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Background

Sometime after we replaced our furnace and added an evaporator coil for a heat pump, the ground level return vent on our HVAC system started making a ringing sound. I was investigating and noticed that the actual duct opening (red dashed line) is quite a bit offset from the grille cutout (blue dashed line) in the wall: Dimensions of return vent

Because the actual duct opening is higher than the grill, the air has to perform a sort of S-turn: first having to make a sharp 90° turn up within the stud bay cavity to go from the grille to the actual duct opening , then having to make a 180° for the air to go back down before making a final 180° to go back up into the filter and then furnace: Air flow diagram of existing installation

When I stick my hand in the overlap area, there's quite a bit of velocity, so I suspect that's what's causing the grille to ring.

Other Details

  • The existing duct cutout is 16x13" (WxH)
  • The existing grille is 14x20" (WxH)
  • Furnace is a Bryant Evolution Model 880TA36070C144-A
  • There are a total of two return vents

Questions

  1. Is this normal? (I suspect not, but wanted to ask)
  2. What's the best way to correct this?
  3. Would correcting this likely resolve the ringing? (I suspect yes, but wanted to see if there are other things I might need to address...such as the size of the of the upper level grille and duct cutout)

Possible Solutions

Here's some solutions I've brainstormed. FWIW, my background is electrical engineering and I'm not afraid to get my hands messy; I do have a wife and kids I need to keep happy though.

Option A

Cut into the drywall to relocate the grille (green dashed line) over the existing duct cutout (blue solid line). I would probably need to use a 14"x14" grille because the duct cutout goes behind the plane of the door, so I can't make it 14"x16". I would put a piece of blocking at the top and bottom of the new grille section (magenta dashed lines) and patch the old opening with drywall (white rectangle).

Pros

  • Relativity easy to do
  • Doesn't require modification to existing metal duct work

Cons

  • Requires the air to make a 90° and then a 180° (still better than current airflow though)
  • Return air location would be about 18" from the floor...not sure if that's too high up or not

Diagram for Option A

Option A

Option B

Trim duct work down (baby blue dashed line) and put fire-blocking (magenta dashed line) in to reduce the opening to be the same height as the horizontal passage to the furnace (for better airflow...less turns). The return grille would be about 14"x14". Patch dry wall above (white rectangle).

Pros

  • Better airflow, I think

Cons

  • Requires cutting in to duct work

Diagram for Option B (and sort of Option C)

Option B and sort of Option C

Option C

Same as option B, but don't patch drywall. Just put fire-blocking in and use existing 14x20" grille.

Pros

  • Better airflow, I think...though maybe slightly worse than Option B
  • Doesn't require drywall patching

Cons

  • Requires cutting in to duct work

Option D

Call a professional...this is way out of the league for DIY. I don't think this is the case here, but I want to leave it on the table because I don't know.

Pro

  • I can sit back and do nothing

Con

  • Costs money

Option E

Something else...what do you suggest?

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  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
    – Community Bot
    Aug 4 at 21:21

1 Answer 1

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Turbulence and resonance are mostly voodoo. I think your best approach is to find out precisely what part of the HVAC system is ringing (vibrating / resonating) and then find a way to damp the vibration.

Start touching parts, frames, panels and ducts to see if the ringing sound changes. Watch out for moving parts and keep loose clothing away. When you find what specific part is ringing, do whatever is necessary to damp the resonance.

There's a black, sticky butyl caulk used in HVAC contracting that comes in strips and sheets. It sticks to any clean surface and can damp vibrations. If a large panel is ringing you can screw or pop rivet a weight to it and change its resonant frequency, and the ringing will stop. Wedges or rods propped or jammed between fixed but vibrating parts can work too.

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  • The ringing/vibrating is definitely coming from the louvers on the grille, particularly toward the top of the grille. I can lightly touch them with my finger and feel it, and if press a bit harder the ringing stops.
    – fergbrain
    Aug 5 at 18:24
  • @fergbrain Great! Problem almost solved. If cost is not an issue, you could replace the grille with another model in the same size. It will almost certainly have different resonant properties. To attempt fixing the one you have, try ever-so-slightly twisting each louver to a different angle using smooth jaw pliers. The stress from twisting may change the resonance. Or rotate the grille 180 to change the turbulence. Or take the grille off temporarily and fill the back of each louver with hot glue or silicone caulk to change its mass and therefore its resonant frequency. Imagination: Engage!
    – MTA
    Aug 5 at 19:51

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