Furnace Clunking

[Mike Lamb]

This furnace was clunking when it was run in the heat mode. It did not clunk when I ran the AC. It seemed like it was coming from the plenum and was being transmitted loudly through the condensate drain. Any ideas?

You can listen to it here:

http://youtu.be/572D9Jl-CrU

Click to Enlarge

41.09?KB

[Nolan Kienitz]

Sounds like water "clunking" ... not sure that water clunks, but does sound like water dripping and the echo being amplified by something.

Now ... that is just my set of ears and I do have hearing loss at the high frequencies.

[Steven Hockstein]

My guess is that there is water in the vent piping. The reason that you only hear it when the heat is running is because the exhaust fan for the furnace combustion gases does not run when the system is in the air conditioning mode.

Is the vent pipe properly pitched to drain condensate water?

[Ben H]

Does it start the noise as soon as the burners fire, or do you have to wait a minute for the sound to start?

[Mark P]

I had a furnace last week that was very very loud, I determined it was air being sucked into through the combustion air intake pipe ontop of the furnace - similar to the one in your picture. Somehow turblance was being created and if i placed just one finger into the pipe the sound went away.

[Bain]

Too, there's an internal trap on the flue-gas condensate drain. Since the furnace hasn't been fired for a few months, the trap was probably dry or nearly dry. Flue gases pushing their way through a partially-filled trap would create a gurgling sound like the one heard in your video.

[Mike Lamb]

Does it start the noise as soon as the burners fire, or do you have to wait a minute for the sound to start?

It took about a minute.

The first fire-up set off the smoke detector. New construction.

I shut it down and waited awhile. No smoke the second start-up but the plenum became extremely hot to the point where it could cause burns. The furnace then short cycled off after about 4-5 minutes.

[emalernee]

Something in the inducer motor?

[Garet]

Possibly a foreign object in the ductwork? Could be different between heat and AC due to different blower speeds.

[Rob Amaral]

Mike.. That's the condensate in the trap goigling.. I run into that every once in awhile..

Recently had the same thing at a 'hard' PVC condensate drain that was going into a laundry-drain pipe... same exact sound... the trap was in the attic, but the noise was transmitted down to the end of the pipe..