Space under door for air return

3337 Views 18 Replies 9 Participants Last post by Edmiester, Jun 10, 2020

rdy2go

Discussion Starter · Jun 7, 2020

We have a bedroom that does not have air return and does not get conditioned properly. The door has a 1/4" bottom crack to the carpet. We had the door replaced a few years back, after one of or kitties was inadvertently left closed in the room and did a job on the inside veneer layer trying to get out. I suspect the door used to have a wider opening on the bottom.

The question is how wide the crack should be. The bedroom has two feeding vents that look like 3x12, but the feeder duct from the basement, if I got it right, is a 6" duct that switches to 3x7 in walls.

1 - 19 of 19 Posts

yuri

· #2 · Jun 7, 2020

Mine are 1/2" to the flooring in the bathroom and look OK.

1/4" to the carpet but I got extra thick underlay and with regular underlay would be 1/2"

I would try a full 1/2".

rjniles

· #3 · Jun 7, 2020

I would cut a vent in the bottom of the door.

Colbyt

· #4 · Jun 7, 2020

I always thought that 1" clear was enough. According to this article https://www.energyvanguard.com/blog/can-door-undercuts-work-as-return-air-pathways 0ne inch clear will pass 60 cfm but a 6" duct can deliver 100 cfm.

Before I would leave more than an inch I would leave the door open or add a return.

I have 4 ducts delivering air to the MBR / MB / closets and even with a return air in that section I have enough negative pressure to pull the door closed when it is only cracked a bit.

Reactions: 1

yuri

· #5 · Jun 7, 2020

I would use 1" unless you cannot stand the look of it. Then again you may have no choice if you want comfort.

Reactions: 1

SeniorSitizen

· #6 · Jun 7, 2020 (Edited)

Maybe someday someone will build a door with a 4 inch pressure activated louver at the top that will pass the all important -- like the looks of -- so the door can be closed, locked and cold and warm heated air won't be forced into the attic via all those little places and out the leaking windows.

EDIT: EDIT:

Bud9051

· #7 · Jun 7, 2020

To pass that 100 cfm Colbyt mentioned there is a large fan pushing that air. With limited return area that creates an increase in pressure inside the bedroom necessary to push the return air along whatever path is supplied. Design parameters I've seen is that the room should never be pressurized above 3 pascals which is a very small pressure. If a return duct is present is helps out by pulling return air back to that large fan.

Without a return duct the open return area needs to be HUGE to maintain a room pressure below the 3 Pascals. When pressure increases in the room it forces conditioned air out through all the leaks in the framing and that lost volume will be replaced by outside air coming back into the house somewhere, expensive.

Long winded but my point is a passive undercut return path is a very poor option as 3 or 4 inches (my guess) would be needed to keep room pressure below 3 pa.

Note hvac usually references air pressure in inches of water column and
1 inch wc = 250 pa.

Is there any place where you can add a return duct.
If not consider a jump duct or high to low transfer duct through a qall to a common hallway.

Bud

disclosure, I did not read this link: https://www.cedengineering.com/userfiles/How%20to%20Size%20and%20Design%20Ducts.pdf

and the energyvanguard link above is good.

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SeniorSitizen

· #8 · Jun 7, 2020

Bud9051 said:
If a return duct is present is helps out by pulling return air back to that large fan.

In some instances it may help very little because if that return is at the far end of a house and there are more larger returns closer to the blower nearly 0.0 air may be returned through that duct.

That's a real touchy situation.

Bud9051

· #9 · Jun 7, 2020

As long as multiple returns share a common space they will experience the same room pressure. A long return duct will have a pressure loss over the length of the duct, but still better than no return.

I'm sure the answer lies somewhere in those two links, but I'm getting old.

Bud

SeniorSitizen

· #10 · Jun 7, 2020

But wouldn't it be disappointing to install a return to discover the hollow cord door still begins to pull closed at a 2" opening with a 3/4" under cut at the bottom?

57TinkerMan

· #11 · Jun 7, 2020

rdy2go said:
I suspect the door used to have a wider opening on the bottom.

If you leave the door open a bit does the room get "conditioned" properly?
If it does, (I) would use high/low return grilles using a wall stud cavity (or 2). Two holes in the drywall, screw in a couple of grilles and your done. No light transfer under the door and reduced sound travel. Your local building dept. may not allow easy effective fixes like this anymore.

Reactions: 1

Bud9051

· #12 · Jun 7, 2020

SeniorSitizen said:
But wouldn't it be disappointing to install a return to discover the hollow cord door still begins to pull closed at a 2" opening with a 3/4" under cut at the bottom?

Yes that would be disappointing, but that is why they have calculations to determine the size duct needed for the distance required. Not my background as energy auditors were banned by my state from doing any hvac work. I only get to view it as it relates to my energy needs. I can't mention on this forum what I think about my state and their policies.

Bud

SeniorSitizen

· #13 · Jun 7, 2020 (Edited)

57TinkerMan said:
If you leave the door open a bit does the room get "conditioned" properly?
If it does, (I) would use high/low return grilles using a wall stud cavity (or 2). Two holes in the drywall, screw in a couple of grilles and your done. No light transfer under the door and reduced sound travel. Your local building dept. may not allow easy effective fixes like this anymore.

That would work very well on a wall of one bedroom of our three and that would be a sure way of the end result of being about as close to neutral pressure as possible. The only available area of our other two bedrooms would be grilles in the transom area above the door.

But here we go again; the -- how does it look problem --

EDIT: EDIT:

yuri

· #14 · Jun 7, 2020

Why the heck did anybody allow rooms to be built w/o return vents??

I guess there is no actual code to follow with HVAC ductwork. Just a gas code which pertains to gas lines and venting. After that you can slap the minimum ductwork on and let er fly.

Rooms w/o returns are as rare as hens teeth where I am. Only some very cheap tract homes had one central return at the top of the stairs and they are very rare where I am.

Kinda sad to see how sleazy some builders are when it comes to saving a few bucks here and there and people don't know what they are not getting for their hard earned money.:vs_mad::vs_mad:

chandler48

· #15 · Jun 7, 2020

I agree with the vents, but usually cut the doors off 1/2" AFF when possible. Hey, Yuri, welcome back. I thought they had shipped you off to Siberia

Reactions: 1

57TinkerMan

· #16 · Jun 7, 2020

SeniorSitizen said:
how does it look problem

Looks interesting but not sure you'd relieve enough air.

https://www.buildingscience.com/documents/reports/rr-0408-the-dekieffer-bypass/view

SeniorSitizen

· #17 · Jun 8, 2020

57TinkerMan said:
Looks interesting but not sure you'd relieve enough air.

https://www.buildingscience.com/documents/reports/rr-0408-the-dekieffer-bypass/view

I'm pretty certain a 14 x 24 transom grill would flow more air than a 1/2" or even 6" cut under the door would.:smile:

rdy2go

Discussion Starter · #18 · Jun 8, 2020

I will start by cutting the door to leave 1/2" or even 5/8" under. I will check other doors to see what they have and how they look. Then see how it works out.

Opening HI/LO returns to open the space is problematic because it is just above the house entry area and I am afraid the low one would look odd. The high one could probably be acceptable because it would be very high.

Running a new dedicated return might be possible, but it may have to cross one or more joists. I would have to take measurements. It would have to go from basement to second floor through a closet in the first floor, just parallel to the run of the water heater exhaust which goes enclosed in drywall on the right side of the closet. Obviously the return duct would be in its own drywall enclosure, but in the basement, the duct elbow would be a few inches away from the metal exhaust, and pretty much above the water heater. I am not sure about code implications on that.

Edmiester

· #19 · Jun 10, 2020

My bedroom doors all have a 2" gap at the bottom. They used to have carpet, which has since been removed and hardwood flooring. The Master bedroom still has carpet and has a 1" gap plus a air relief gap at the top 4 x 6 with a grate covering on both sides. This allows the warmer air to escape and get drawn back to the unit for circulation.

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