isolating furnace intake

As part of duct sealing process, I need to improve the air intake port show in the photo so that it doesn't suck air from under the staircase:
https://dl.dropboxusercontent.com/u/55756213/IMG_0341.JPG
The round port is 14 inch diameter. The grill cover is 15.5 inch x 25.5 inch. The white box on the upper right corner is a house alarm and can be moved.
My initial thought is to get a 14-inch round to square vent boot like this (Amazon.com product link shortened)
and somehow attach it to the back of the grill cover, then connect this to the floor vent with a 14 inch flex duct.
However, the edge of the floor vent is only 6 inches away from the grill cover, so the back of the square to round boot will be over the floor vent, so there is no room to connect the flex duct.
Could someone suggest a solution that works?
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*Go to a sheet metal shop and have a duct boot made to fit into the space. You will need to give them a drawing with the dimensions. Make it a 1/8" shorter on each side to slide in easily and have them add a 3/4" lip all around so that you can secure it to the wall. You can have them add a duct collar so that you can attach the flex directly to that. Ask them to make the duct collar attachable from the inside so that you can remove it, slide the boot in, pull the flex up, attach it to the collar and then drop the collar into place.
I have seen this set up many times. The original installers are using the wall cavity as a return plenum.
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On Wednesday, October 16, 2013 7:46:28 AM UTC-4, John G wrote:

Which is typical. So before going to all that trouble, how airtight is the area under the stairs and if any air is getting in there, where is it coming from? If the leakage is from heated inside air space, it doesn't matter. It might be easier to seal up any leakage that's coming into the cavity from the basement, etc. instead of custom fitting sheet metal.
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On 10/16/13 3:05 AM, bob wrote:

Assuming the round pipe is not meant to also pull air down the wall cavity from an upper level, consider "boxing in" the space behind the vent cover with something like 1/4" plywood. Cut 3 sides and a top. Use 1x2 to glue and screw together and to sub-floor behind vent.
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That would be my vote too, But, as mentioned, only if the round pipe is not also meant to pull air down the wall cavity from an upper level.
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Good point about the upper level. However there is no opening anywhere else in the wall cavity.
Building a box is probably easiest for me since I have the necessary tools. The trickest part is probably sealing the box against the drywall opening.
As to why I need to do this, it's because I'm thinking of getting the ducts "Aeroseal"ed and the contractor recommended I do this first.
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On 10/17/13 1:33 AM, bob wrote:

Consider caulking, or foam weatherstripping
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On Wednesday, October 16, 2013 2:05:51 AM UTC-5, bob wrote:

ow in the photo so that it doesn't suck air from under the staircase: https ://dl.dropboxusercontent.com/u/55756213/IMG_0341.JPG The round port is 14 i nch diameter. The grill cover is 15.5 inch x 25.5 inch. The white box on th e upper right corner is a house alarm and can be moved. My initial thought is to get a 14-inch round to square vent boot like this http://www.amazon.c om/Speedi-Boot-SBH-202014-SRA-Diameter-Adjustable/dp/B0085UYOGS and somehow attach it to the back of the grill cover, then connect this to the floor v ent with a 14 inch flex duct. However, the edge of the floor vent is only 6 inches away from the grill cover, so the back of the square to round boot will be over the floor vent, so there is no room to connect the flex duct. Could someone suggest a solution that works?
What is wrong with sucking air from that location? Too dusty?? Are thre o ther cold air returns on that floor level? What registoers will be allowin g return air to flow if you close that one off. Is it also drawing air fro m the floor above, using the wall space as a plenum as another poster quest ioned?
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On Wednesday, October 16, 2013 3:26:25 PM UTC-4, snipped-for-privacy@sbcglobal.net wrot e:

show in the photo so that it doesn't suck air from under the staircase: htt ps://dl.dropboxusercontent.com/u/55756213/IMG_0341.JPG The round port is 14 inch diameter. The grill cover is 15.5 inch x 25.5 inch. The white box on the upper right corner is a house alarm and can be moved. My initial though t is to get a 14-inch round to square vent boot like this http://www.amazon .com/Speedi-Boot-SBH-202014-SRA-Diameter-Adjustable/dp/B0085UYOGS and someh ow attach it to the back of the grill cover, then connect this to the floor vent with a 14 inch flex duct. However, the edge of the floor vent is only 6 inches away from the grill cover, so the back of the square to round boo t will be over the floor vent, so there is no room to connect the flex duct . Could someone suggest a solution that works?

other cold air returns on that floor level? What registoers will be allow ing return air to flow if you close that one off. Is it also drawing air f rom the floor above, using the wall space as a plenum as another poster que stioned?
He's not trying to close it off. He wants to make it so it can only pull air through the vent grill. It's a triangular space below stairs and the way it is right now, it can pull air anywhere from the cavity. Probably 95% or so is coming from the open front. But, I earlier raised the question of what is on the other sides of this enclosed little space? If it's all interior space, basement below, then the only unheated air issue would be from an unheated basement. In that case, if he can, he could just seal the bottom. If there is an outside wall on one side, then it's a different story. Per his last post, the "aeroseal" contractor told him to get it done. I'm wondering why an aeroseal contractor, who's in the business of sealing up duct systems, isn't capable of fixing this for the customer, since he's the one saying it needs to be fixed. As someone else pointed out, all that's needed is a custom made sheet metal boot. You would think this would come up all the time. It's a very typical duct situation and if it's your business to seal up ducts..... I mean what's more professional, to offer that service to your customers or leave them to go find another contractor, or find a duct fabricator, take measurements, etc. If the contractor did it, they measure it, have the fabricator they do business with make it, put it in, add it to the bill, make more money. As a customer, I'd sure prefer that instead of leaving me to figure it out.
But if it were my house, no exterior wall, and I could see inside that there are no holes to the basement, I'd just leave it be. Or if there are a couple holes, then I'd seal them. Or better yet have aeroseal, seal them. IMO, the difference in any possible energy savings beyond that isn't worth all the trouble. At some point you can get carried away beyond the point of reasonable return.
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Two sides of the space is outside walls, below the floor is crawlspace.
The aeroseal contractor did offer to build a box for $450. I think it's way too high so I'm doing it myself.
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On Thursday, October 17, 2013 1:57:04 PM UTC-4, bob wrote:

I agree, that's a lot of money. In that case either getting a sheet metal boot fabricated or building a wood enclosure around it and sealing it with duct sealant sounds like the way to go.
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