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 1875 days

#1 posted 07-12-2018 10:57 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

I agree that AC units should be matched to the load and arbitrarily adding air conditioned space can be an issue. But if your AC was running continuously, that is good, if it was keeping it cool enough. The ideal AC sizing is if the AC unit is runnign continuously during the hottest hours of the hottest days. AC units are a lot like cars. Best millage is a small engine running at a steady rate. Rather than a large engine stopping and starting frequently (short cycling).

Most AC systems tend to be way oversized because they don’t put the time in to properly calculate the heating and cooling load. Most often they just use rule of thumb values based on square footage and this is much too simplistic. They just make sure to put in a system that they know is large enough. My house should have about 3 Tons of AC, instead it was built with two 5 Ton units (10 Tons total). Way overkill and extremely inefficient. This type of thing is not unusual.

And it’s not all just about cooling. Short cycling makes a house uncomfortable in humid areas. It’s the classic cold yet clammy feeling. The unit is so large it cools the house quickly then shuts off. But it’s on so short a time, the air doesn’t get dehumidified.

-- Clin

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