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justinNC 10-29-2009 09:14 PM

Help with gas pack trunk sizing
I need to replace the transition pieces from my 3 1/2 ton 1400CFM Trane gas pack to the trunks inside my finished basement. The existing pieces are taped together and just resting against the gas pack openings with no flange to secure them. Just a little tape.
It looks like someone made the mess when they retrofitted the gas pack years ago. Possibly it was a split unit before the gas pack was installed.
The trunk is also pretty poorly constructed and leaky.

The opening for the supply on the gas pack is 9"x17".
My supply trunk is a metal 16"x20". Is there any reason for the trunk to be that large? I assume it was a plenum for a split unit at some point in the past. The return is slightly smaller.

The supply trunk comes in the house and it immediately turns 90 degrees and goes up 4' to an end cap. 3 round 6" flex ducts come out of the end cap and another smaller trunk (8"x14") feeds out the upper side of the trunk to rooms farther away.
The unit is for the 2 story part of my split level, about 1800SF.
The main ground level has another gas pack, I'll get to that one later.

I have some room to make length changes if needed.
To replace the 16x20 is a custom sheetmetal job.
Can I downsize my trunk to an off the shelf size?
8x24 is the largest I can find locally.

Having someone come in to design a Manual D system isn't in the bank account right now. I just want to get it working and sealed up correctly.

I hope this all makes sense, I was up pretty late last night mixing concrete
to fix the mess under the gas pack :)

Let me know what I have left out...



beenthere 10-30-2009 04:44 AM

8X24 is small for a 3.5 ton unit.
The rest of your duct work is also small for a 3.5 ton unit.

8X28, or 10X22 would be a bit more appropriate.

justinNC 10-31-2009 08:19 PM


At a supply house I got a pair of "blowouts" to attach to the gas pack.
They flare out to 16x20 so I can attach them to my existing trunks.

I'll see what can be done with upsizing my existing ducts.
The ducts to my upstairs bedroom go from the basement ceiling,
down a chase to the crawlspace and then up another chase to the upstairs bedroom. There must be 100' of mixed rectangular and flex duct in that run.
The house has had several major remodels in its 55 year life. The basement was a carport and the upstairs is a post and beam addition.

I'll see if I can seal up my gaps in my trunks with mastic before I put them back in. I tore out the mechanical closet in my basement so I can get in to work on this stuff. No big deal since I was pulling down the 6" pine T&G boards to put drywall up. Just had to knock out a half dozen studs.

I poured an enlarged slab for my unit, it had a lightweight slab hanging off all 4 sides of an old small slab that must have been for a previous condensor unit. Laying 2 courses of block on newly enlarged slab to get my gas pack up to the same height as the hole in the foundation wall. Trying to get rid of the huge offset that was part of the problem. I had a bunch of blocks and some surface bonding cement laying around anyway.

I wish I knew if the homeowner was a cheapskate or if the installer is to blame or both. I guess someone someday probably won't like what I am doing to the system either :)
thanks again,


beenthere 10-31-2009 08:38 PM

Your welcome.

it was probably a combination of both previous owner, and the installer.

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