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I have a gas-powered forced air furnace. The last time I had a repair man look at it, he said it's 30 years old, so it's getting harder to find replacement parts.

It recently developed an intermittent problem. When the thermostat switches on the heat, I can hear something turning (presumably the induced draft fan). Normally, the blower fan would start soon after that (a few seconds, sometimes as long as a minute) - but recently, it gets into a state where the blower won't come on at all, no matter how long I wait. I can make it work again by turning off power to the furnace for a few minutes, then turning it back on. But doing that several times a day all through the winter is not really practical.

Normally, I'd just hire a pro to fix it. But at this time of year, all the HVAC companies have a weeks-long backlog, so if it's something relatively simple, I'd prefer to do it myself. Any pointers on how I can diagnose this?

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The normal startup should be a sequence like this:

  1. inducer fan comes on
  2. pressure switch verifies fan operation and flue non-obstruction
  3. hot surface igniter comes on
  4. gas valve opens
  5. gas ignites
  6. flame-proving sensor confirms ignition
  7. blower motor turns on
  8. pressure switch, flame proving sensor, and high-limit temperature switches continue monitoring for abnormal conditions that could develop during operation

All of those things are prone to failure: fans wear out, pressure switches wear out or connecting tubing develops leaks, igniters fail, flame-proving sensors get dirty.

You'll have to create a call for heat and observe how far into the sequence the furnace goes. If the inducer fan comes on and nothing more happens that's a sign there's something truly wrong with the flue or there's something wrong with the pressure sensing. If you hear a click and light hiss (gas valve opening) but never see the igniter glow then the igniter has failed (unlikely: igniter failure is usually permanent, not intermittent). If the gas ignites but then shuts off after a moment, and the controls possibly run through the ignition cycle again, then the flame sensing mechanism isn't working.

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