Furnace Not Heating
Furnace runs but produces no heat, or blows cold air instead of warm air.
Quick Overview
DIY: 30 minutes | Professional: 1-3 hours | Heat exchanger: 4-8 hours
Winter
Common Symptoms
You might be experiencing this problem if you notice any of these signs:
- Furnace blowing cold air
- Blower runs but no heat produced
- Pilot light won't stay lit (gas)
- Burners not igniting
- Furnace starts then shuts off
- Thermostat calling for heat but no response
- Flame present but no warm air
- Furnace cycles without heating
Step-by-Step Diagnosis
Follow these steps to identify the exact cause of your problem:
- Check thermostat set to HEAT mode
- Verify temperature set above room temperature
- Replace thermostat batteries if applicable
- Check furnace power switch (looks like light switch)
- Verify circuit breaker hasn't tripped
- Inspect air filter for severe clogging
- Look for flashing error codes on furnace
- Check gas valve is turned on (gas furnaces)
- Verify pilot light is lit (older gas furnaces)
- Listen for igniter clicking (modern furnaces)
- Feel for warm air at furnace - may take 3-5 minutes
- Check for blocked exhaust or intake vents
Most Common Causes (Ranked by Likelihood)
Thermostat set wrong or not working
HIGH LIKELIHOODDirty or clogged air filter
HIGH LIKELIHOODFurnace power switch turned off
HIGH LIKELIHOODFailed igniter or flame sensor
MEDIUM LIKELIHOODTripped limit switch from overheating
MEDIUM LIKELIHOODGas supply issue or closed valve
MEDIUM LIKELIHOODFailed blower motor or capacitor
LOW LIKELIHOODCracked heat exchanger
LOW LIKELIHOODDIY Solutions
What You'll Need:
- Replacement air filter
- Flashlight
- Thermostat batteries
- Screwdriver
Time Required: 20-40 minutes
Step-by-Step Instructions:
- Verify thermostat is on HEAT, not COOL or OFF
- Set temperature 5 degrees above room temp
- Replace thermostat batteries
- Locate furnace power switch near unit - flip to ON
- Check breaker panel for tripped breaker
- Replace air filter if dirty
- Wait 5 minutes for furnace to warm up before testing
- For gas furnaces: verify gas valve handle is parallel to pipe
- Reset furnace by turning power off 30 seconds, then on
- Check for and record any error codes on display
- Verify vents are open throughout home
- Clear any obstructions from furnace intake/exhaust
⚠️ Safety Warnings
- If you smell gas, leave immediately and call gas company
- Never attempt to light pilot manually if you smell gas
- Carbon monoxide detector is required with gas furnace
- Don't bypass safety switches - they prevent dangerous operation
When to Call a Professional
Don't attempt DIY repairs if you experience any of these situations:
- Gas smell near furnace
- Yellow or flickering pilot flame (should be blue)
- Furnace makes loud banging or rumbling
- Carbon monoxide alarm sounding
- Burners won't ignite after troubleshooting
- Pilot light won't stay lit
- Repeated error codes
- Soot around furnace or vents
- Furnace is more than 15 years old
Prevention Tips
Stop this problem before it starts with these preventive measures:
- Replace air filters monthly during heating season
- Schedule annual furnace tune-up before winter
- Test furnace in early fall before needing it
- Keep furnace area clear of storage items
- Install carbon monoxide detectors on every floor
- Clean flame sensor annually
- Never close more than 20% of vents
Frequently Asked Questions
Why is my furnace blowing cold air?
A furnace blowing cold air usually means the blower is running but burners aren't igniting. Common causes include thermostat on fan mode "ON" instead of "AUTO," failed igniter, flame sensor issue, or the furnace is between heating cycles. If set correctly and cold air persists, call a professional.
How do I know if my furnace igniter is bad?
Signs of a failed igniter include: clicking sounds without ignition, visible cracked ceramic element, burners not lighting, or error codes related to ignition failure. Most igniters last 3-7 years. Replacement costs $150-400 and typically takes 1-2 hours.
Why does my furnace start then stop immediately?
Immediate shutoff indicates a safety lockout from problems like: flame sensor too dirty to detect flame, overheating from clogged filter, pressure switch issue, or faulty limit switch. Check and replace filter first, then call professional if problem continues.
Related Problems
Furnace Not Turning On
Furnace completely unresponsive with no sounds, no ignition attempt, or no blower activation.
Furnace Short Cycling
Furnace starts and stops in short bursts instead of running full heating cycles.
Strange Smells from HVAC
Unusual odors from vents including musty, burning, chemical, rotten egg, or other strange smells.