Skip to main content
True Trade Path

Diagnostic guide

Furnace Blowing Cold Air

The furnace runs but the air from your vents is cold or lukewarm — diagnose the cause before calling for an expensive repair.

System
Heating
Causes covered
8
You can check
4 without a technician

Stop and read this first

  • If a carbon monoxide alarm sounds, leave the building and call 911 from outside.
  • If you smell gas, do not operate switches indoors. Leave and call the gas utility from outside.
  • Do not repeatedly reset a furnace after lockout — unburned gas can accumulate.
  • Never bypass a safety switch to keep the furnace running.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Thermostat fan set to On instead of Auto

    CommonYou can check
    How to check
    Air blows continuously even when the furnace is not heating. Fan setting is On.
    The fix
    Set the fan to Auto. On mode circulates unheated air between cycles and is the most common cause of perceived cold-air complaints.

  2. Furnace still in cooldown after a heating cycle

    CommonYou can check
    How to check
    Cold air blows for one to three minutes after the burner shuts off, then stops.
    The fix
    This is normal. The blower runs to extract remaining heat from the heat exchanger. If it persists more than five minutes, investigate further.

  3. Pilot light or igniter failure

    CommonYou can check
    How to check
    No flame visible through the sight glass. Inducer runs but burner does not light.
    The fix
    Relight the pilot per tank instructions, or call a technician for igniter replacement — typically $150 to $300.

  4. Clogged filter causing limit switch shutdown

    CommonYou can check
    How to check
    Furnace starts, blows briefly, then blows cold as the burner shuts off on overheat.
    The fix
    Replace the filter. A restricted filter causes the high-limit switch to cut the burner while the blower continues.

  5. Dirty flame sensor

    CommonNeeds a pro
    How to check
    Burner lights, runs a few seconds, shuts off, and repeats before lockout.
    The fix
    A technician cleans the flame rod — typically $100 to $200. One of the most common and least expensive furnace calls.

  6. Gas supply interrupted

    PossibleNeeds a pro
    How to check
    No other gas appliances work, or the furnace gas valve is closed.
    The fix
    If no gas appliance works, contact the utility. If only the furnace is affected, a technician checks the valve and gas pressure.

  7. Failing inducer motor or pressure switch

    PossibleNeeds a pro
    How to check
    Inducer does not start, or runs but ignition never begins. Flue termination may be blocked.
    The fix
    Clear a blocked flue if safely reachable. Otherwise a technician tests the inducer and pressure switch — $200 to $600.

  8. Cracked heat exchanger

    UncommonNeeds a pro
    How to check
    Carbon monoxide alarm, flames that move when the blower starts, or soot around burners.
    The fix
    Shut the furnace off and ventilate. Replacement is required — typically $3,500 to $6,500 installed.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

Furnace Replacement cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms