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Old 06-24-2008, 06:14 PM   #1
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Adding new duct/Register

I have a 3br 1989 single-story "track" home in hot Fresno, CA. All the usual assumptions for a home of type/age in this area. I'm a DIYer w/no a/c exp except tons of reading online. I'm adding a new register for a 10x6 walk-in closet that requires cooling. The bedroom it's attached to is the hottest room in the house, b/c it faces East+South and the feed is simply not big enough to cool such a hot room that lacks any real windows, has no cross-ventilation and is at the "end" of a 50' duct run.

Imagine the home layout as a U. The a/c is the left side of the U and has 1 return + feeds 5 registers very close to it. <nice> However, it has to travel almost 50' all the way around the U to get to the other side -- which is the bedroom + closet in question. The a/c is a roof mounted American Standard 2.5ton 48nlt 10seer 80% and when it has the proper freon level, it cools the house acceptably well except the master bedroom. All the ducting is insulated flex w/sheet metal joints inside the attic only (none down walls, no crawl space, etc).

The plenum has the following:
* 14"x10' return.
* 6" x 5' run to bedroom.
* 6" x 10' run to bedroom.
* 5" x 8' run to bathroom.
* 8" x 5' run to living room.
* 6" x 15' run to family room.
* 10" (maybe 12) x 30' Supply Line to feed rooms furthest away:
* 1st Wye branch (10"->7"x9"):
* 7"x8' to Kitchen, 9"x15' toward master area.
* 2nd Wye: (9"->9"x6"),
* 6"x3' to master bath, 9"x7' to master bedroom.

1st Question: Are the register sizes important or aesthetic only (unless severely large/smaller than duct feed into it)? Almost all the registers are 14"x8" (except for bathrooms) which I can't find at HD anywhere. When I add a new register, the only "real" consideration is the duct feed on it?

2nd Question: Is the 2nd Wye split to the master area of 9" feed to 9"+6" common and appropriate? My layman assumption is a 9" source should Wye to smaller ducts, esp so far away from the plenum.

3rd Question: After tracking all the ducting + checking leaks/joints I decided to ignore the U layout of the house and take a "short cut" across the U. The house may be a U but the attic isn't. The attic space does exist over the top part of the U, but it's over unconditioned space outside in the patio. I put 8" R6 duct from HD over the joists above the patio, making a direct run from the Plenum to the master b/r only 25'. I'll be adding more insulation + radiant barrier to that part of the ducting later.

So I decide "hey, if I'm adding a register to the closet, might as well add another register to the hot master bedroom".

I'm capping the 5" collar feed to the bathroom (we had the register blocked off w/cardboard anyway) and cutting a new 8" hole into the plenum. So adding this is a net of 3" diam draw of cfm from the other ducts (well, not really 3", true volume loss is a different formula - but you get my drift).

The Q: I'll take the new 8"x25' duct and Wye to 8"->6"x6" and feed the closet with a 4x12" reg and mbr a 6x12" reg. Or would it be better a 8"->8"x6" wye for more cfm to the bedroom.

Does this sound OK?


Last edited by CEbury; 06-24-2008 at 06:23 PM. Reason: added a/c info
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Old 06-25-2008, 10:51 AM   #2
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I'm going to assume my original plan is OK -- except maybe to make the closet a 4" register instead.

The more I look at everything, the more I think there is definitely not enough RETURN air. There is only one return in the house and it's far away from the master area -- and it's 14" which is 150sqft supposedly too low for the 2.5 ton unit.

I'll try to add a 8" return duct in the master bedroom also -- since air movement is the real issue here, not just cold supply.

I wish I knew where to put it -- concrete floor, all walls are exterior facing, and I'm already putting supply into the tiny closet... not much real estate except on the ceiling.

Question: Should I add the return on the ceiling (which would make 3 registers/grills now in a 11x15 room) or just outside the master door in the hallway or on an exterior facing wall?


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