Furnace blowing cold air? Quick diagnostic checklist

The furnace is running, the blower is moving air, but what's coming out of the registers is room temperature or worse. Before you grab another sweater, walk through this checklist. The quick wins are at the top.

1. Thermostat: heat mode and a fresh battery

Confirm the thermostat is set to HEAT and the set point is well above room temperature. If the screen is dim, blank, or showing a low-battery warning, swap the batteries. A weak thermostat battery is a surprisingly common reason a system gives up mid-cycle.

2. Filter, filter, filter

Same drill as cooling season. A clogged filter chokes airflow across the heat exchanger; the high-limit safety trips and shuts the burners off, but the blower keeps running and pushes cold air through the ducts. Replace the filter and let the system rest for 20 minutes before re-testing.

3. Check that the gas is on

If you have a gas furnace, find the shut-off valve on the gas line at the unit. The handle should be parallel to the pipe (open). If you've had recent work in the basement, a plumber or another contractor may have closed it.

4. Look for an error code

Modern furnaces have a small status light visible through a window on the front panel. It blinks in patterns to tell you what's wrong. Take a video with your phone if you call us — "three short, two long" tells a tech a lot about what to bring on the truck.

5. Power-cycle the furnace

Find the furnace switch on the wall near the unit (it looks like a normal light switch, often red). Flip it OFF. Wait 30 seconds. Flip it ON. Wait two full minutes for ignition. If you hear the blower start before the burner ignites and warms up, that's a control board sequence working correctly. If the blower stays on but you never hear the burner fire, that's our call.

What we'll probably find

If the basics check out, common gas-furnace failures we see are: a dirty flame sensor (often cheap and quick), a cracked or bad ignitor, a bad pressure switch, a failed inducer motor, or a control board issue. Heat pumps have a different list — most often a bad reversing valve, low refrigerant, or a failed defrost board.