Forced-air heating ducts - should they meet flush with vent grates?

This is probably a dumb question, but I know nothing about this sort of thing.

We’re in a new apartment, a small 3-flat brick building that was bought probably not all that long ago (maybe a few years?) by the current landlord. We’re on the top floor.

The other day I’d removed one of the forced-air heating vent covers in the bedroom (the hot air “depositing” kind, not the air return duct) and noticed that there’s about an inch gap all around between the duct and the actual hole where the vent cover goes. I’d never done this before, so I had no idea if this was normal/common/no big deal/very bad thing/what. I guess I was just kind of thinking things like how anything inside the top of the ceiling could just get in through the vent (bugs, dust, grit, cold air from inside the roof) and how that might not be the most energy-efficient thing. Also, our bedroom was chillier than the rest of the apartment last night but since the very high winds were smacking right up against that window all night due to the incoming cold front, that’s also a very good explanation…

Since my landlord doesn’t pay for heat, he doesn’t care about fixing simple energy-efficiency things. When we mentioned that we’d put extra weatherstripping and draft stoppers around the apartment entry doors since there were significant gaps and the stairwells are cold in winter, he sniffed that since we were at the top floor we’d have a lesser heating bill than the other floors anyway. :rolleyes: So we’re probably going to have to patch it if this is not a Working As Intended situation. (My hypothesized fix was duct tape from duct to edge of ceiling.)

Are you saying the gap is open to the ceiling cavity? If so, not only can outside air get in, but your forced air will get blown into that space. It’s not unusual for the size of the hole to not fit the vent exactly, but it’s typically sealed with spray foam or caulk.

See this duct picture in this article. The duct has yet to be sealed and will let air get into the ceiling cavity. The article talks about how to seal it up.

Not normal. Anything you can do to seal that gap will help, tape may be the easiest way to do it.

Yes, it’s open to the ceiling cavity. I immediately assumed that was not a good thing, but then thought, ‘self, you know nothing about HVAC, that may be dumb to assume.’ I see the linked article suggests metal tape, which was my second thought after duct tape. Thanks for confirming my suspicions.

Metal tape is expensive (around $15 a roll) but there is NO substitute for it. It will stick, and form, and last for years where things like the badly-misnamed “duct tape” will fail.

It’s easier to use than this may sound: Be sure the surfaces are as clean and free of dust as you can get them before you start to work, and use short lengths in overlapping segments. It’s like forming heavy aluminum foil with an extremely sticky adhesive on one side. Keep the tape edges inside the outline of the grate, and don’t in any way block the interior of the duct.

It’s times like these that I’m glad there’s a hardware store less than a half-mile walk from here! (Dear husband Shoujin is at work with our only car.)

The vent cover isn’t a plain grate per se, it’s a register cover (I think that’s the name) - they have a little lever to open/close the flow of air by moving flaps underneath the vent’s grated surface (edit: you know, like one of these). I unscrewed it from the ceiling and was greeted with an inch or so gap of dark opening between ceiling and duct.

Usually the duct is even with finish surface of the drywall or floor. The heat register has a sleeve that should fit firmly inside this about an inch. Ducting is usually rough work - the holes for floor ducts are even cut by chainsaw sometimes, so the duct often fits in its hole loosely and is fixed with screws. The heat register sleeve is designed to fit snugly in the duct though so this minimizes leakage, and the flange of the register hides the ugliness.

The heat register / duct fit does not need to be air tight by any means but they should mate. The suggestion of using adhesive metal duct tape is a good one. You may also be able to pull the duct tight and fasten it properly in place with a screw if there is a stud or wood backing (like there is supposed to be).

You can also add your own backing - a piece of 2x4 for example, by gluing and screwing it to the back of the the drywall. Drive the screw in close to the edge where it can be hidden by the flange of the register. This gives you a nice firm surface to attach the duct to, and is the proper fix rather than tape or foam(tsk).

Upon removing the register again, it appears the problem is not quite as bad as I first saw. The duct appears to have the wooden backing (perhaps a stud) on at least 2 sides, the sides that were closer to me and thus harder to see. There is no wooden backing on the side with the obvious gap, which was directly across from me when I first removed the register. It’s hard to see if there is backing on the remaining (narrow) side due to the position of the duct.

Unfortunately the open side is the side closest to the roof edge, etc. - but at least there is some insulation in there (peeking with a flashlight) and it only feels a little cold when I put my fingers in the gap, not crazily cold.

Called the landlord to tell him. It honestly sounded like he was as surprised as I was - as I told my husband, I’m pretty sure he’s never gotten up on a ladder and moved a register that was stuck to the ceiling with paint. (Need to fix that too, I took a few layers off in small chunks around it. Starting out with some sparkle to fill it in…)

Uh, spackle, not sparkle. (Thanks, autocorrect.) I’m not fixing up a little girl’s room… :stuck_out_tongue:

It’s pretty standard to seal the edges of ducts to the wall so that here is no loss of ducted air (hot or cold), no increep of external air, and no intrusion of dust, insulation fibers, etc. This is true even if the collar of the register fits into the duct.

I taped the whole thing anyway, since I didn’t know how securely anything had been sealed (or not) on the other side of things.

Some times the guy installing the duct work gets a little lazy. The can may have metal ears the screws for the register screw into the ears to hold the register in place. If two people are working the instillation and taking short cuts the one in the attic will hold the can in place while the second guy in the space screws in the register. Every thing works well and is sealed, that is until someone removes the register, the can can move up and out of place.