Furnace will not start? Work through this before you call

A furnace that will not start is the most common call we take between November and March, and a fair share of them end on the phone, with the homeowner finding the cause before a truck is sent. Go through this list in order. Nothing on it needs tools or a panel removed. If you smell gas at any point, stop, leave the house, and call Enbridge Gas first.

Two minutes upstairs

• The thermostat. Set it to Heat, not Cool or Off, and push the setpoint three or four degrees above the room. Wait a full minute; most thermostats hold a delay before they call. If the display is blank or faint, change the batteries. A thermostat with flat batteries is indistinguishable from a dead furnace from the living room.
• The fan setting. If it says On rather than Auto, the blower runs continuously and you may be feeling room-temperature air from the vents while the burner waits for a call it never gets. Set it to Auto.
• The breaker panel. Find the breaker labelled furnace. If it sits between On and Off, push it fully off and then on. If it trips again at once, stop; that is an electrical fault, not something a reset fixes.

Five minutes in the basement

• The furnace switch. There is a light switch on the furnace cabinet or on the wall next to it. In a basement used for storage it gets knocked off, and it is the first thing we check when we arrive. Make sure it is up.
• The filter. Slide it out. If it is grey, packed or bowed, replace it. A loaded filter starves the heat exchanger of air; the exchanger overheats, the high-limit switch cuts the burner, and the furnace waits for things to cool before it tries again. Give it ten minutes after a new filter.
• The condensate drain. A high-efficiency furnace makes water every hour it runs, and it leaves through a trap and a small plastic line to a floor drain or a pump. If the pump reservoir is full, the pump has tripped or failed; reset it if it has a button. If there is water at the base of the furnace, the trap or line is blocked. A blocked drain keeps the pressure switch from closing and the furnace will not light.
• The reset. Flip the furnace switch off, count to thirty, flip it on. The control board clears its lockout and tries once more. Watch what it does, because the point where the sequence stops is the diagnosis.

Outside: the Ottawa step

Go out and find the two plastic pipes on the exterior wall. One is the intake, one the exhaust. After a snowfall, especially a wind-driven one, a drift can bury them on the sheltered side of the house, and the exhaust can grow an ice cap from its own plume on a night at minus 25. Either one blocks the airflow the pressure switch is waiting for, and the furnace locks out. Clear them by hand. On a long cold spell the condensate line can freeze where it runs near the pipes or along an outside wall, with the same result; a hair dryer on the exposed section thaws it, and then it needs rerouting or insulating so it does not happen again.

This step does not apply to an older mid-efficiency furnace that vents up the chimney. Those stop for other reasons.

Reading the start-up sequence

When the thermostat calls, a gas furnace runs a fixed order: the inducer fan starts and runs for a few seconds to prove the vent is clear; the igniter glows orange; the gas valve opens and the burners light; the flame sensor confirms the flame; and about a minute later the blower starts. Where it stops tells you what is wrong.

• Nothing at all: thermostat, switch, breaker, a blown low-voltage fuse on the board, or a failed transformer or board.
• Inducer runs and nothing follows: the pressure switch is not closing. Blocked intake or exhaust, a frozen or blocked condensate line, a failed inducer, or a failed switch.
• Igniter glows, burners do not light: the gas valve or the gas supply. Check that the valve on the gas line to the furnace is open, and on a propane home that the tank is not empty.
• Burners light and go out after two to five seconds, three times, then lockout: the flame sensor. Fouled with oxide, it cannot prove the flame and the board shuts the gas. This is the most common no-heat fault of all, and cleaning the rod is a ten-minute job.
• Clicking and no glow: a cracked igniter.
• Burners run a few minutes and shut off, again and again: the high-limit switch is tripping. Filter, blower, closed registers or, in the worst case, a cracked heat exchanger.

Most furnaces have a small LED on the control board, visible through a window in the door, that flashes a code when the furnace locks out. Count the flashes and the pauses and tell us when you call; the code is printed on the inside of the door.

Stop and call

• Any smell of gas: out of the house, then Enbridge, then us.
• A carbon monoxide alarm sounding: everyone outside first.
• A breaker that trips again as soon as it is reset.
• Scorching, a persistent burning smell, or a bang on ignition.
• Anything beyond this list. Safety controls exist for a reason, and jumpering one to get heat is how combustion gas ends up in a bedroom.

A furnace repair visit in the Ottawa area is typically $109 to $159 to diagnose, credited against the repair, and the common parts are carried on the truck so most no-heat calls are finished on the first visit. Every technician who touches the gas side is TSSA-certified. If the furnace does start but the air stays cool, the causes are in cold air from the vents. The phone is answered every day of the year, and a furnace that gets a tune-up each fall is the one that does not appear on this page.