Why is my furnace blowing cold air?

Short answer: The most common reasons are a thermostat fan set to ON instead of AUTO, a clogged filter that makes the furnace overheat and shut off its burners, a dirty flame sensor, or a blocked vent pipe on a high-efficiency furnace. Check the thermostat and filter first. If the burners won't stay lit, call a pro.

Safety first: If you smell gas (rotten eggs), leave the house, then call your gas company from outside. In Utah that's Enbridge Gas, or call 911. If your carbon monoxide alarm goes off, get everyone outside and call 911.

1. The fan is set to ON

With the thermostat fan on ON, the blower runs nonstop, even between heating cycles, so you feel cool air. Set it to AUTO.

2. It just started

Most furnaces run the burners for a short time before the blower turns on. If the air turns warm within a minute or two, that's normal.

3. A dirty filter

A clogged filter chokes airflow. The furnace overheats, a safety switch shuts the burners off, and the blower keeps pushing unheated air. Replace the filter and see if heat comes back.

4. A dirty flame sensor

If the burners light, then shut off after a few seconds, the flame sensor may be coated and can't "see" the flame. A tech can clean or replace it. It's one of the cheaper fixes.

5. The igniter or pilot isn't lighting

Newer furnaces use a hot-surface igniter that glows orange. If it doesn't glow, or glows but the gas never lights, the furnace will try a few times and lock out. Older furnaces have a standing pilot that can go out.

6. A blocked vent or drain on a high-efficiency furnace

High-efficiency furnaces vent through white PVC pipes, often out a side wall. Snow, ice, or a bird's nest can block them, and a clogged condensate drain can also stop the burners. Clear snow away from the pipes gently and don't push anything into them.

7. Gas supply

Check that the gas valve to the furnace is open (the handle runs in line with the pipe). If other gas appliances aren't working either, call the gas company.

8. Leaky ducts

If the furnace is making heat but some rooms get cool air, ducts in an attic or crawl space may be leaking or disconnected.

What the blinking light means

Most furnaces have a small LED behind the lower panel that flashes a code. The code chart is usually printed inside the door. Write the code down before you call. It saves diagnostic time.

When to call a pro

Call if the burners won't light or stay lit after a new filter, the furnace keeps locking out, you see an error code you can't clear, or you smell gas or burning. Typical repair prices are on Furnace repair cost. If an older furnace needs a major part, compare the quote with the furnace replacement cost before you approve the repair.

Checked these and it's still blowing cold? Request furnace help. One local heating company that serves your ZIP will contact you. No heat and it's below freezing? Call a local 24 hour heating company now instead of waiting.

FAQ

Why does my furnace blow cold air, then warm air? Usually the startup delay or the fan on ON. If it cycles cold, warm, cold, an overheating filter or flame sensor is the likely cause.

Can I reset my furnace? Turning the power switch off for about 30 seconds and back on clears a lockout. If it locks out again, something needs fixing, so don't keep resetting it.

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