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True Trade Path

Diagnostic guide

Furnace Not Heating

Diagnose a gas furnace that will not fire, short cycles, or blows cold air, starting with what you can safely check.

System
Heating
Causes covered
9
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. Do not re-enter to investigate.
  • If you smell gas, do not operate switches or phones indoors. Leave and call the gas utility's emergency line from outside.
  • Do not attempt to reset a furnace repeatedly after a lockout. Lockouts exist to prevent unburned gas accumulating.
  • Never bypass a safety switch to keep a furnace running, no matter what you read online.

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 settings or dead batteries

    CommonYou can check
    How to check
    Mode set to Heat, setpoint above room temperature, display lit and responsive.
    The fix
    Replace batteries and correct the mode. Raise the setpoint five degrees and listen for the system to call.

  2. Clogged air filter causing limit shutdown

    CommonYou can check
    How to check
    The furnace fires, runs briefly, then shuts down and repeats. The filter is visibly loaded.
    The fix
    Replace the filter. A restricted filter causes the high-limit switch to trip on overheat, which is the most common cause of short cycling.

  3. Furnace power switch or door safety switch open

    CommonYou can check
    How to check
    Check the switch on or near the furnace, and confirm the blower access door is seated fully.
    The fix
    Turn the switch on and reseat the door. The door interlock will not allow the furnace to run if the panel is not fully closed.

  4. Blocked condensate drain on a high-efficiency furnace

    CommonYou can check
    How to check
    Water near the furnace, or a unit that locks out with the inducer running but no ignition.
    The fix
    Clear the condensate trap and drain line. A blocked trap trips the pressure switch and prevents ignition.

  5. Failed igniter

    CommonNeeds a pro
    How to check
    The inducer runs, the gas valve does not open, and you never see a glow inside the burner compartment.
    The fix
    A technician replaces the hot surface igniter. This is a routine wear item, typically failing every five to ten years.

  6. Dirty flame sensor

    CommonNeeds a pro
    How to check
    The burner lights and then shuts off after three to seven seconds, repeating several times before lockout.
    The fix
    A technician cleans the flame rod. This is one of the most common and least expensive furnace service calls.

  7. Pressure switch or blocked flue

    PossibleNeeds a pro
    How to check
    The inducer motor runs but ignition never begins. Check the exterior flue termination for a bird nest, snow or ice.
    The fix
    Clear an obstructed termination if it is safely reachable. Otherwise a technician tests the switch and verifies draft.

  8. Gas supply interrupted

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

  9. Cracked heat exchanger

    UncommonNeeds a pro
    How to check
    A carbon monoxide alarm, flames that visibly move when the blower starts, or soot around the burners.
    The fix
    Shut the furnace off at the switch and ventilate. This is a carbon monoxide hazard and almost always means the furnace is replaced, not repaired.

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.

Estimate the repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms