Furnace not turning on? A ten-minute checklist

A furnace that will not start is the most common no-heat call we take, and perhaps one in four of them is fixed on the phone or by the homeowner before a technician leaves. Work through this list in order. Each step takes a minute. If you smell gas at any point, stop, leave the house and call your gas utility first.

1. The thermostat

Set it to Heat, not Cool or Off, and raise the setpoint a few degrees above the room temperature. Wait a minute; many thermostats have a built-in delay. If the screen is blank or faint, replace the batteries. A dead thermostat looks exactly like a dead furnace. If the fan is set to On rather than Auto, the blower runs but the burner may not, and you feel cold air from the vents.

2. The furnace switch and the breaker

There is a switch that looks like a light switch on or beside the furnace. It gets knocked off more often than anyone admits, especially in a basement with storage near the furnace. Check it is on. Then check the breaker panel for a tripped breaker on the furnace circuit; flip it fully off, then on. If it trips again immediately, stop and call: that is an electrical fault, not a reset.

3. The filter

Pull the filter out. If it is grey, dense or sagging, replace it and give the furnace ten minutes. A blocked filter overheats the heat exchanger, trips the high-limit switch and shuts the burner off; on many furnaces the blower keeps running, which is why the vents blow cold. A furnace that has tripped on limit a few times will restart once it cools and has airflow.

4. The vent pipes outside

On a high-efficiency furnace, find the two white PVC pipes on the outside wall. After a snowfall, a drift or an ice cap over the intake or exhaust shuts the furnace down through the pressure switch. Clear them by hand. Look for a bird nest or leaves in the fall. On an older mid-efficiency furnace that vents up the chimney, this step does not apply.

5. The condensate drain

A high-efficiency furnace makes water, which runs through a trap and a drain line to a floor drain or a pump. If the line is blocked or frozen, the pressure switch will not close and the furnace will not light. Look for water pooling at the base of the furnace or a condensate pump with a full reservoir. If the pump has a reset or has tripped its outlet, reset it. If the line is frozen, a hair dryer on the exposed section will usually free it; then call to have it re-routed, because it will freeze again.

6. The reset

Turn the furnace switch off, count thirty seconds, turn it back on. The control board clears its lockout and tries again. Watch what happens: the inducer fan should start, then the igniter glows, then the burner lights, then the blower starts a minute later. Where the sequence stops tells the technician a lot.

What the symptom means

• Nothing happens at all. Thermostat, switch, breaker or a failed control board or transformer.
• The inducer fan runs but nothing else. Pressure switch not closing: blocked vent, blocked condensate, or a failed switch.
• The igniter glows but the burner does not light, or lights and goes out in a few seconds. Dirty or failed flame sensor, a failed igniter, or a gas valve problem. The flame sensor is the single most common cause of a furnace that lights and then shuts off.
• Clicking without ignition. Often the igniter has cracked.
• The burner lights, runs for a few minutes and shuts down, over and over. Overheating on the high-limit switch: filter, blower or a cracked heat exchanger.
• Blower runs constantly with cold air. Thermostat fan setting, or a limit trip, or the burner is not lighting at all. See why is my furnace blowing cold air?.

When to stop and call

• Any smell of gas: leave the house and call the gas utility, then us.
• A carbon monoxide alarm: get everyone outside first.
• A breaker that trips again immediately.
• Scorch marks, a burning smell that does not clear, or a loud bang on start-up.
• Anything beyond the six steps above. The furnace has safety controls for a reason, and bypassing one is how houses fill with combustion gas.

A furnace repair visit in the Hamilton area is typically $99 to $149 to diagnose, credited against the repair, and the common parts are on the truck so most no-heat calls are fixed on the first visit. If it is below freezing and you cannot wait for business hours, the emergency line is answered around the clock.