Furnace blowing cold air instead of heat

Cold air from a furnace is often a fan setting, the normal warm-up delay, a clogged filter, or a power or fuel problem you can check yourself. If those check out, the fault is in the ignition or controls, which is a technician's job.

Also searched as: furnace blowing cold air, furnace blowing cold air instead of heat, furnace not heating but fan running, furnace blowing room temperature air.

Before you start. If you smell gas or rotten eggs, stop here. Leave the house, don't touch switches or use your phone inside, and call 911 or your gas utility once you are outside.

A cozy living room on a winter evening with a floor register in the foreground

Every question and answer on this page, in order

Question 1: Do you smell gas or rotten eggs anywhere in the house?

Yes, or I'm not sure: Gas smell: leave the house now
No: go to question 2

Question 2: What is the thermostat set to?

Heat, fan on Auto, set above room temperature: go to question 3
The fan is set to On, or the mode isn't Heat: Likely cause: the fan is set to On

Question 3: How long had the furnace been running when you felt cold air?

Stand at a vent and time it from when you hear the furnace start.

Less than a minute after it started: Probably normal: the warm-up delay
Several minutes or longer: go to question 4

Question 4: Is the filter clogged, or are a lot of supply vents closed or blocked?

Slide the filter out and hold it up to a light. Then walk the house and check the vents.

Yes, one or both: Likely cause: a clogged filter or closed vents
Filter is clean and vents are open: go to question 5

Question 5: At the furnace, which of these is true?

Look only; don't remove panels. The power switch often looks like a light switch on or near the furnace. The gas handle is on the pipe going into the furnace.

The power switch was off, or the front door panel wasn't fully seated: Likely cause: the furnace switch or door panel
The gas handle sits across the pipe, or the propane or oil tank is empty: Likely cause: the gas is off or the tank is empty
Switch on, door seated, fuel on: Likely cause: an ignition, flame sensor or control fault

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.