Register on existing system

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csp0202
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Register on existing system

Postby csp0202 » Tue Sep 15, 2015 6:26 pm

My friend wants to add a register in his basement to his existing ductwork. He wants to know if he can do that. He asked me if there is enough room on his existing system. Is there an equation out there I can use to determine that?

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CEO1
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Re: Register on existing system

Postby CEO1 » Tue Sep 15, 2015 7:40 pm

csp0202 wrote:My friend wants to add a register in his basement to his existing ductwork. He wants to know if he can do that. He asked me if there is enough room on his existing system. Is there an equation out there I can use to determine that?


One opinion... ;)

The real question is whether or not the system has enough cooling/heating capacity to maintain the present conditioned space, after sharing some unknown (for the moment) volume of air with the basement area. The answer to that question, would require a load calc (Manual J), compared to the system capacity...which is a bunch of equations. :mrgreen:

The simpler approach is a practical evaluation of how things work now...does the cooling (heating) system run all the time, to maintain the desired temp? If "yes", then the existing conditioned space may suffer. If "no", it likely wouldn't be a problem. The other question...how much basement area is he hoping to condition? 10' X 12', or 30' X 50'? I would estimate a typical 8" flex residential run-out (register) at 150 CFM, give or take (remember, 400 CFM ± per ton), which would do some good in a 10 X 12 room...but not much more.

Another point...most finished basement areas don't need a lot of "cooling" (maybe more dehumidification). They typically don't have windows and are to some extent, below grade, so heat gain isn't so much an issue. For that matter, heat loss is less, so heating requirements are also reduced.

I have, over the years, added one or two run-outs to existing systems for basements, many times. The net results ranged from "decent performance" to a "waste-of-time-and-duct-material". :P

One of the MFIC's...

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CEO1
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Posts: 2875
Joined: Mon Feb 17, 2014 11:20 pm
Trade Experience: 30
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Re: Register on existing system

Postby CEO1 » Tue Sep 15, 2015 7:57 pm

..forgot one point. A supply added to the basement, needs an accompanying return added to the system return duct...if there isn't an existing "pathway" between the basement supply air and system return. Otherwise, the upstairs area could go negative pressure, which sucks in outside air through all the cracks.
One of the MFIC's...


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