Loud HVAC Return Duct

Discussion in 'HVAC Heating & Cooling' started by s10010001, Nov 9, 2020.

  1. s10010001

    s10010001 Member

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    So, I'm starting to research something that's been bothering us since we moved into this house about 2 years ago. The HVAC return is in our main room, high ceilings, large room, tile floor. I know that's not the best for acoustics but it is what it is.

    The AC is loud, it's a newer unit. Runs great, sounds smoother but it's loud. So loud it hard to hear the TV when it's on. (Texas, so a lot)


    I'm wondering if the return/intake is too small for the bigger unit the previous owner had installed a few years back. It sounds like getting a lot of win noise. It's a 24x24 opening. Maybe a better grate cover that's straight ribs not louvered?

    Any thoughts?
     

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  2. fitter30

    fitter30 Well-Known Member

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  4. s10010001

    s10010001 Member

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    ok, ill go up and see how big my unit is in a bit. The box is insulated, well, it's some kind of foam board, painted with some grey stuff. so I guess that counts.

    The round duct looks to be 18" and not very "smooth" on the inside lol. Think I should build up a new box to go straight to the air handler? Doesn't seem that hard to do.
     

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  5. fitter30

    fitter30 Well-Known Member

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    Look at the condenser brand & model, air handlers can vary with blower motor hp and ac coil.
     
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  6. Dana

    Dana In the trades

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    In order for flex to meet it's flow specs it has to be stretched tight, with only wide (yet still stretched) arc turns. What you've got there looks a bit like a ball of mating snakes (not best practice, but all too typical.)

    [​IMG]

    There should be no kinks and minimal sags on runs that are suspended by straps.

    Does the rest of it look like this?

    [​IMG]

    or more like this:

    [​IMG]
     
  7. s10010001

    s10010001 Member

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    umm the rest is pretty ok looking I guess. Most of it goes up into long straight runs I cant see. (ceiling is only a few feet form the roof.) I don't really have much of a problem with air on the cold side in all the rooms. Just the noise level in the main room from the intake (air filter side). Just make a lot of swhoosing air noise.
     
  8. Dana

    Dana In the trades

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    Even the straight runs can create a lot of impedance and noise if not pulled tight, but fixing all sags and tight turns would be part of the first round of abatement. Replacing tight turns with radiused (not sharp throated) hard duct ells can sometimes help. Be sure to seal all seams in any hard piping with duct mastic.

    If there's any high frequency whistle & hiss it's likely that there are leaks at the register boots, air handlers, and hard-duct seams & joints. Ideally all seams (even those that look tight) should be sealed with duct mastic. The duct boots themselves need to be caulked/glued to the wallboard or subfloor etc where it enters/leaves the room. Polyurethane caulk is good for sealing duct boots to heterogeneous materials for any gaps up to 1/2". If the paint is smooth and the interior of the duct boot is clean it's often possible to use housewrap tape to seal those transitions. I personally prefer to use a "belt and suspenders" approach with tape over caulk-sealed boots-to-wallboard/subfloor joints.

    What type of air filter do you have? Cheap 1" fiberglass air filters usually have lousy air sealing at the edge slots where they are installed/removed. The sloppy fit makes replacement easy, but also contributes to noise when air is being sucked into the installation slot. Larger pleated filters do a better job, and usually have gasketed doors/panels to open when replacing filter media.
     
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  9. s10010001

    s10010001 Member

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    Yea the noise sounds like it's right at the filter. I will try to come up with a better filter setup first, with a gasket to seal it all up and see how that helps.

    and yea I generally use cheap 1" filters and replace them often. and yea I can see how air shoots through the sides.
     
  10. Dana

    Dana In the trades

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    When you have time & interest installing a better filter system really is "worth it" for most people. Large pleated media doesn't need to be changed very frequently at all, and does a much better job of cleaning up the indoor air than 1" fiberglass filter.

    More that 2 decades ago in my home I installed an Aprilaire 2400 air filter unit that takes 16" x 28" x 6" deep pleated media. The media gets changed at most 1-2 time per year, though there have been times where it's gone 18+ months between changes. Aftermarket MERV 13 replacement media runs between $30-$40, MERV 11 are usually under $30 any of them with free shipping from. I had to touch up/replace some of the gasketing after the hamster that got away ended up on the return side and chewed some of it up (sadly, he didn't make it :() but it's a very reasonable system, injects no noise and very little pressure drop at flow even at higher MERV #s, even after 6+ months of service. As long as we don't leave windows and doors open for long periods dust & pollen levels in the house stay extremely low even with MERV 10 media.

    I usually swap the media after the very end of the fall shoulder season. The higher humidity levels from the summer promotes enough mold growth on the crud trapped in the filter during the 30-50 days of no air movement that the exit air at the registers smell ever so slightly off during the first few heating cycles- that's my reminder. I've taken to writing the date of installation on a piece of tape on the access hatch just in case I sleep through the reminder, which has happened more than once, where there was no hint of mold smell at all early in the heating season.
     
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  11. s10010001

    s10010001 Member

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    Any good info on what I can use to rebuild my existing box to supports a deep pleated filter like that? I would guess it would need anew frame and front grate too,
     
  12. fitter30

    fitter30 Well-Known Member

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    Can't size duck work without knowing how much air is moving. Need model and brand of outside unit.
     
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  13. s10010001

    s10010001 Member

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    ill go get it.

    Does the return duct size matter, as long as it's big enough? I'm just looking at the return duct here, that's whats making the wind through a crack noise.
     
  14. Dana

    Dana In the trades

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    The key operative phrase is "...as long as it's big enough...".

    Oversized ducts are OK- low duct velocity reduces friction & noise. This is also true for supply ducts. Th only time oversized low-velocity ducts create a problem are when they run outside of conditioned space (such as those located above the insulation layer in an attic), made even worse if leaking and/or uninsulated. But that's a system efficiency problem (due to high parasitic losses/gains), not a noise problem.

    High static pressure systems typically run 700-900 feet per minute duct velocity, but higher efficiency (and quieter) systems usually have a design target velocity in the 300-400 fpm range, though that applies primarily to ducts fully within the pressure & insulation boundary of the house (for efficiency reasons). A home efficiency & comfort company out in CA called Energy Docs made this handy nomograph for sizing duct systems for efficiency, including for low static pressure, low noise:

    [​IMG]
    Do you have a model number on that air handler? (And condenser too, assuming a multi-speed air handler).

    The opening size on the return is 24" x 24". A square duct that size would deliver a velocity of only 400 feet per minute @ 1200 cfm, which would normally be pretty quiet. Using the 400 cfm/ton rule of thumb for high static air handlers that duct would be pretty quiet with a 3 ton air conditioner, but could be singing a song or even screaming with a 4 or 6 ton system. (Being an engineer I hate using rules of thumb in lieu of real design, but many HVAC installers disagree.)

    Not that it's going to be something you would change just to fix the noise, the odds are pretty good your existing system is way oversized for your actual cooling load (including the parasitic load.) An HVAC consulting company in the Atlanta GA area made this graphic plotting the square feet of space per ton of load ratio on few dozen Manual-J load calculations against house size. Most of those were in the Gulf coast states. (It does not include parasitic loads of ducts outside of conditioned space, but those are generally no more than 20% of the total load.)

    [​IMG]

    Rules-of-thumb designers typically use something like 700' per ton and spec the next size larger "just to be sure". For houses in the 2000-4000' range that's going to be 2x oversized, leading to lower comfort and sometimes lower efficiency and poor latent load handling. Ideally it would be no more than 1.2x oversized for the 1% design load, which would be sufficiently oversized to handle peak loads during heat waves, yet still deliver long comfortable cycles during normal hot weather.

    Right sizing a replacement AC or heat pump onto an existing duct system designed for twice the volume yields lower duct velocities, less noise, and a higher, more comfortable duty cycle during hot weather.

    To get a handle on your actual oversize factor it's useful to measure the duty cycle on the condenser on afternoons that dwell near the 1% outside design temp. It's amazing how often that turns out to be less than a 50% duty cycle (for single stage systems), which is an oversize factor >2x. Measuring the duty cycle of a multi-stage system becomes a more complicated, but is still possible by tracking amperage on the compressor across time, not just timing then on/off cycles.
     
  15. WorthFlorida

    WorthFlorida The wife is still training me.

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    You have an excellent air handler (Trane) and most need larger duct work to get the most efficiency out of it. The short piece of round flex duct is placing a pinch on the air flow. The square duct work is insulated board. About 1" thick rigid fiberglass with foil on one side. They are sold in preformed seams for making straight ducts, preformed boxes or 4'x8' sheets to custom cut & fit. Usually at the air handler to transition from the old duct work to the air handlers the duct is cut from sheets. All techs who install air handlers will have this stuff since the old is almost always cut out.

    You need to get an AC guy to rebuild the duct from the grill to the air handler the same size all the way. 18" round is too small that you have but for this short run it should have been one straight duct. Might have been OK for the old unit, By changing it out it should allow the fan in the air handler to be slowed down some thereby reducing noise. Some techs will increase the fan speed to overcome duct work that offers too much resistance. As suggested above going to a larger filter that can be incorporated in the new duct work can reduce the noise some.

    When air handlers are installed in closets or garages, the intake side of the air handler is wide open to a grill through the wall. They sit on a framed box lined with fiber board or drywall, depending when the home was built. That is not smaller than the square inches of the air handler. Your air handler looks like is about 24" x 24". That is 576 square inches, an 18" round is 255 square inches. Most returned grills installed are 18x24, 20x20 and many variants but with an area of 400 square inches minimum. Thought the air handler might be built as 24x24 the actual size for the air to flow through the coils is less.
     
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  16. s10010001

    s10010001 Member

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    Thanks for all the info guys. I just got my water quality issues sorted out So now it’s time to start planing the hvac noise. I’ll work on better ducts for sure but I’m going to start with the return air. I’m gonna make the box go all the way to the handler without the smaller round duct. And I’m looking to install a bigger (thicker) filter. 24x24x4-6

    Anyone recommend a good place to get the return air grill (frame part) and filter mount for theses thicker filters?
     
    Last edited: Dec 26, 2020
  17. WorthFlorida

    WorthFlorida The wife is still training me.

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    You would want to be larger than the air intake of the air handler. 24x24 grill is too small. As Dana explained the larger the grill work the slower the air will flow to reduce the noise level. The old work remaining is a good size to meet up to.
     
  18. DavidDeBord

    DavidDeBord Member

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    Correct me if I'm wrong, but, is that Flex mounted with out adjustable 90 degree Elbows?
     
  19. DavidDeBord

    DavidDeBord Member

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    One Return,... is going to make a lot of noise, no matter what You do.

    Somehow, you need to split that duct in to at least one, hopefully more, so as to pull Return Air, from another location, which would cut down the volume of air, being pulled in, as well as increase the efficiency of the system.
     
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