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Our house has a 15 year old gas furnace. When it fires up, you can hear this series of events:

  1. A fan blowing
  2. A few clicks
  3. Steady blue flames
  4. A big whoosh (like the sound of something going up in flames)
  5. Then it blows hot air, cycling off and on every 5-10 minutes, unable to heat above 67 degrees.

I went in to do a little winter maintenance--changing the filter, removing debris, and so on. Now, when firing up, the first three steps are the same (blowing air, a few clicks, then blue flames). But, now there's no big whoosh and the hot air blows steadily until the house reaches the desired temperature.

In short, we're happier with it now. But, I wanted to make sure that we didn't disable something important by eliminating the big whoosh noise and the cycling off and on.

If you've got a hunch, please weigh in.

Thanks!

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    What make/model is said furnace? – ThreePhaseEel Dec 13 '16 at 4:30
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    Sounds like your blower is not initiating. Does it turn off after a short period without blowing hot air? Make sure all the panels/hatches/doors on the unit are securely in place, many have switches that prevent the unit from operating if a panel is not properly in place. – Jimmy Fix-it Dec 13 '16 at 6:31
  • It blows plenty of hot air. In terms of heating, it works better now that I've cleaned it. This is our first winter in the house, so I'm trying to understand if it was supposed to sound like it did (with the whooshing noise) or if it was running poorly (and the absence of the whoosh is a good sign). – user2258740 Dec 13 '16 at 17:00
  • At first, I was going to suggest the same as 'Jimmy Fix-it' but after reading your follow-up comment, I'm wondering if that big whoosh was the sound of the unit going off. Sometimes a change in pitch of a burner going off is more noticeable than a steady burner. I notice our gas furnace's big whoosh when it shuts off....if I'm in the room above it when it happens. – James Olson Dec 13 '16 at 18:34
  • Works better in what way? More air? Warmer temperature? – isherwood Dec 13 '16 at 21:50
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Disclaimer: I am not a HVAC technician, and this is speculation on very little information.

I am guessing that what you describe as “like an amplified version of burners flames on a stove” is mostly the sound of the gas flowing through the piping and burners, not the combustion itself.

The “big whoosh”, then, is when the not-yet-burning gas reaches the igniter, and a bunch of gas ignites all at once (causing a noise as the gas and air becomes very much hotter and expands).

If there is less “big whoosh” but the furnace is doing its heating job, that suggests that the gas is igniting more promptly with less accumulation of unburned gas, which is, all else being equal, a good thing.

I would suggest that to further settle your mind you should look at the flames. They should be blue (with perhaps occasional orange flickers but not continuous ones) and in a neat row, and when they ignite they should not spill back out of the burner and heat exchanger area even for an instant. Don't remove anything other than the usual outer access panels to get a look, as it might change the airflow away from normal.

  • Thanks for this. The flames are blue and in neat rows. To clarify my notes above, the furnace used to cycle like this: 1. Blowing air 2. A few clicks. 3. Blue flames appear. 4. A big whoosh sound. 5. Then it would heat the house, cycling on and off every 5-10 minutes, unable to heat above 67 degrees. Now, after cleaning, the first three steps are the same: blowing air, clicks, then blue flames. But, after that there's no whoosh. Instead it blows hot air steadily without cycling off until it hits the desired temperature. It's great, I just want to make sure I didn't disable something important. – user2258740 Dec 13 '16 at 23:07
  • @user2258740 It'd be great if you could edit that information into your question! – Kevin Reid Dec 13 '16 at 23:27
  • What Kevin mentions may well be the reason there is no big woosh and the reason it is heating better. Some of the jets may have been plugged and your cleaning opened them and now the burners are lighting and burning better.+ – Ed Beal Dec 14 '16 at 22:21

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