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I have a bonus space that is attached to the garage that we use as a home gym. I want to be able to run a portable air condition when working out. It will only run 2-3 hours a day and probably only 4-5 days a week. Can I vent it through the drywall into the garage? It would be easier to do that versus drilling through the stucco to the outside.

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    An A/C unit doubles as a dehumidifier so the expelled air is warm and humid. If you're okay with a warm and humid garage then there should be no issue. The expelled air is not toxic if that was your concern.
    – MonkeyZeus
    Mar 8 at 15:13
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    Would be better if you can ventilate the garage when running. Have door/s windows open or else it could be like a sauna in there, if walking from an A/C room.
    – crip659
    Mar 8 at 15:20
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    Thanks. The garage doors would be open most of the time as well when running so I am thinking this will be fine and I was concerned with any health issues so sounds good. Thanks again!
    – Jon
    Mar 8 at 15:21
  • @crip659 make an answer out of that so Jon can give you a check mark and close out the question.
    – FreeMan
    Mar 8 at 15:31
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    @blacksmith37 Is your garage attic well ventilated with gable vents or soffits?
    – MonkeyZeus
    Mar 8 at 16:11
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Would have some ventilation in garage so humidly does not build up. No problems with venting A/C to garage except what humidly might do to stuff inside(drywall, storage of cloth) if not vented outside, open doors/windows.

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    Your AC will also produce water, which will drip or run onto the garage floor. Most garage floors are routinely wet so that should be ok but think about where the water will start and where it will flow to and make sure it's what you want, or else set up a hose to carry it where you want it.
    – jay613
    Mar 8 at 17:04
  • @jay613 Portable A/C units generally exhaust all of the moisture via the vent; sometimes called a hot-air discharge. If it's a particularly humid day then the pan could fill up and prevent the A/C from running until it is emptied. If you have a simpler (cheaper) unit then it would need frequent emptying.
    – MonkeyZeus
    Mar 8 at 18:38
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The problem with venting the AC into the garage is that you are then venting the garage into your workout room. I'd worry about CO poisoning. Garages are typically air sealed from the rest of the house to prevent and CO from moving from the garage into the house.

I can't imagine a damper that I would trust enough to vent the AC into the garage and take the risk. You could mitigate a bit by install a CO detector in your workout room but I'd still go with the drill to outside through stucco.

Drilling through stucco isn't such a huge deal.

The other thing to consider is that by venting the house into the garage which is presumably sealed from the house, you can depressurize your house (depending on your make up air vents). A depressurized house can result in back drafting from combustion appliances. In new construction there is typically a sheet metal expert who runs that calculations and ensures there is enough make up air available. This factor would be similar whether you vent into the garage or outside though.

While you aren't using the garage for your car you may be storing things in the garage that off gas - paints, gas, etc. If you depressurize the house and pressurize the garage there will be a propensity for the garage to want to leak into the house.

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    AC units don't move air from inside to outside, or vice versa, so there's no actual air movement unless you screw up the sealing. But, yes, install a CO detector. Mar 8 at 18:19
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    Helpful. We do not park cars in the garage so i am thinking that is not an issue.
    – Jon
    Mar 8 at 18:35
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    @Jon you may want to look up if it's code though. where I live, anything going through the wall of an attached garage requires a special fire break (expands and seals the hole in case of garage fire). Even if it's not a big deal with your use case, it may not be up to code if / when that becomes important.
    – Aaron
    Mar 9 at 0:13

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