Door undercut maximum?

[tims982345]

To help with a return insufficiency, what's the most you would undercut a bedroom door with tile on one side, shortish carpet on the other, such that it wouldn't look stupid, compromise privacy, etc? The current undercut is 1", and just by partially shutting the door as a test, I'm not sure that less than an additional 2" would solve the problem. I think a total of 3" would be way too much. Comments? I know other alternatives include a jump duct, but I've spoken to at least three HVAC guys over the last few years, and none have ever suggested it, and none have been enthusiastic about installing a proper return. They always go to the undercut.

[udarrell]

Well, how many CFM is supposed to be delivered to & returned from that room?

May need a properly sized Return Air Grille installed in the door or on the wall connected to the hallway.

[energy_rater_La]

there are options.

jump ducts are not that difficult unless you have a really low roof.
2 supply boxes and grills and a few feet of duct.
it does just what it says..jumps from one room to the next.

or you could do transfer grills.
above the door on the hallway one grill is
installed.
above the door on the bedroom side of the same
wall another transfer grill is installed.

both have to be in the same stud bay, but
if you put them opposite of each other the noise/light
will transfer also.
off setting them (again with in the same stud bay)
with one higher and one lower will reduce noise and light
from one area to the other.

hollow core doors only have about 2-3" of solid wood
at top and bottom. cut too deep and you destroy the door.

undercutting doors is easy. but it doesn't always work well.
a 1" undercut will return one supply.
what if you have 2 supplies in the bedroom
or 2 in br, one in each closet and one in the bathroom?

then you get insufficient return air, and it 'ghosts' at
the undercut of the door.
one type of ghosting is when the air is being
sucked thru the carpet fibers as it tries to get back to the return.
this will make a dark mark on the carpet.

if carpet ghosts at wall to floor it is notice that the house
is under a negative pressure and sucking between
slab to sole plate.

some hvac folks will fuss at you for burning candles..even if you don't.
but it is a sign of insufficient return air pathway. candles always
get the blame... you just put them away before hvac co was called..LOL.

I hate it when any tradesperson takes the easy way
rather than the right way to do a job. just speaks
poorly of their ethics IMO.
just a personal observation.

best of luck.

[tims982345]

There are four 7" supplies behind that door, which is a hollow door with 2 column by 3 row decorative rectangular carvings embedded in it. I don't think a grille would work in it. I did a coarse test in which I measured the distance the door had to be cracked before the air noise level would return to normal, and it had to be cracked a full inch. So with the door 80" tall but only 30" wide, I don't think undercutting is going to work. Frustrating.

The transfer grille idea is interesting. They would be in wall maybe a foot behind two of the supply registers which blow out into the room. Would that short circuit the air flow? Any problem hiding them behind furniture, maybe with a couple inches clearance? What about dust or attic smell? I guess it wouldn't be worse than what I'm already pulling when the door's shut.

I think the jump duct may be the best bet. I guess it would go on the opposite side of the room, away from the door and supplies, but again I wonder about short circuiting the air flow. Is that an issue? It's a bedroom with an attached bathroom and large closest, and the jump duct would sort of be in the middle of the four supplies, two in the bedroom near the door, and one each in the bathroom and closet, all in the ceiling.

[energy_rater_La]

The transfer grille idea is interesting. They would be in wall maybe a foot behind two of the supply registers which blow out into the room. Would that short circuit the air flow?

supply registers have fins that direct the air flow. are the fins adjusted to blow to the door? I can't imagine that they are. if necessary change to a 4 way grill with adjustable fins. then you can pretty much put the air where you need it to go.
Any problem hiding them behind furniture, maybe with a couple inches clearance?

just like with any return you would want the area to be free in front of them.
if they are above the door you won't have that issue.
you could always paint them the color of the wall to blend in.
if you don't locate above the door where would you put them?
just asking cause I can't see it..LOL.

What about dust or attic smell?

you are only passing the air thru the wall, which shouldn't be connected to the attic.
walls have top plates, unless balloon framed. balloon framing was popular before
hvac when houses cooled passively.
the air moving thru the transfer grills won't be as pressurized as it was under
the undercut of the door, because you will have more area for it to move through.


jump duct quesions..I've never installed a jump duct to jump a long area. usually from
a couple of feet inside the door to the hall beyond.
the closets and bathroom doors are kept closed?
they should return to the bedroom thru the undercuts of the doors, then to the hall
via the jump duct.

think about it. returns are usually in a hall and supplies in the bedrooms beyond them
and air returns fine. (we don't have returns in every room here as the norm)
air doesn't think like you are doing..it simply takes the path of least resistance.


best of luck.