Reply by Kazooman

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Posted on Tapping into Plenum for temp AC in Fl garage shop

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1540 posts in 2829 days

#1 posted 07-12-2018 10:49 PM

Not a good idea. An A/C system, in order to work efficiently has to be balanced room to room within the house. Tapping off the house A/C to feed the garage, will through the balance way off. Remember if you divert air to the garage, there has to be an air return to achieve balance. If the house has been properly sized for an A/C system, you may not have enough BTU s for the garage to share. I speak from experience. I had ductwork extended to a large room that was formerly a garage. When I did, my A/C system couldn t keep up with the added load. It ran continuously (not good), so I had to upgrade the A/C for a bigger unit.

- MrRon

You are spot on correct, IMHO. In my former life as an organic chemist in the pharmaceutical industry I was heavily involved in the design, construction, and validation of laboratory facilities where air handling was a huge concern (lest we poison ourselves with our own noxious chemicals!!!). Balancing the air supply with the draw of the exhaust hoods is an art and we paid dearly for experts to design and implement the system with sophisticated control systems. Seemingly minor concerns such as what happens when the door to the hallway is opened have a huge impact on how the air flows.

A dead end run to the garage seems unwise. As you pointed out, that air has to come from somewhere back at the house and the system was not designed for it. If you put in a return, then all of the vapors from the shop will be distributed throughout the entire house. I understand that such a set up would not be in continuous use, but that only exacerbates the problem. When you shut down the garage, then the house is unbalanced.

Even lacking a return from the shop that could contaminate the house air, there could well be a problem. The AC system is not on constantly and there is not always a positive pressure in the duct work. The air can flow both ways. Open a window in the house and you create a path for the air flow to reverse from shop to the house. Whether it is car exhaust, finishing fumes, or those nasty bottles of left over weed killer, I don’t want to be breathing that stuff in my house if I can help it.

I do not have the luxury of a dedicated out building for a shop and I know all too well the limitations that the problems of fumes, especially from finishing, can impose on my hobby. I used to love Deft, but gave up on it because although I could apply it with a respirator and try to exhaust the fumes outdoors with fans, the smell always made its way upstairs.

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