Furnace return plenum "bangs" when blower motor starts/stops

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wrote:

Im where houses are 200 yrs old, but AC is the only reason you actualy need returns on every floor, sure its probably more even heat with returns but I think his are undersized, even a small 2" vent in the basement would help and circulate heat better, but if he has AC he need a pro to evaluate his ductwork sizing, whith my new unit thats exactly what the installer did and found my returns to small, we added alot more and it help balance temps.
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Parts of our house are over 200 years old in fact... and the central heating was probably put in about 100 years ago.
We do plan to put in AC which as you note requires returns on the higher floors to work properly -- in fact, one of our reasons for not investing in the HVAC work now is that we want to do it all right when we add AC and do renovations rather than doing it half-assed multiple times...
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Actually, I think the banging problem became much worse because I recently closed up an "inadvertent" basement return. Specifically, the bottom end of the plenum was open but sitting on the rough basement floor. I had previously used metal tape to try to seal it off but recently did it right with a sheet metal cap. So, in a sense you are right.
But my understanding is that it is not a good idea to do that for several reasons: 1. It draws in the cooler (unheated) basement air which needs to be heated and ultimately results in either heating the basement or drawing in more cold air from outside since the air has to be replaced from somewhere.
2. The negative pressure could draw in more radon (and we have that here in granite-laden New England). This is a concern particularly since our basement floor is 150 years old so it is not fully intact and we also have a crawl space with only a loose fitting vapor barrier covering the raw earth.
3. If there are any open combustion chambers in furnaces or water heaters, the negative pressure may draw combustion gasses and CO into the basement and from there through the return into the house.
3. More generally, you will draw in fumes and other potentially undesirable odors, gasses, etc. from the basement.
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1. 2. 3. 3. sure if you want a cold unused basement,
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Doesn't sound like you'd want to use his basement even if it was warm.
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blueman wrote:

You don't necessarily need to stiffen the duct. If you can get inside you can wedge a piece of wood between middle of the sides that 'oil can' (pushing them apart).
You can put blocks of wood on the center of the sides that 'oil can' and tie a rope around the duct to push the blocks in. (With a little thought you could the same thing much better.)
--
bud--

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