Heating Emergency
Heater Running But Blowing Cold?
The vents are pushing air, but it's not warm. Here's what's going wrong and what you can check.
What's Going On
Understanding the Problem
You've set the thermostat to heat, the fan is running, but the air coming from the vents is cool or cold instead of warm. The blower is working, but the heating element — whether it's a gas burner, heat pump, or electric heat strips — isn't doing its job.
Diagnosis
Common Causes
1Fan set to ON instead of AUTO
When the fan is set to ON, it runs continuously — even when the furnace isn't actively heating. Between heating cycles, it blows unheated air through the ducts, which feels cold.
2Furnace overheating and shutting off
A dirty filter or blocked vent causes the furnace to overheat. The safety switch shuts the burner off, but the fan keeps running to cool the heat exchanger — blowing cold air into your house.
3Pilot light or ignitor failure
The furnace goes through its startup sequence — the inducer motor runs, the gas valve opens — but the ignitor doesn't light the gas. The safety system shuts the gas off, and the fan blows cold air.
4Heat pump in defrost mode
If you have a heat pump, it periodically enters defrost mode to melt ice off the outdoor coil. During defrost, the system temporarily blows cool air. This is normal and should only last 5–10 minutes.
5Thermostat set wrong
Double-check that the thermostat is in HEAT mode, not COOL or FAN ONLY. Also check that the set temperature is higher than the current room temperature.
DIY Troubleshooting
What You Can Try
Switch the fan from ON to AUTO
This ensures the fan only runs when the furnace is actively producing heat. Immediate improvement in most cases where this is the cause.
Check and replace the filter
A dirty filter is the most common cause of furnace overheating. Replace it and give the system 10–15 minutes to reset.
Listen to the furnace
Go to the furnace and listen. Do you hear it trying to start? Click-click-whoosh means it's igniting. Click-click-silence means the ignitor is failing. No sound at all means a different issue.
Wait 10 minutes (heat pump owners)
If you have a heat pump and it's blowing cool air on a cold day, wait 10 minutes. It may be in defrost mode. If it doesn't return to heating after 15 minutes, something else is wrong.
Know When to Call
When to Call a Pro
If the fan-to-AUTO switch doesn't fix it and the filter is clean, the problem is likely an ignitor, flame sensor, or gas valve issue. These aren't DIY repairs — they involve gas and high voltage. Call us before the house gets dangerously cold.
Pro Tip
Here's a quick test: set the thermostat to HEAT, then go stand at a supply vent. You should feel warm air within 3–5 minutes of the thermostat clicking on. If you get cold air for more than 10 minutes straight, the heating cycle isn't firing properly.
More Troubleshooting
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