Why Is My Furnace Not Heating?
A furnace that runs but fails to heat your home -- or one that will not turn on at all -- is a stressful situation, especially in cold weather. Many causes are simple to identify. This guide walks through the most common problems in order of likelihood.
Check These First
- Thermostat: Set it to HEAT, at least 5 degrees above room temperature. Confirm it is not in a schedule or hold mode that is overriding your setting. Replace batteries.
- Air filter: A severely clogged filter restricts airflow and can cause the furnace to overheat and shut down on its safety limit switch. Replace if overdue.
- Circuit breaker: Check the furnace breaker. Also check the furnace switch (looks like a light switch, usually near the unit or at the top of the basement stairs) -- it is easy to accidentally turn off.
- Gas supply: Confirm other gas appliances (range, water heater) are working. If they are not, call your gas utility.
- Pilot light or ignition: Older furnaces have standing pilot lights that can go out. Modern furnaces use electronic ignition -- if the furnace is clicking repeatedly but not igniting, the igniter may have failed.
Common Causes
Failed Igniter
The hot surface igniter glows to ignite the gas. When it fails, the furnace will try to ignite, fail, and lockout. You may hear clicking. Replacement cost: $150-$350.
Bad Flame Sensor
The flame sensor detects whether the burner lit. A dirty or failed sensor causes the furnace to ignite briefly then shut off -- you may notice the furnace starts, runs for a few seconds, then shuts down. Cleaning or replacing a flame sensor: $75-$200.
Failed Blower Motor
The blower motor circulates heated air through the ductwork. If it fails, the furnace may heat the heat exchanger but you will get no air from the vents. Replacement: $300-$600.
Cracked Heat Exchanger
A cracked heat exchanger is a safety issue -- combustion gases can leak into living spaces. Signs: yellow or flickering flame (should be blue), unusual odors, or a CO detector going off. This is grounds for immediate shutdown and replacement evaluation. Do not operate the furnace.
Dirty Burners
Burners clogged with rust or debris may not light fully or may shut down on the flame sensor. Annual maintenance prevents this.
When Is It an Emergency?
Call for emergency service when: outdoor temperatures are below freezing with no alternative heat source, pipes are at risk of freezing, or there are elderly, very young, or medically vulnerable people in the home. Carbon monoxide issues are always an emergency -- evacuate and call 911.
Frequently Asked Questions
- Why is my furnace running but not producing heat?
- Common causes: failed igniter (furnace tries to start but does not ignite), bad flame sensor (starts then shuts off after a few seconds), failed blower motor (heat exchanger heats but no air reaches vents), or a tripped high-limit switch from a clogged filter. Check the filter and thermostat settings first.
- Why does my furnace turn on but blow cold air?
- If the furnace starts and blows air but the air is cold, the burner may not be igniting. This is often a failed igniter or a dirty flame sensor. It can also happen when the furnace is in a restart delay after a lockout -- wait 30 minutes and try again before calling.
- Is a furnace not heating an emergency?
- It depends on conditions. If outdoor temperatures are at or below freezing with no alternative heat, pipes are at risk, or vulnerable people are in the home, yes -- call for same-day emergency service. In mild weather with alternative heating available, a next-day appointment is usually fine.
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