The thermostat is set to heat but everything coming out of the vents is cold. Furnace is running – or seems to be – but no heat.
You probably came here because…
- Cold air coming from vents when thermostat calls for heat
- Blower runs but the air stays cold
- You may hear the ignitor click but no flame catches
- It may have worked earlier today and quit
What’s actually happening
Cold air from a heating system usually means one of three things: the fan is on but the burners aren't lighting, the fan setting is on ON instead of AUTO (so it runs even when there's no heat call), or the furnace is overheating and shutting the burners off but leaving the fan running for safety.
The overheating case is dangerous long-term because it means the heat exchanger is under stress. Almost always caused by a clogged filter or a blocked return.
If you have a heat pump, cold air is normal at first – heat pumps deliver warm-ish air, not hot air like a gas furnace. But if it's genuinely cold, the reversing valve or defrost cycle is stuck.
What we do about it
Check the fan setting
On the thermostat, make sure Fan is set to AUTO, not ON. On ON, the blower runs constantly even when there's no heat call – you'll get cold air between cycles.
Change the filter and clear returns
Clogged filter = furnace overheats = safety cuts the burners = fan keeps running to cool the heat exchanger = cold air. Change filter, open all return grilles, wait 30 minutes, retry.
Watch for ignition failure
Stand by the furnace during a call for heat. You should hear a click, then a whoosh of ignition, then the main blower 30-60 seconds later. If you hear click but no whoosh, ignitor or flame sensor failed.
When to just call
If the filter is fresh and it's still blowing cold, or if you can smell burning plastic, or if the burners aren't lighting at all, call (423) 637-1846. Repeated overheating cycles can crack the heat exchanger, and a cracked heat exchanger is a carbon monoxide risk.
Ready to fix it?
Call the family that’s been doing this in the Sequatchie Valley since 1996.