Sealing up old furnace ducts

Hi,
I've got a couple of old furnace ducts near the ceiling in a couple of rooms in the house. I've since had new ducts put in the floor, and these upper ducts were simply closed off (but not sealed).
It's time to repaint, and I'd like to close up the upper ducts permanently and plaster over the openings. I figure I'll have to seal them off some way; otherwise I'll end up pumping hot air into the walls and eventually into the attic. That would be a bad thing, as I live in Canada. Probably a bad thing anywhere...
So, my question is, what is the best way to seal up these ducts? Is there something I can spray into them to block the air flow? There is really nowhere that I can apply metal tape.
Thanks.
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Brian wrote:

If you plaster over them how are you going to be getting any significant amount of air in them?
Seems adequate to me.
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I guess a picture would be better, but not knowing how to add one, I'll try to describe what they look like.
The duct opens to 12"x6" flush with the wall face and is 4" deep. Looking up and in, the duct seems to extend up about an inch or two, where it joins with a 12"x3" duct that feeds down from the attic.
1) One option is to try and rip the larger duct completely out of the wall, without leaving a huge mess behind. Then I'll have to cap the 12x3, but it will be tough trying to work in a cramped area.
2) The other option, as I see it, is to hammer back the edges of the duct that are flush with the wall, and then fit a small square of wallboard over the old opening using the hammered edges as a back support. I'll then plaster and tape the sides.
I'm leaning towards option #2, since it has the potential to be less messy. But either way I'll need to seal up the duct.
Thanks.
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Brian wrote:

Well, that is basically what I thought you said in your original post -- put a piece of wallboard over the opening and plaster/tape it and were wondering if you really needed to have a sealed end for the ducts themselves independent of the wallboard/plaster.
If it is "real" plaster instead of just wallboard so it is more of an issue in making an opening, you could simply take a sawzall and cut them back. Or, as you say, just bend the tin edges back so they don't reach the surface. You could if there is some real problem in doing that simply screw a backer piece on the inside of the duct itself as the support for the patch and set it inside the duct and tape over the edge as long as the actual duct itself is behind the surface. It doesn't need anything other than some way to hold a patch in place to finish over.
If it's just wallboard/sheetrock, cut it back to the midpoint of the joists and use them for the mounting points.
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Are you sure you want to seal them off? If you use central AC, it's better for it to come in from the ceiling than the floor. Since you're in Canada, I doubt this is a concern for you, but I thought I'd mention it.

Can you get into the attic and disconnect the duct from there, and then put an endcap on it? I imagine the closer you can get to the furnace, the less unwanted duct you'll be paying to heat. Especially if it's going through an unheated attic.
If nothing else, you can just shove some insulation batt into the opening. That should stop that section of the wall from being over-heated or whatever it is you're worried about. You could also tape some plastic on the inside of the duct to stop all airflow (but note that I'm not an HVAC person and don't really know what the long-term consequences of this is).
Good luck with it all :)
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Thank you Kitep and dpb.
You've certainly given me something to think about.
I'll post back here in a week or so to say how things worked out.
Brian Francis.
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Brian wrote:

I think the first thing is to find out for sure what that duct is doing (return air maybe?)
If it truly dead, then I would want to remove the necessary parts and patch the wall to totally erase any evidence of them.
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Joseph Meehan

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What, you mean the entire duct? That would require removing a bunch of wall board.
If you want to eliminate or re-locate a vent, I would suggest just cutting some sheet Al to fit over then opening. Drive some sheet metal screws to hold it in place and "seal" it with METAL duct tape.

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