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kib49 06-05-2012 06:54 PM

an a/c plenum question. please advise me.....
i built an insulated room in my garage in florida. my brother has fallen on hard times and he will be living in there very soon.

the ductboard box in the attic has no more room on the sides, to cut a new hole for a new drop.

my question is;:confused: can i cut a hole in the top of the ductboard box? then connect the duct to the new top hole and gradually bend it over to the new room's ceilng register, only about 6' away?

at home depot i've also seen inline duct booster fans. (a fan inside a short length of duct). it would hopefully pull enough air upwards from the plenum box and into his room to make it comfortable.

or should i use a larger duct? i was thinking 10" but maybe 12" because the garage gets hot. the soon to be conditioned space is about 160 sq ft. the central HVAC is 3 ton and freezes the house in a couple minutes, so it is easily enough to cool another room.

thank you for your knowledge and for taking time to reply to my question. it will help tremendously. kib49

kok328 06-06-2012 05:36 PM

Welcome aboard.
Yes, you can tap into the top of the main trunk to supply another room.
Stay with the same diameter duct that is already up there for other off-shoots.
A duct booster may rob the other rooms of needed heat & cooling.
I'm no expert so you may want to consult with an HVAC engineer regarding CFM's and associated dynamics of your proposal.

kib49 06-07-2012 12:45 AM

hi kok,
thanks for the welcome and for answering my question!

since the ductboard box top hadn't been used i wasn't sure if i could. those duct fans are cheap but the bend to the register will be so slight that i doubt i'll need one. but doable if need be.

take care and thanks again, kib49

nealtw 06-07-2012 12:49 AM

Don't forget, return air.

kib49 06-07-2012 03:20 PM

thx neal!
the return opening inside the house is 30"x16" = 480 sq". the reccommended return opening for a 3 ton central ac is 20"x20" = 400 sq". i'm pretty confident that cooling an extra room will affect the rest of the house very much. it's the coldest ac i've ever seen and i'm sure the tile and wood floors throughout help.

thx for your input and have a great day! kib49

nealtw 06-07-2012 03:42 PM

Yes ;I ment from that room.

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