How to Quiet a Noisy Forced-Air System | This Old House

How to Quiet a Noisy Forced-Air System | This Old House

when a thermostat calls for heat the burner comes on and starts to fire at the same time there's a fan which comes on and pulls air back from the return air duct right here from the building now it's pulled across this pleated filter I'm really glad to see that and now the fan sits right here and now it pushes that air up across the heat exchanger where it picks up the temperature now the heat of the air now goes up into this box right here this is called a plenum now the plenum has a couple of branches you can see one right here and this goes off to the second floor and another branch comes off the side right here and goes up to the first floor in each one of these branches you'll notice this this is a motorized zone damper now this is the pipe that sits inside the ductwork now in the thermostat is satisfied this is what it looks like okay with a call for heat now the motor how opens these blades right like that and it's open thermostat satisfied it closes like this now imagine what happened when this first install it did the ductwork system here let's say that he designed it for 50% of the air flow to go in the second floor and 50% to go to the first floor now you put this damper in and now it's closing off half the ductwork you've got a furnace putting out 100% of its force what happens it's now trying to jam that air into much smaller ductwork and it can lead to two things one is it can build up the pressure and have a velocity noise inside the open zone but it can also push against the closed damper so you might get a little whistle against these these damper blades right here and that's where I think it's happening to you okay so what do you think we can do to correct this well because it is a buildup in pressure we actually need to put in the equivalent of a thing called a relief valve in this case is called a bypass damper we need to cut in off the supply right here and relieve the pressure that's building up inside the plenum now to tie into the ductwork we're going to use these start collars now these are seven inch round it has a rubber gasket right here underneath the paper with a little bit of sticky back on it and it has a crimped connection on this side to accept our fitting and this one is going to install right there on the supply side on the return side we're going to use another start collar again with a gasket but this doesn't have any crimp right here it's going to accept the crimp fitting right there so now it starts by where I want to mount it and I'd like to get it on the flattest portion of this ductwork away from these creases so right about there looks good right there and now I want to find a center point I can I that up right there and now we want to drill this pilot head all right for cutting a perfect circle into ductwork there's no better tool than this this is a circle jig that's the center point that's adjustable pilot hole in I want to be sure I go clockwise okay that's it for the return side now the supply alright now we can install the start college with some zip screws these are eight by two quarter-inch self-tapping screws made for sheet metal now we're ready for the return side all right this is your bypass duct two elbows and there's a straight piece of pipe in the middle that is your bypass damper as a door inside of it it has a weighted arm to hold it shut and that seals against this gasket now it's going to install right here when you have that condition you talked about closing that first full of zone you build up the pressure here in the supply plenum now we have a way to relieve the pressure that higher pressure air pushes and can open this door now that air can divert between the supply and the return now what that does is it drops the pressure over here you no longer pushing against that closed damper no more whistle no more annoying noise I like to use some foil tape to seal up these seams alright we have sealed all of our new connections I've adjusted the weight on this bypass damper arm first of all thermostat is closed second floor is open now it's time to turn the furnace on we do the bypass damper should open you


  1. Works great.>>>    I was able to wire this up and install it in about an hour in an existing setup with a very similar fan and motor that was quite old. So far its been working about a year, we'll see how it does this summer.

  2. Sound-Proofing Furnace Doors – Tapping on my Goodman furnace doors makes a drum sound; Adding some kind of sound dampening material would soundproof the doors helping to eliminate furnace noise; I crumpled a bunch of black duck tape into a 3 inch round ball and then duck taped that to the back/inside of the bottom furnace door (which made the most noise); It does help however I do worry about that duct take coming lose someday and getting sucked into the blower fan up into the furnace catching on fire; I wonder what could safely be used; One thought is painting duct sealant on the inside of the furnace doors [Duct sealant is a rubberized sealant that would mute vibration noise well and would probably stick better/longer than duck tape would]. I had two 1 inch round clear plastic inserts that were making rattling sounds [The one in the top door is for seeing the flame; The one in the bottom door is for seeing an LED light display]; I removed those two clear circle pieces and applied a small amount of super glue to the top and bottom edges and popped them back into the furnace doors; That eliminated those from rattling when the furnace would run. The GOODMAN plastic sign on the front of the furnace would also rattle; I pulled it off and put five 1/2-inch round foam/rubber adhesive circles [You can buy at Walmart] on the back side of that GOODMAN plastic sign [one in each corner and one in the center] and popped the sign back on the furnace door [The adhesive foam/rubber circles sit between the plastic sign and the furnace door preventing the rattling sound].

  3. He should’ve mentioned that a 7” duct won’t work on all systems, the bypass should be sized correctly for each system. You should also have the collar on the return duct further from the furnace to prevent it from overheating and shutting down on a high limit fault.

  4. I always wonder why women don't get into HVAC jobs, they pay pretty well. It is almost 99.9% male dominated profession.

  5. I'm surprised a 7 inch could work, but it would have been real tough to put in a 10 inch. At least it's 150 – 200 CFM being bypassed. Hopefully she has a good zone control panel, not an old relic. Need a dump zone and especially with a bypass it has to regulate the discharge air temp. Bypassing supply to return the furnace will have to cycle on a discharge air limit or you'll be cycling on the hi temp limit in the furnace.

  6. Goes to show that comfort from climate control does not just come from temperature control… there are noise considerations too. A good contractor should be able to fix that.

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