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In the master bedroom, there is one of four intake vents in the house, and this particular one is very loud.

Our house has two vertical air shafts that run parallel to each other, one for supply and one for return. The intake in the master bedroom is connected directly to the return shaft - no duct between it and the shaft. Directly behind the vent is a 3-story drop straight down into the air handler.

I have held a piece of paper next to the offending intake and the paper gets drawn strongly against it.

I tried this on the middle floor as well, as its intake is the largest but still right on the return shaft. Similar effect, but not quite as strong. This vent is also somewhat loud, but not as loud as the master bedroom one.

I then tried the paper on the intake directly outside my room, which has a long duct connecting it to the return shaft. Not nearly as much suction as the one in the master bedroom.

I did the same thing on the basement return, which is the smallest of them all. That one had more suction than the top floor hallway one, but nowhere near as much as the middle or master bedroom intakes.

Vent sizes:

Basement: roughly 14x14 inches, estimated 10 feet of duct that is 8-10 inches wide, connects to bottom of return air shaft. Basement vent

Middle floor: roughly 18x18 inches, directly on return air shaft. Middle floor vent

Top floor - hallway: roughly 18x18 inches, estimated 20 feet of duct that is 14 inches wide, connects to top of return shaft. Hallway vent

Top floor - master bedroom: roughly 14x14 inches, directly on return air shaft. The problematic one. Room vent

Here's where the two vertical shafts connect to the air handler in the basement: Air handler

The two shafts are more visible here: Air shafts

How can I make the vent quieter? Also, how can I give the one in the hallway more suction?

Here's a not very professionally made diagram of what the vents are like: Diagram

EDIT: The sound is an air rushing sound, not whistling or any other noise. Either that or it could be the sound of the fan.

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  • What is the cfm of the fan. Can you get us pictures of how the three return air ducts are attached to the air handler?
    – Kris
    Commented Oct 21, 2021 at 17:02
  • All four are attached to the main vertical shaft. It is a roughly 20x24 inch duct that goes from the basement to the attic. This connects via a right angle and goes directly into the air handler, in the basement. I'm not sure the CFM, but the system is a 4 ton system. Two intakes are directly connected to the shaft, and the other two are connected via short duct sections. There is only one air shaft serving all four returns. Similarly, there is a supply shaft that runs parallel to the return air duct.
    – Proxy303
    Commented Oct 21, 2021 at 17:28
  • You prob have 1600 cfm so the three RA vents total 807 square inches which is appropriate. Are each of the RAvents filtered? Or does the unit pull all that air through a single filter at the air handler?
    – Kris
    Commented Oct 21, 2021 at 20:57
  • The air handler has a single filter in the path of the return air. There are four vents, but one was easily overlookable. Edited the post to make it more easily readable.
    – Proxy303
    Commented Oct 21, 2021 at 21:33
  • 1
    Added a diagram of the system (return side only)
    – Proxy303
    Commented Oct 22, 2021 at 14:52

1 Answer 1

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In this case “size matters”. (Sorry, but I couldn’t wait to use that phrase.)

Your 4 ton unit pushes a certain amount of air to various spaces. Therefore, that air must come from somewhere, so the unit will draw sufficient air through the existing ductwork.

If the existing ductwork is sufficient size it will easily draw a sufficient amount of air. If the existing ductwork is too small, it will draw air extra fast through the ductwork causing a “whistling” sound. This size and sound can be calculated. However, as a rule of thumb, the return air should add up to be about the same size as the supply air size ductwork.

There are many factors that go into determining the size of the return air ductwork, including number of elbows, register sizes, inline filters, etc. However, be aware it’s based on ductwork from the unit all the way to the registers.

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