Furnace makes farting noises

Mr_TA

And I'm not even kidding.

Over the past few days, every so often, sometimes once a day, sometimes every 15-20 minutes over a couple of hours, we hear this FAARRRRTT from the corner of our house while being out and about on the 1st floor, typically in the evening but also during the day (rarely). In that corner, there is absolutely nothing on the 1st floor that can produce any sounds: 1) a closet with clothing and underwear; 2) a wardrobe which is mostly full of children's toys, and an accordion. A bathroom is nearby but the sound is not coming from that direction, and again, there is nothing in the bathroom that can do it.

In the basement right underneath the area where the sound comes from, there are: 1) scrap wood from a bedroom renovation project I completed a few months ago; 2) split and seasoned wood logs; 3) log splitter (very similar to http://www.logsplittersdirect.com/Powerhouse-FX128-Z-Log-Splitter/p3215.html but not the same make/model); 4) our central HVAC system. The outdoors AC unit is right across the wall in the yard, but I don't think its sounds would be so audible inside the house, with the windows closed and all.

It's a Trane XV95, with gas heating and AC, central air system. It works perfectly, has a new high quality filter from Home Depot as of ~1 month ago. I'd like to say that I've never heard the sound while the heat is on, but can't guarantee that to be a 100% accurate observation.

Basically, while it's off, it farts for no good reason, on no set schedule - like irregular fast-food/beer/popcorn consumption causing flatulence? Except it's an HVAC system.

Any help would be GREATLY APPRECIATED.

BobC

It could be a pipe rubbing against an opening in a floor or framing member - or else your trying to hide something

Bob

Fred

Plenium and/or duct work sometimes contracts after a heating cycle, especially if it's large enough that a bottom or side panel can pop in or out.

Mr_TA

BobC,
What kind of pipe are you talking about? Is the sound made by the rubbing itself? It sounds more like some gas is released by a valve.

Right, I'm hiding something by POSTING ON A FORUM!!!

Mr_TA

Fred,
What is "plenium"? Ductwork is partially sheet metal and mostly flexible insulated tubing (probably fiberglass insulation, wrapped with black plastic). Even if it contracts, how would that explain the farting sound? What part of it do you think may be contracting, the metal or the flexible tubing?
Thank you.

Fred

The plenium is typically the galvanized metal box on the top of the furnace that feeds the branches (round piping) to each register, depending on how it was designed. Many, if not most pleniums tie into longer, square/rectangular truck lines and the round ducts run off of various locations on that main trunk line. Both the Plenium and/or the trunk line can makes noises as they fill or empty of heated air. It's just the sides or bottom/top popping in or out during or after a heating cycle. I'm not saying that is your issue but it is one to listen for. It may be.

Mr_TA

Fred,
That sounds very compelling actually. I'll experiment tomorrow with turning it on and off and standing next to the plenium and the trunk line to watch. Thanks a lot!

BobC

@Mr_TA

I'm sorry if I offended you, that was not my intention.

Any pipe or duct will change dimension when temperature changes and that can cause it to rub against anything is goes through. That rubbing can cause any number of sounds, if you can localize it I suggest cutting up some milk jug material to line the openings.

Bob

Mr_TA

BobC,
Hahaha you offended me by apologizing!
I don't think it's a rubbing sound though. It has a completely different pitch to it; I can tell something rubbing against wood - and this is not it.

JUGHNE

Could it be the inner liner on the flex duct has a small hole in it and pressurizes the outer vinyl, and it slowly "passes " the air (gas) out thru a small hole in the outer vinyl covering. WAG

You could turn on the heat, listen when fan starts and stops during one heating cycle. Then switch system switch to off and turn on fan only, this will run the fan at the AC speed building more pressure and while you are in the basement you can control the furnace power switch to see if the sound shows up and when.

SWEI

JUGHNE said:

Could it be the inner liner on the flex duct has a small hole in it and pressurizes the outer vinyl, and it slowly "passes " the air (gas) out thru a small hole in the outer vinyl covering. WAG

I've seen something similar on wrapped commercial duct. Abandoned, un-patched hole or gap in the sheetmetal covered by ductwrap that's just sort of ballooning out.

Mr_TA

Thanks for the suggestions, I'll check it out tonight.

kcopp

Could it be the condensate pump draining?

JUGHNE

Good idea! But does he have one?.........I know the sound......a failed check on the pump and washback after a pumping cycle.