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I have a 3 year old home and the builder installed a return vent on the main intake trunk that connects to the furnace. This is super annoying for a few reasons:

  • It pressurized the first floor. This is very noticeable on the first floor when opening the door to the basement. You feel the rush of air being sucked into the basement.
  • It pulls odors and dust from the basement into the first floor.
  • It is heating the basement unnecessarily by pulling hot hair through the underside of the basement door

I tried capping the return vent but it resulted in the blower motor making a slight noise. I mentioned this to my HVAC guy and he said this is a sign of the motor working too hard and i should remove the cap. He said there is insufficient return air coming from the first floor. I will note that prior to the cap, i have heard this same noise coming from the unit from time to time.

We have 2 return air registers on the first floor for about 1500 square feet of living space. They measure 11x11 and 10x13 but the 11x11 one seems to be doing most of the work.

What is the best way of solving this problem? My HVAC guy offered an approach of turning the basement into a full zone and utilizing dampers to have that furnace operate both the basement and first floor zones. This would be useful for finishing the basement which is something I am interested in. He was a bit hand wavey about how he would solve for the pressurization problem. I would really need to see a full system diagram to be confident. Is there a simpler solution to this problem? Basement return

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    It sounds to me like one or both of your 1st floor return ducts are at least partially blocked. I'm not sure on the calculations for the vent/duct sizing, but you shouldn't be drawing so much in from that basement one that the furnace struggles if it's blocked. – FreeMan Dec 2 '20 at 14:58
  • what is the best way to fix this? – bradforj287 Dec 2 '20 at 15:16
  • Remove the grate & check for obvious things in them. Otherwise, look for info on duct cleaning. I don't know how DIY it really is vs hiring professionals. Wait for others to chime in to see if there are other suggestions/answers. – FreeMan Dec 2 '20 at 15:32
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    Your statements seem contradictory. If this is depressurizing then opening the basement door would pull air from the basement into the first floor. If dust etc. is being blown into the first floor, then the system is obviously pulling air into the ductwork from the basement. So, is there any grate or return in the basement which feeds the system? By the way, is there a second floor? – Carl Witthoft Dec 2 '20 at 15:55
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    WHoops my bad. I updated the post. It is pressurizing the first floor and depressurizing the basement. – bradforj287 Dec 2 '20 at 16:04
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Your main trunk duct may be undersized for the return air on the main level. If this is a new system that used the existing ductwork the fan speeds may be set for the highest efficiency. In my opinion the system is not properly sized or set up correctly not enough info but one of the 2 is true.

For a fix: It looks like a Rheem but not sure, the Rheem furnaces / air handlers I have installed have multiple motor speed taps. Running the system at a lower speed should help reduce the pressure imbalance.

Adding a larger intake trunk duct to pull air out of the upper floors would be a option since there are already 2.

If you add heat to the basement this will eliminate most if not all of the imbalance but I would absolutely want a damper on the supply duct for the basement so you can balance the flows or much the heat / ac will end up in the basement.

Even a single heat duct down there may reduce the pressure imbalance.

I would check to see if you have a multi tap motor that could be run at a slower speed if you are not ready to add heat downstairs.

I just re read and the system is 3 years old so you are past the point where you can usually get something done for free.

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  • when you say the main truck duct, are you referring to the one oriented vertical adjacent to the furnace or the one horizontal along the joists? – bradforj287 Dec 2 '20 at 16:38
  • The one along the side appears to be the intake as the electrostatic filter? is there and that is the one the hvac guy added the return to causing the upper floor to pressurize. – Ed Beal Dec 2 '20 at 16:43
  • I don’t know if we have an electrostatic filter. – bradforj287 Dec 2 '20 at 17:55
  • Ok that looks like one between the duct and the furnace. Similar size to the Honeywell’s I use and in the right location. – Ed Beal Dec 2 '20 at 18:33
  • Yes, the white box near the floor connecting the vent and the furnace contains a general aire filter. Not sure if it is electrostatic or not. – bradforj287 Dec 3 '20 at 14:38
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The only good news here is the opening isn't terribly large. You might be able to add a flexible 6" line to the entry and add another intake from the first floor. If you elect to do so, remember to use foil tape and not duct tape to seal the hole.

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  • "foil tape" = "duct tape" (designed to actually seal duct work) while "duct tape" = "Duck™ tape" (designed for making wallets or decorating your daughter's stuff). – FreeMan Dec 2 '20 at 17:13

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