Why Is My Furnace Blowing Cold Air? 7 Causes, Ranked by Likelihood

The blower is running, air is coming out of the vents — but it's cool or lukewarm, and the house temperature is going the wrong direction. Here's what's actually happening, ranked from "check this yourself right now" to "call a pro."

1. Thermostat set to FAN ON instead of AUTO

When the fan is set to ON, the blower runs constantly — including between heating cycles, when it's pushing unheated air. Check this first; it takes ten seconds. The fan should be set to AUTO so it only runs while the furnace is actively heating.

2. Dirty filter choking the system

A clogged filter restricts airflow, the heat exchanger overheats, and the high-limit safety switch shuts the burners off while the blower keeps running to cool things down. The result: cool air from the vents. In the Treasure Valley, fine dust and summer wildfire smoke load filters fast — if you can't remember the last change, do it now and give the system 30 minutes.

3. Dirty flame sensor (the Boise special)

The flame sensor confirms the burners actually lit. When it's coated in residue — common after a long, dusty summer of sitting idle — it kills the gas valve within seconds of ignition. The furnace lights, dies, lights, dies, then gives up and runs the blower alone. This is the single most common no-heat call we run, and it's usually a $90–$250 fix.

4. Failed ignitor

If the ignitor is dead, the gas never lights at all. You might hear the inducer fan start, a click, and then nothing — followed by cool blower air. Ignitors are wear items with a 5–7 year typical life, and they love to die on the first night of an inversion cold snap. Replacement runs $150–$350.

5. Duct leaks pulling in crawlspace or attic air

If your ducts run through a crawlspace (common in older Boise and Nampa homes) or an unconditioned attic, disconnected or leaky joints can dump your heated air where nobody lives and pull freezing air into the supply stream. The furnace is working fine — the heat just isn't arriving. If some rooms are warm and others get cold air, suspect ducts.

6. Pilot light or gas supply issue (older furnaces)

Standing-pilot furnaces (generally pre-1990s — still common in the North End) can lose the pilot to drafts or a failing thermocouple. Also worth checking: is the gas actually on? After service work or a meter swap, valves get left closed more often than you'd think.

7. Cracked heat exchanger

The serious one. A crack can trigger safety rollouts that kill the burners mid-cycle — and it can leak carbon monoxide into your air. If your CO detector has ever alarmed, if you see soot around the cabinet, or if burner flames look yellow and lazy instead of blue and steady, shut the system off and call immediately.

When to Stop Troubleshooting

Anything involving gas valves, ignition components, or the heat exchanger is pro territory — both for safety and because a wrong move can turn a $200 repair into a $2,000 one. If the quick checks don't solve it, book a diagnostic and we'll find it fast.