Furnace Not Working? A Homeowner's No-Heat Troubleshooting Guide

Safety first

If you smell gas, leave the house and call your gas utility from outside. If a carbon monoxide alarm sounds, get everyone out and call 911. Do not troubleshoot either situation yourself.

Step 1: Thermostat

Set it to HEAT, raise the setpoint five degrees above room temperature, and replace the batteries. Check that a programmed schedule or vacation hold is not overriding you. A surprising number of no heat calls end here.

Step 2: Power and gas

Check the breaker labeled furnace or air handler. Find the switch on or near the furnace - it looks like a light switch and gets flipped accidentally all the time. Confirm the gas shutoff valve is open and other gas appliances work.

Step 3: The filter

A clogged filter overheats the heat exchanger and trips the high limit switch, which is the classic cause of furnace short cycling. If you can not see light through it, replace it.

Step 4: Watch the start sequence

A modern furnace runs a set sequence: inducer motor spins, pressure switch closes, igniter glows, gas valve opens, flame proves on the sensor, blower starts. Note where it stops. Igniter glows but no flame points to the gas valve. Flame lights then dies after a few seconds means a dirty flame sensor. Nothing at all suggests the board, transformer or a safety lockout.

Step 5: Vents and returns

Open all supply registers, clear furniture off returns, and check the outdoor flue and intake pipes for snow, ice, leaves or nests. A blocked intake will lock the furnace out on the pressure switch.

Step 6: Reset once - only once

Cutting power for 30 seconds clears some lockouts. If the furnace fails again, stop. Repeated resets flood the burner area with gas and stress components.

Furnace runs but blows cold air

Usually a failed igniter or flame sensor, a closed gas valve, or a furnace stuck in fan mode. On heat pumps, lukewarm air is often a defrost or reversing valve issue instead.

Furnace short cycling

On and off every few minutes means the furnace is overheating and hitting the limit, or the flame is not proving. Causes: dirty filter, closed registers, undersized ducts, dirty sensor, cracked pressure switch hose, or a badly oversized furnace.

Strange noises decoded

Boom on ignition means delayed ignition - shut it down and call. Squealing is usually blower bearings or a belt. Rattling metal is often loose ductwork or the blower wheel. Clicking with no start points to ignition control.

Repair or replace?

Under 10 years old: repair. Over 15 with a major part failure, rising bills, a cracked heat exchanger, or rooms that never warm up: get replacement numbers. We provide both, in writing, at no charge.

Prevent next winter's no-heat call

One annual furnace tune-up in the fall catches weak igniters, dirty sensors, cracked hoses and blower problems before the first cold snap - and keeps your warranty valid.