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No heat

My Furnace Won't Heat the House

Most winters in the region are mild, so a furnace may sit idle for months before a cold snap puts it to work. That is often when hidden problems show up.

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Technician inspecting the blower and wiring inside an indoor furnace cabinet

Quick answer

A furnace that won't heat is often held back by a thermostat setting, a dirty filter, a tripped breaker or switch, or a furnace door that isn't fully closed. Deeper causes include a failed igniter, a dirty flame sensor, a gas supply issue or a blocked vent triggering a safety shutdown. If you ever smell gas, leave the home first and call for help from outside.

What may be going on

Possible causes

  1. Thermostat setting or batteries

    A thermostat left on Cool from summer, set too low or running on dead batteries will never call for heat.

  2. Clogged filter

    A heavily clogged filter can cause the furnace to overheat and shut itself off as a safety measure, then cycle again.

  3. Ignition failure

    Modern furnaces use an electronic igniter. If it cracks or wears out, the burners never light even though the system tries.

  4. Dirty flame sensor

    When the flame sensor gets coated, the furnace lights briefly and then shuts off because it cannot confirm a flame.

  5. Safety switch or venting problem

    Blocked flue pipes, intake vents or a door switch that isn't pressed will keep the furnace from running.

Safe checks you can make

  • Set the thermostat to Heat and raise the setpoint a few degrees above room temperature.
  • Replace thermostat batteries if the screen is faint or blank.
  • Check the furnace breaker and the switch near the furnace, which looks like a light switch, and reset the breaker once if it tripped.
  • Replace a dirty air filter.
  • Make sure the furnace's front door is pushed fully closed so its safety switch is engaged, without opening any panels.

What not to do

  • If you smell gas, don't flip switches or try to relight anything. Get everyone out of the house, then phone the gas utility or 911 once you are standing outside.
  • Don't remove the furnace panels or work on the burners, igniter or flame sensor yourself.
  • Don't keep resetting the breaker or repeatedly cycling the furnace after it fails to light.

When to call a professional

When the thermostat, power, filter and door all check out and the furnace still won't produce heat, the issue is in the ignition, gas, venting or controls. Those are not homeowner repairs, and a technician should handle anything inside the cabinet.

Cold snaps in Birmingham, AL can arrive quickly, so it helps to test the heat in fall. Call to get connected with an independent local HVAC provider. Ask about furnace repair or a diagnostic visit.

  • The furnace tries to light several times and then shuts down.
  • A flashing light on the furnace control board.
  • Any smell of gas, or a carbon monoxide alarm sounding.
  • Heat comes on briefly and then cuts out repeatedly.

What an HVAC provider may inspect

  • Igniter and flame sensor condition
  • Gas valve and burner operation
  • Flue and intake venting for blockages
  • Limit and pressure switch function
  • Error codes on the control board

Keeping it from happening again

A fall tune-up before the first cold night, a clean filter and a working carbon monoxide alarm near sleeping areas are simple ways to protect the home. The winter heating prep guide covers a short checklist. If the furnace runs but the air feels cool, see furnace blowing cold air.

Where to go next

Related serviceFurnace Repair

Frequently asked questions

Furnace Not Heating: questions homeowners ask

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Leave the home right away with everyone inside, and don't flip any switches, use phones indoors or try to find the leak. Once outside, call the gas utility or 911. Only after the area is declared safe should a provider be called about the furnace.

Short cycling like this often comes from a dirty flame sensor, a clogged filter causing overheating, or a venting issue. Replacing the filter is a fair first step. If the pattern continues, a technician should check the sensor and safety switches.

Yes. Because winters in the region are often mild, a furnace can sit unused for months. Running it on a cool fall day reveals problems before a hard freeze, and gives time to arrange a repair if needed.

Still not sure what's wrong?

Call and describe what you're seeing. You'll be connected with an independent HVAC provider serving homes in the Birmingham, AL region.

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