What to Do When Your Heater Stops Working

A furnace or heater that stops working in winter is a legitimate emergency, especially with elderly family members, children, or pets in the home. Before calling a technician, work through these checks - they resolve the problem about a third of the time.

Step 1: Check the Obvious Things First

Thermostat: - Is it set to "Heat" mode? Not "Cool" or "Fan only"? - Is the temperature set above the current room temperature? - Are the batteries fresh? Many thermostats have a low-battery indicator. Replace batteries even if they seem fine. - Has it been reprogrammed or accidentally changed?

Circuit breaker: - Find the breaker for your furnace (usually labeled "Furnace" or "HVAC" or "Air Handler") - Check if it's tripped (moved to the center position) - Reset it by switching fully to "Off" then back to "On" - If it trips again immediately, there's an electrical problem - call a tech

Furnace power switch: - Most furnaces have a dedicated on/off switch on the unit or on a wall nearby that looks like a light switch. It gets accidentally switched off. - Make sure it's in the "On" position

Gas supply (gas furnaces): - If you have gas appliances in the house, check whether the stove burners light normally. If they do, gas supply to the house is fine. - Check the gas shutoff valve on the line going to the furnace - should be parallel to the pipe to be "open"

Step 2: Check the Filter

A severely clogged filter can cause the furnace to overheat and trigger a safety shutoff (high limit switch). The furnace may try to restart and fail repeatedly if the filter isn't replaced.

Pull out the filter. If it's dark gray or completely packed, replace it. Then let the furnace cool for 20-30 minutes before attempting to restart.

Step 3: Check for Error Codes

Most modern furnaces have an LED indicator on the control board that blinks an error code pattern. Open the front panel of the furnace (turn it off first) and look for a blinking LED. Count the blinks - they follow a pattern like "3 blinks, pause, 3 blinks."

There should be a legend on the inside of the furnace door or panel that decodes each pattern. Common codes include: - Pressure switch fault (may indicate a clogged condensate drain) - Ignition lockout (tried to ignite and failed 3+ times) - High limit fault (overheating - usually from filter or airflow issue)

Write down the code before clearing it. Tell the technician when you call.

Step 4: Check Pilot Light or Ignitor

Standing pilot systems (older furnaces): If the pilot light is out, relighting it is often straightforward. Follow the lighting instructions typically printed on the furnace label. The sequence is usually: turn gas valve to "Pilot," push and hold, light the pilot, hold for 30-60 seconds, then release and turn to "On."

Electronic ignition systems (modern furnaces): You should hear a clicking sound and see a spark when the furnace tries to start. If you hear the inducer motor run but no ignition happens, the ignitor may have failed. This requires a technician.

Step 5: Check Condensate Drain (High-Efficiency Furnaces)

High-efficiency (90%+ AFUE) gas furnaces produce condensation as a byproduct and drain it through a plastic tube. If this drain is clogged, a safety switch shuts the furnace off.

Locate the condensate drain (a white PVC pipe) and trace it to where it drains. Look for visible blockage. Clearing the condensate drain is sometimes as simple as flushing it with water or a little white vinegar.

Common Furnace Problems That Need a Tech

After checking the above, if the furnace still won't start:

Ignitor failure: The hot surface ignitor glows orange to light the burner. A failed ignitor needs replacement ($150-$300 for the part and labor). Fast fix once diagnosed.

Flame sensor dirty or failed: The flame sensor confirms that the burner lit. If dirty, it doesn't "see" the flame and shuts off the gas. Technicians sometimes clean flame sensors as a maintenance step; replacement is needed if it's failed ($75-$150).

Pressure switch failure: Monitors airflow. If it fails or the condensate drain is blocked, the furnace won't run.

Control board failure: More expensive ($400-$900). Diagnosis usually rules out simpler causes first.

Heat exchanger crack: The most serious problem - a cracked heat exchanger allows combustion gases (including CO) to mix with household air. If your CO detector goes off in winter, this is a possible cause. Replace the furnace if the heat exchanger is cracked on an older unit.

Staying Warm While You Wait for a Tech

• Use electric space heaters in occupied rooms (keep away from curtains, never leave unattended)
• Close off unused rooms to concentrate the heat from remaining sources
• Dress in layers
• If temperatures are dangerous (below 40F inside), take vulnerable family members elsewhere

Bottom Line

Before calling a tech: check the thermostat settings and batteries, circuit breaker, furnace power switch, gas supply, filter, and error codes. If these don't solve it, you've narrowed it down to a component repair and can give the technician useful information.

Sources and editorial notes

Prices and permit requirements vary by location, property, equipment, and scope. Treat price ranges as planning estimates and confirm current requirements with the relevant authority and contractor.

• US Department of Energy - Energy Saver
• US Consumer Product Safety Commission - home safety

Related guides

• When to Repair vs. Replace a Water Heater
• Garbage Disposal Not Working: Common Fixes Before Calling a Plumber
• Water Heater Maintenance: What Homeowners Should Do Each Year

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