Reply by clin

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

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1121 posts in 1873 days

#1 posted 07-12-2018 10:40 PM

The issue isn’t that bad stuff (CO etc.) is going to come back through the duct that is blowing air into the garage, its that the garage will be at positive pressure and the house at negative. So air from the garage will be getting back into the house through whatever means it can. Certainly any door seal, but also things like power outlets, and any sort of crack. Attic is also a common way it would. And attic air is usually pretty bad stuff, lot’s of dust, pollen, possible fiberglass fibers. Maybe even stuff from bird and bat excrement.

Also, when the AC unit is off, air can actually go backwards through the duct if the garage became pressurized relative to the rest of the house. This can happen if you have wind creating pressure differentials.

Since we are talking using the garage space for a wood shop, you would also be encouraging fine dust to migrate from the garage shop into your house. Fine dust is a health hazard.

Another issue is if the house has gas appliances like a water heater or cloths dryer. If this is in the negative pressure area of the house, it could draw combustion products like CO2 and I supposes some CO back into the house by making the air flow in the flue go backwards.

To avoid this you would have to vent the garage to the outside world so it doesn’t become pressurized, and in turn open the house up to the outside so it doesn’t become negatively pressurized. This of course would be very inefficient for AC. But would avoid the problem.

This in fact is effectively what I used to do in my old house. But that used an evaporative cooler, a form of displacement cooling, and those require you to vent the air to the outside. I would just close up the house and open the garage door to the house, and crack the main garage door. But, of course evaporative coolers aren’t the thing to use in humid places like Florida.

-- Clin

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