Furnace Not Heating
Your furnace is running or won't start but isn't producing heat.
Furnace Not Heating: A furnace not heating is commonly caused by a dirty filter, thermostat issues, a failed ignitor, or pilot light problems. Check your filter and thermostat first. If the pilot light is yellow (not blue), turn off the gas and call for service immediately due to carbon monoxide risk. Repairs typically cost $125–$500.
What Causes This?
Dirty Air Filter
A clogged filter restricts airflow, causing the heat exchanger to overheat and trigger the high-limit safety switch. The blower runs but no heat is produced.
Failed Ignitor or Pilot Light
Electronic ignitors have a 5–7 year lifespan. A cracked ignitor won't light the burners. Older furnaces with standing pilot lights may have a blown-out or dirty pilot.
Thermostat Problems
Dead batteries, incorrect settings (set to COOL instead of HEAT), or a faulty temperature sensor can prevent the furnace from firing.
Gas Supply Issue
A closed gas valve, empty propane tank, or gas line problem prevents the burners from igniting. Check that the gas valve near the furnace is in the ON position.
Flame Sensor Dirty
The flame sensor verifies that the burners are lit. A dirty sensor can't detect the flame, causing the gas valve to shut off after a few seconds of operation.
Blower Motor Failure
If the blower motor fails, the furnace may fire but cannot circulate warm air through the ducts. You'll feel heat near the furnace but not at vents.
Can It Wait?
While Northern California winters are mild, nighttime temperatures can drop into the 30s. No heat is uncomfortable and can be dangerous for elderly residents and families with young children. A yellow pilot light or gas smell requires immediate attention.
What Does It Cost?
| Item | Cost Range |
|---|---|
| Air Filter Replacement | $15–$50 |
| Ignitor Replacement | $150–$250 |
| Flame Sensor Cleaning/Replacement | $80–$200 |
| Thermocouple Replacement | $100–$200 |
| Blower Motor Replacement | $300–$600 |
| Gas Valve Replacement | $200–$400 |
Repair vs. Replace
Repair
Repair if: the fix is an ignitor, flame sensor, or thermocouple (all under $250) and the furnace is under 15 years old.
Replace
Replace if: the heat exchanger is cracked (safety hazard, $1,500+ repair), the furnace is 20+ years old, or it has needed 3+ repairs in the past 2 years.
Rule of thumb: A cracked heat exchanger is a carbon monoxide hazard. If diagnosed, replace the furnace immediately — repair is not safe or cost-effective.
When to Call a Contractor
- ✓You smell gas near the furnace (leave the house and call your utility first)
- ✓The pilot light is yellow or flickering instead of blue
- ✓The furnace starts but shuts off after a few seconds repeatedly
- ✓You've replaced the filter and checked the thermostat with no improvement
- ✓There's no heat and nighttime temperatures are below 40°F
Related Problems
Furnace Not Heating — FAQ
Why is my furnace blowing cold air?
Check the thermostat (set to HEAT, not COOL), check the fan setting (AUTO, not ON — the ON setting blows air even when the burners aren't firing), and replace the air filter. If those don't fix it, the ignitor, flame sensor, or gas valve may need service.
Is a yellow pilot light dangerous?
Yes. A healthy pilot light is blue. A yellow or orange flame indicates incomplete combustion, which produces carbon monoxide. Turn off the furnace, ventilate the area, and call for professional service immediately. Install CO detectors near all sleeping areas.
How much does furnace repair cost?
Most furnace repairs cost $125–$500. Ignitor replacement ($150–$250) and flame sensor cleaning ($80–$200) are the most common fixes. Blower motor replacement runs $300–$600.