Common Reasons Your Furnace Isn’t Heating
A furnace can stop heating your home for several reasons. Start with these common problems before assuming your system needs a major repair.
1.
Check Your Thermostat Settings
Make sure the thermostat is set to Heat and the temperature is set several degrees above the current room temperature. If the thermostat has a fan setting, use Auto instead of On. If you have a programmable or smart thermostat, also check that the current schedule isn't preventing the heat from running.
What to look for: After raising the temperature, listen for the furnace to begin its heating cycle. Depending on the furnace, it may take a short time before warm air begins coming from the vents.
2.
Check the Air Filter and Airflow
A dirty or clogged air filter can restrict airflow through the system and reduce your furnace’s ability to heat the home. Check the filter and replace it if it appears heavily covered with dust or debris. Also make sure the supply vents and return-air grilles inside the home aren't blocked by furniture, rugs, or other objects.
What to look for: Weak airflow from multiple vents can indicate an airflow problem. If the filter is dirty or clogged, replace it with the correct type and size for your system, then give the furnace time to operate before checking the temperature again.
3.
Check That the Furnace Has Power
If the furnace isn't running at all, make sure it has power. Check your home's electrical panel to see whether the furnace breaker has tripped. Some furnaces also have a power switch located near the unit that looks similar to a regular light switch. Make sure it hasn't accidentally been turned off.
What to look for: If the breaker is tripped, reset it once. If it trips again, stop troubleshooting and contact an HVAC professional. Do not repeatedly reset a breaker that continues to trip.
5.
Check the Air Coming From Your Vents
Once the furnace has been running for several minutes, check the air coming from a supply vent. The air should feel noticeably warmer than the room. If the blower is running but the air stays cool or only slightly warm, the furnace may not be completing its normal heating cycle.
What to look for: Check several supply vents to see whether the problem affects the whole home or only certain rooms. Weak airflow throughout the home may indicate an airflow problem. If the airflow seems normal but the air remains cool or only slightly warm, continue to the next check.
4.
If your furnace uses PVC intake and exhaust pipes that terminate outside your home, check the openings from the outside. Snow, ice, leaves, or other debris can sometimes block the pipes and prevent the furnace from operating properly.
What to look for: Make sure the exterior pipe openings are clear and unobstructed. Remove loose snow, leaves, or debris from around the openings, but do not insert tools or objects into the vent pipes.
6.
Check the Furnace Vent Pipes Outside
Some furnaces have a small viewing window where you can see a diagnostic light without removing any panels. If your furnace isn't heating, look through the window to see whether the light is steady or flashing in a pattern.
What to look for: If the light is flashing, write down the number or pattern of flashes. The pattern may help identify what part of the heating cycle is having a problem. Do not remove the furnace panels to access the controls or diagnostic light.
Check for a Furnace Error Light
6.
5.
4.
3.
2.
1.
When to Stop Furnace Troubleshooting and Get HVAC Help
Some furnace problems should not be handled as a homeowner troubleshooting project. Stop troubleshooting if you smell gas, notice burning odors or damaged wiring, the breaker repeatedly trips, the furnace repeatedly shuts down, or the problem requires removing equipment panels or working around gas or electrical components.
If the basic checks above didn't identify the problem, FixWithAPro can help you take the next step. During a live video session, an experienced HVAC technician can review what your system is doing, guide you through additional safe checks, and help determine whether the problem requires an in-person HVAC professional.
Furnace Not Heating FAQs
Why is my furnace running but not heating?
Your furnace may run without producing enough heat because of incorrect thermostat settings, restricted airflow, a dirty air filter, blocked venting, or another issue that requires further diagnosis.Why is my furnace blowing cold air?
A furnace may briefly blow cooler air when it first starts or after the heating cycle ends. If it continues blowing cool air while the thermostat is calling for heat, check the thermostat settings and air filter. The furnace may not be completing its normal heating cycle.Can a dirty air filter cause my furnace to stop heating?
Yes. A dirty or clogged air filter can restrict airflow, reduce heating performance, and may cause the furnace to shut down during operation. Replace a dirty filter with the correct type and size for your system.When should I call an HVAC professional for a furnace that isn't heating?
Get professional help if the basic checks don't solve the problem, the breaker repeatedly trips, you smell gas or burning odors, notice damaged wiring, or diagnosing the problem would require opening the furnace or working around gas or electrical components.
One safety detail: if you smell gas, STOP troubleshooting, leave the area, and contact the gas utility/emergency services from a safe location rather than continuing with the furnace.