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We put an addition on a couple of years ago. At that time we installed a second HVAC unit to service the upstairs. Our original unit still services the basement and main level. The original contractor only sealed off the vents in 2 of the bedrooms.

We are ready to install hardwood flooring in the bedrooms and just realized they didn't seal all the other rooms (instead the vent is covered tightly with aluminum foil to stop the airflow)!.

So my questions are.... 1. Can we just put sub-flooring over the vents and then the hardwoods? 2. Is there a way to seal off the other vents closer to the heating unit so it isn't blowing air so far into the house that it isn't servicing?

Any ideas will be appreciated! Karen

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    The second question can only be answered if we see the layout of the ducts, without more information we'd be guessing. – Tester101 Aug 6 '15 at 1:19
  • How are the first vents sealed? Are the other vents actually aluminum foil, or HVAC foil tape? – gregmac Aug 6 '15 at 15:54
  • The addition was put on 8 years ago. I know it is amazing that we didn't know that all vents were not capped, but there was no air flow or sound coming out so we didn't question it. The two rooms that are capped.....1 vent is above the kitchen ceiling and I am guessing they capped it at that point when they were adding can lights and doing other work in the ceiling. The second room was probably capped in the same way, but we haven't figured out why they would have been in walls/ceiling in that area. The vents not capped are just cover in a heavy duty piece of aluminum foil. – KSD Aug 7 '15 at 2:53
  • Do you have photos of a capped vent? What may look like heavy duty aluminum foil could be commercial duct sealing product. – BrownRedHawk Aug 13 '15 at 20:28
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You can seal off a vent anywhere: the basic goal is to have zero air leakage. So to answer your question, yes it's possible to seal a vent below a subfloor.

If you don't seal it up properly though, you'll be blowing conditioned air into the space under your floor, which is a big waste of energy and will do basically nothing.

It's better to cap off closer to the blower unit, mainly to minimize the amount of air leakage along the run. The challenge is of course finding out which runs go where, and ensuring that you aren't blocking off a vent that feeds another area as well. It's pretty common to see 5 or 6" round duct used to feed each individual vent, and larger square duct as the main runs. If you can see the round duct for the room right by the furnace, there's a good chance it's only feeding that vent.

You can get round and square ("blind") end caps of varying sizes:

Round end cap blind end cap

If you're pulling up the subfloor anyway, I'd take the boot right off and just put a cap on the line (as far back as possible to get to). Seal all seams around the cap (and anywhere else you can see that isn't sealed) with foil tape or mastic.

If you can't remove the boot (the part that comes up through the subfloor), the next best thing would be to seal it by placing a blind end cap over the opening and sealing it up. This would basically be butted up against the new floor. If there is minimal or no air leakage through the boot, it shouldn't really heat up much, but if air is allowed to move through the vent, you'll end up with a warm or cold spot on the floor, which may be weird.

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