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The return "ducts" in my house are run through building cavities, and I've just about had it dealing with them.

I do have a laundry chute to sacrifice. I am toying with the idea of using that space for a single, large return duct; which would have grates on the first and second floor (and maybe a smaller one in the basement where the furnace is located?).

That way, my entire HVAC system would be ducted, and I could have some hope of actually sealing it. I am also not above putting vents in the doors, so that there is still air circulation when the bedroom doors are closed.

So my options: (a) Continue with the building cavity "ducts". This sucks because they seem to be returning air from just about everywhere except where they should be (e.g. pulling air through fiberglass, making it dirty and moist; pulling air from the garage because it's not sealed correctly) and so on.

(b) Seal off all of the existing return cavity "ducts" and have one massive duct that takes the place of the laundry chute (it looks like I have room for a 16x16" duct in there). It would have two or three large grates on it. Rooms with doors might have some form of grate added to allow airflow with the door shut.

My question is: which option is less bad? Is this a valid solution?

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  • "opinions and experiences" make for a very poor fit at Home Improvement. Check out What types of questions should I avoid asking?
    – FreeMan
    Commented Dec 14, 2021 at 18:19
  • Do your internal doors all have vent panels in them to allow air movement between rooms? Also, can I have your laundry chute? If I were your laundry person I would have something to say about its deletion.
    – Harper - Reinstate Monica
    Commented Dec 14, 2021 at 22:18
  • @Harper-ReinstateMonica, I am unfortunately the laundry person. I don't want to give up the chute of course, but it seems the lesser evil. If I had unlimited money and time, I'd gut the house entirely, run proper ducts and re-wire/re-plumb the place, but... have to pick and choose my battles. (Internal doors do not have vents - I will either add them, or as MonkeyZeus suggests cut an inch off the bottom of the relevant doors).
    – negacao
    Commented Dec 15, 2021 at 0:07
  • Unless there's something in the fire codes I am missing, I don't see why a laundry chute couldn't do double duty. Change the upstairs doors to ones with gratings. Certainly better than fiberglass microfibers in your lungs. (just because asbestos gets all the press, doesn't mean the other mineral wools are harmless.)
    – Harper - Reinstate Monica
    Commented Dec 15, 2021 at 1:54
  • @Harper-ReinstateMonica That sounds like a neat idea. Right now the laundry chute has a 8" duct for the laundry to go down to a wooden box in the basement. Unfortunately that door is in the upstairs bathroom. But maybe with some work I could leave that setup alone, and use the space around the laundry duct as the return supply (e.g. put metal or thermopan on the inside walls and add grates).
    – negacao
    Commented Dec 15, 2021 at 11:10

2 Answers 2

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I do have a laundry chute to sacrifice. I am toying with the idea of using that space for a single, large return duct; which would have grates on the first and second floor (and maybe a smaller one in the basement where the furnace is located?).

How big of a return would this space allow? If your furnace blower motor is 2,000 cfm then you need a 20" round pipe or 30"x12" rectangle or 18"x18" square.

Duct Sizing Chart

I am also not above putting vents in the doors, so that there is still air circulation when the bedroom doors are closed.

If you cut off an inch from the bottom of your doors then that would likely suffice for air circulation.

Continue with the building cavity "ducts". This sucks because they seem to be returning air from just about everywhere except where they should be (e.g. pulling air through fiberglass, making it dirty and moist; pulling air from the garage because it's not sealed correctly) and so on.

Wow, that sucks. HVAC work is expensive so I hope you'll be able to DIY a solution.

Seal off all of the existing return cavity "ducts" and have one massive duct that takes the place of the laundry chute (it looks like I have room for a 16x16" duct in there). It would have two or three large grates on it. Rooms with doors might have some form of grate added to allow airflow with the door shut.

Seems like a fine solution to be honest, you just have to check your furnace's CFMs to make sure you're not choking your motor and sending the furnace to an early grave. At worst you might have to maintain a wall cavity or two.

Joining all of the sheet metal to your main return in the basement is not for the faint-of-heart.

Which option is less bad?

Which option are you capable of executing or paying someone to do?

Is this a valid solution?

Yes, one central return is valid and used in modern construction.

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    But whether a single central return is the most beneficial solution for good HVAC performance is another question entirely. May be the least expensive, but not necessarily the best for the homeowner for even temperature distribution.
    – Milwrdfan
    Commented Dec 14, 2021 at 22:07
  • @Milwrdfan don't get me wrong, I'd prefer to have proper return ducts in each room. But that's not an option right now without extensive work - walls would have to be moved, ceilings lowered, and so on.
    – negacao
    Commented Dec 14, 2021 at 23:00
  • @MonkeyZeus, after actually poking some holes and looking in there - I have more like 16 x 32, so it sounds like plenty of of room. From my perspective, the single main return duct is better because it's something I can actually do, without tearing down a bunch of stuff.
    – negacao
    Commented Dec 14, 2021 at 23:02
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    @Milwrdfan What's the lesser of two evils: dirty and moist fiberglass air which might be pulling in air from outside the building's thermal envelope or a large central return duct?
    – MonkeyZeus
    Commented Dec 14, 2021 at 23:22
  • @negacao Well that's 42% larger than the requirement for a 2,000 CFM motor. Somehow I doubt you have a 3,000 CFM motor but you should check your furnace model number to be sure. The hole in the wall per floor only needs to be big enough to accommodate the CFMs of that floor.
    – MonkeyZeus
    Commented Dec 14, 2021 at 23:26
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Some pros and cons:

  • Generally you may initially cause temperature imbalance between the bedrooms and need to correct that.
  • If the thermostat and return are both in the hallway outside bedrooms with closed doors, the system will behave differently and you'll need to find adjustments to make the bedrooms comfortable.
  • Sounds like you do not have a zoned system. If you do, especially upstairs/downstairs zones with the downstairs off at night, you'll find that moving the upstairs returns into the upstairs hallway will increase energy usage, create balancing problems, and if the upstairs thermostat is in the hallway will cause bedrooms to be too cold until you learn to adjust and balance. All this is because you'll be mixing in more unconditioned air rising from downstairs at night.
  • You can move the air filter from the air handler to the main intake grill, making it easier and more inviting to change at the correct frequency.
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  • Thank you. The system is not zoned; thermostat is on the first floor in dining room, well away from the planned returns. I really like the idea of return grates with filters in them.
    – negacao
    Commented Dec 15, 2021 at 0:10

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