Test yours when you change the furnace filter.
What to tell the technician
A clear description shortens the diagnosis. Say what the furnace is doing, when it started and what you have already checked. Give the error code if there is one, and take a photo of the rating label inside the furnace door. Mention the kind of house and whether it runs on natural gas or propane.
The gas valve, burners, heat exchanger and venting are not homeowner work. Work is performed by TSSA-certified gas technicians, and you get a written price before any part is replaced. If the furnace is older and the fault is a big one, ask for both numbers, repair and replacement, and decide from there.
Many of the faults above, from clogged condensate lines to coated flame sensors, are caught early by a seasonal furnace tune-up.
If the checks above do not bring the heat back, book a visit through furnace repair.