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Old 04-13-2012, 06:03 PM   #1
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How to terminate an AC vent?


In my remodeling I combined two areas into one.

From the master bedroom I used to have a small closet size space that is a "dressing area" then next to it was the bathroom. I have combined these two areas into the new master bathroom.

However, each of these areas used to have an individual AC vent, as marked 1 and 2 in the image shown below.



From another vantage point you can see both vents came off the same duct.



This is an older house so there are return and supply vents in each room.

I think it makes no sense to have two supply vents in the same room so I think I will just terminate one of them.

How would one go about doing that? Is there some sort of a cap you buy? Or you cut off the extra duct and tape it off? Or do I need to somehow merge the two ducts into one big one inside the attic and increase the terminal vent size?

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Old 04-13-2012, 07:59 PM   #2
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How to terminate an AC vent?


Every time I come across that idiot mike holmes show on tv they show them cover ductwork/wiring/plumbing with that nasty spray foam crap,and I always say whats going to happen when somebody has to come back in there and fix something?
Well now I know!
I suppose you could chisel that crap off of the pipe at a joint and stick a cap in there

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Old 04-13-2012, 08:38 PM   #3
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How to terminate an AC vent?


Couldn't you just cover the hole with some galvanized steel screwed in and cover with mastic sealent and metal tape?
So if your plans change in the future and you need it it's there?

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Old 04-13-2012, 08:40 PM   #4
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How to terminate an AC vent?


what a mess.... why on earth is it spray foamed.... you could leave as is and install adjustaber diffuses to balance airflow
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Old 04-13-2012, 11:09 PM   #5
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How to terminate an AC vent?


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Originally Posted by plummen View Post
Every time I come across that idiot mike holmes show on tv they show them cover ductwork/wiring/plumbing with that nasty spray foam crap,and I always say whats going to happen when somebody has to come back in there and fix something?
Well now I know!
I suppose you could chisel that crap off of the pipe at a joint and stick a cap in there
I can't see where there is a joint with this spray foam.

I supposed at some point in time in the 70s they like to spray everything to keep the ducts cool? I don't know. The entire run of supply and return ducts are all spray foamed. I don't even know if I cut it with a sawzall whether those foam will create dusts with asbestos.
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Old 04-13-2012, 11:14 PM   #6
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How to terminate an AC vent?


There should be some sort of tap collar right there where it comes out of the other pipe
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Old 04-13-2012, 11:36 PM   #7
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How to terminate an AC vent?


If you want to get technical then most likely yes, you'd have to upsize the one duct run that you plan on using to accomodate proper air flow, measured in static pressure.

I'd take a saw zaw to it right at the trunk line and use some duct board for the patch (duct board is an insulation, 1"-2" thick) which you're going to need to prevent that area from sweating and dripping water on the ceiling. The cold air inside the duct meets the warm air of the attic and it'll condensate like when you take a cold can of soda out of the fridge. That's a lot of why ducts are insulated to begin with.

You can probably buy a half of a sheet of duct board for a few dollars at an hvac supply house, if they sell to anyone, and easily cut it with any blade to fit the needed patch.

Use mastik (hardcast) to seal the patch.
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Old 04-13-2012, 11:44 PM   #8
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How to terminate an AC vent?


I should have said cold dry air meets warm humid air and it condensates. Never use only metal without insulation.

Duct board. Used for various reasons up to and including entire trunk lines and duct systems to patching holes in them.



You can pick this up at the local supply house as well. Make sure and get a brush to apply it too. They'll have one there.
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Old 04-14-2012, 10:05 AM   #9
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How to terminate an AC vent?


Doc Holliday thanks!

Reference the condensation issue, this means I couldn't really cap off the line at the vent because condensed water will accumulate there and over time will be problematic right?

If I do a cut with a sawzall on these sheet metal duct, I am not sure how straight a cut I can make it as these ducts are somewhat flexible (the insulation actually make them more sturdy), do I need to cut them fairly clean and flat in order for the duct board to close against them?

I am still trying to understand how the duct board is used to plug the hole.

Do you mean cut the pipe and use a duct board against the end of the pipe, which means the pipe will need to be cut quite evenly.

Or do you mean cut as clean as I can, then cut the duct board into a circle matching the inside diameter of the duct and push the round shaped duct board into the duct a little bit, then use the Hardcast to seal around the seam?

Is there any mechanical fasteners involved at all between the duct board and the duct?

Is it a good idea after the whole cut and patch is done to spray some foam to cover the whole contraption or to leave it exposed?
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Old 04-14-2012, 10:26 AM   #10
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How to terminate an AC vent?


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Doc Holliday thanks!

Reference the condensation issue, this means I couldn't really cap off the line at the vent because condensed water will accumulate there and over time will be problematic right?

If I do a cut with a sawzall on these sheet metal duct, I am not sure how straight a cut I can make it as these ducts are somewhat flexible (the insulation actually make them more sturdy), do I need to cut them fairly clean and flat in order for the duct board to close against them?

I am still trying to understand how the duct board is used to plug the hole.

Do you mean cut the pipe and use a duct board against the end of the pipe, which means the pipe will need to be cut quite evenly.

Or do you mean cut as clean as I can, then cut the duct board into a circle matching the inside diameter of the duct and push the round shaped duct board into the duct a little bit, then use the Hardcast to seal around the seam?

Is there any mechanical fasteners involved at all between the duct board and the duct?

Is it a good idea after the whole cut and patch is done to spray some foam to cover the whole contraption or to leave it exposed?
Unless you are planning on possibly re-using that duct run in the future, re-attaching to it, I'd cut it at the main trunk line itself and patch the hole in the main trunk. I woulnd't leave more than 6" of the cut line sticking out if I absolutely had to cut it there. You can cut the duct to fit tightly inside the hole and simply hardcast the entire thing. Hardcast will dry up like a rock, sticks to anything. No mechanical fasteners involved but you should leave the system off overnight to let the hardcast dry up. Don't be skimpy with the hardcast meaning you shouldn't use a very thin layer where you can see through it.

If you use the duct board then no spray foam will be needed.

Good thing about half of a sheet of duct board is you can make about a dozen patches if you need to practice.
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Old 04-14-2012, 12:25 PM   #11
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How to terminate an AC vent?


I'd put a piece of metal in the boot, then spray foam it so it doesn't sweat or leak air.

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