Furnace problems, by what the furnace is doing

You do not need to know which part failed to call. You need to know what the furnace is doing: whether anything happens at all, whether the burners light and stay lit, whether air comes out, and whether there is a smell or a noise.

Find your symptom

• Not heating

  No heat, or it blows cold

  The furnace will not fire, fires and quits, or blows cold. The four free checks, what each failure pattern means, and why not to keep resetting it.

  What the pattern means

• Won't turn on

  Nothing happens at all

  What to check first

• Keeps shutting off

  Starts, stops, tries again

  Ignition or airflow?

• Igniter and flame sensor

  Lights, then drops out

  How each one fails

• Blower not working

  Burners light, no air

  Motor, capacitor or relay

• Noisy furnace

  Bangs, squeals, rattles

  Match the noise

• Burning smell

  Dust, or something worse

  Normal or not?

• Weak airflow

  Some rooms stay cold

  Filter, blower or ducts

• Electric furnaces

  Lukewarm air, all day

  Elements and sequencers

• High-efficiency furnaces

  Plastic vents, water nearby

  Drain, vents, switches

Gas, electric or high-efficiency?

Look at how the furnace vents. A gas furnace with a metal flue going up is a standard-efficiency furnace. Two plastic pipes out through a side wall mean a high-efficiency, condensing furnace, which has faults of its own. No gas pipe and no flue at all means electric, covered on the electric furnace page. If you are not sure, say what you can see when you call.

What is in scope, and what is not

Boilers, radiant floor heat and wood or pellet stoves are a different trade, and in the case of wood and pellet stoves a different permitting office. If one of those is your problem, a specialist in that system is the right call.