Furnace Running But No Heat: Where to Start
This is one of the most common HVAC complaints. The good news: many causes are simple to fix yourself.
Check #1: Thermostat Settings
Before anything else, verify your thermostat:
Is it set to HEAT? (Not cool, not off) Is the temperature set above room temp? (Try setting it 5° higher) Fan setting: Should be AUTO, not ON
When set to ON, the fan runs continuously — even without heat. This blows cold air.
Try this: Set thermostat to HEAT, temperature to 75°F/24°C, fan to AUTO. Wait 5 minutes.
Check #2: Air Filter
A clogged filter is the #1 cause of furnace problems:
How it causes "no heat":
- Restricted airflow causes overheating
- Safety switch shuts down burner
- Fan keeps running, blowing cold air
Check your filter:
- Locate the filter (usually between return duct and furnace)
- Pull it out
- Hold it up to light
- If you can't see light through it, it's clogged
Replace it. This alone fixes many furnace problems.
Check #3: Pilot Light / Ignition
Standing Pilot (Older Furnaces)
If you have an older furnace with a standing pilot light:
- Find the pilot light window
- Look for a small blue flame
- If it's out, you'll need to relight it (follow manufacturer instructions on the unit)
Note: Many newer furnaces don't have pilot lights — they use electronic ignition.
Electronic Ignition (Newer Furnaces)
If your furnace has electronic ignition, you may see:
- Blinking LED on the control board (indicates error codes)
- Check your manual for what the blink pattern means
Common ignition problems:
- Ignitor failure (part wears out)
- Dirty flame sensor (can sometimes be cleaned)
- Gas valve issues
Check #4: Gas Supply
Is the gas turned on?
- Check the gas valve on the supply line (lever should be parallel to the pipe)
- Check other gas appliances (stove, water heater) — do they work?
If no gas appliances work, you may have a supply issue. Call your gas company.
Check #5: Circuit Breaker
Furnaces have electricity even if they're gas-powered. Check:
- The furnace breaker in your panel (may be labeled "Furnace" or "HVAC")
- The power switch on or near the furnace (looks like a light switch)
Reset if tripped. If it trips again, there's an electrical problem.
Check #6: Vents and Registers
Blocked vents cause problems:
- Check that registers are open throughout the house
- Make sure no furniture is blocking airflow
- Check the return air vent — is it blocked?
What the Furnace Might Be Telling You
Modern furnaces have diagnostic LEDs on the control board:
Steady green: Normal operation Blinking green: Call for heat (normal) Blinking red/amber: Error code — check manual No light: Power issue
The pattern tells a technician what's wrong. Note it before calling.
When to Call an HVAC Technician
Call if:
- Filter is clean but still no heat
- Pilot won't stay lit
- You smell gas (call immediately, leave the house)
- Furnace is short-cycling (turning on/off rapidly)
- You see error codes you can't interpret
- DIY steps didn't help
Don't try to fix:
- Gas components
- Electrical wiring
- Heat exchanger
- Anything inside the furnace cabinet beyond the filter
Temporary Measures
While waiting for repair:
- Use space heaters safely (never leave unattended)
- Close off unused rooms to concentrate heat
- Use extra blankets
- Seal drafts temporarily