# Furnace Not Heating? Complete Troubleshooting Guide
A furnace that won't heat is more than an inconvenience—it can be a safety concern. Here's how to diagnose the problem and when to call for help.
Safety First
Before troubleshooting: - If you smell gas, leave immediately and call your gas company - Ensure CO detectors are working - Don't attempt repairs on gas connections - Know your limits—when in doubt, call a professional
Quick Checks
1. Check the Thermostat - Is it set to "heat"? - Is temperature set above current room temp? - Are batteries fresh? - Is the fan set to "auto"?
2. Check Power - Is furnace switch in "on" position? - Check circuit breaker - Some furnaces have a secondary switch at the unit
3. Check Gas Supply - Is the gas valve to the furnace open? - Are other gas appliances working?
Common Problems and Solutions
Problem: Furnace Won't Turn On **Possible Causes:** - Thermostat issue - No power to furnace - Furnace switch off - Blown fuse in furnace
DIY Solutions: - Check thermostat batteries and settings - Reset circuit breaker - Check furnace power switch - Look for blown fuses on furnace control board
Problem: Furnace Runs But No Heat **Possible Causes:** - Pilot light out (older furnaces) - Ignition failure (newer furnaces) - Gas valve issue - Flame sensor dirty
DIY Solutions: - Relight pilot if applicable (follow manufacturer instructions) - Check for error codes on furnace display - Clean flame sensor with fine sandpaper (if comfortable doing so)
Problem: Furnace Short Cycles **Possible Causes:** - Dirty filter restricting airflow - Overheating - Thermostat problems - Flame sensor issues
DIY Solutions: - Replace air filter (most common cause) - Ensure vents aren't blocked - Check thermostat location (shouldn't be near heat source)
Problem: Furnace Blows Cold Air **Possible Causes:** - Thermostat fan set to "on" instead of "auto" - Pilot light/ignition problems - Overheating causing safety shutdown - Gas supply issues
DIY Solutions: - Set fan to "auto" - Check filter - Ensure vents are open - Allow furnace to cool and restart
Problem: Furnace Makes Strange Noises | Sound | Possible Cause | |-------|----------------| | Banging | Delayed ignition, duct expansion | | Screeching | Belt or bearing issue | | Rattling | Loose panels, components | | Humming | Electrical issue, blower motor | | Clicking | Ignition issue, relay problems |
Most noises require professional diagnosis.
Understanding Furnace Error Codes
Modern furnaces display error codes via LED lights: - Check your owner's manual for codes - Common codes indicate: - Pressure switch issues - Ignition failures - Limit switch trips - Flame sensing problems
Note the pattern and report it to your technician.
When to Call a Professional
Immediately: - Gas smell (leave house first, call gas company) - CO detector alarming - Visible flames outside the furnace - Burning electrical smell - Furnace won't shut off
Soon: - No heat after basic troubleshooting - Error codes you can't resolve - Unusual noises - Frequent cycling - Yellow or flickering pilot light
Preventive Maintenance
Monthly - Check/replace filter - Listen for unusual sounds - Verify even heating
Annually (Professional) - Heat exchanger inspection - Combustion analysis - Safety control testing - Burner cleaning - CO testing
Fall Preparation - Schedule tune-up before heating season - Test system before you need it - Stock up on filters
Bay Area Furnace Considerations
Coastal Areas - Higher humidity can affect ignition - More frequent tune-ups recommended
Older Homes - Many furnaces are 20+ years old - Consider upgrade if frequent repairs needed
Mild Climate - Furnaces may not run enough to self-diagnose problems - Annual tune-ups catch issues before cold snaps
Quick Reference Troubleshooting
- **No power?** → Check breaker, switches
- **Thermostat OK?** → Check settings, batteries
- **Filter dirty?** → Replace immediately
- **Any error codes?** → Note and report
- **Gas supply OK?** → Check valve, other appliances
- **Still not working?** → Call professional
Remember: Your safety is more important than a quick fix. When dealing with gas appliances, always err on the side of caution and call a professional if you're unsure.
Contact us for fast, reliable furnace service. We offer same-day emergency appointments for heating failures.