Furnace Troubleshooting
Furnace Blowing Cold Air — Causes & Repair in Maine
A furnace that runs but blows cold or lukewarm air is one of the most common heating complaints in Maine winter. The cause can range from a simple thermostat setting to a cracked heat exchanger. RG Mechanical diagnoses and fixes the problem quickly throughout Central Maine.
What to Expect
Check thermostat fan setting — should be AUTO, not ON
Dirty filter can trigger high-limit shutoff — replace the filter
Pilot light or ignition failure causes burner to not light
Cracked heat exchanger is a serious issue — CO risk — call immediately
Failed flame sensor causes burner to ignite then immediately shut off
Gas valve failure prevents fuel from reaching the burner
Frequently Asked Questions
Why is my furnace running but blowing cold air?
Most often: the thermostat fan is set to ON (runs continuously even when not heating), a clogged filter triggering the high-limit switch, a failed flame sensor, or an ignition issue. We'll diagnose the exact cause.
Can a dirty filter cause my furnace to blow cold air?
Yes — a severely clogged filter restricts airflow enough to overheat the heat exchanger, causing the high-limit safety switch to shut off the burner. The fan keeps running to cool the furnace — blowing cold air.
Should I be worried about carbon monoxide if my furnace is blowing cold air?
If you see any physical damage to the heat exchanger or suspect a crack, evacuate your home and call both us and 911. Install CO detectors on every floor — we recommend this for every home with a furnace.
How do I know if my furnace flame sensor is bad?
Symptoms include: furnace lights, runs for a few seconds, then shuts off, then repeats. This is a classic flame sensor failure pattern. It's a straightforward repair.
Ready to get started?
Call us or request a project estimate. Contact RG Mechanical to confirm service availability for your community.
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