A/C in the garage

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Skywalker18
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11:53a, 5/15/26
AG
Brand new home. Insulated garage door and walls. I'm trying to figure out the best way to get some A/C in there! No mini split. If you read my concrete forum them you know I'm a DIY cheapo person who just tries to figure it out.

Here's my question for yall. I have a tankless water heater in the garage. Its attached to the vent hole with a PVC elbow coupling roughly 5 inches in diameter. Would it be completely stupid to replace the elbow with a Y coupling and install a flapper valve where it connects to the water heater. This would prevent the A/C exhaust from going to the WH but will allow to WH to vent when in use? I could then connect the vent pipe from the portable A/C unit the the other side of the Y.



Water heater is installed exactly like the diagram on the right side of the picture. Very minimal vent piping to the outside.
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BenTheGoodAg
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In reply to Skywalker18 • 12:35p, 5/15/26
AG
Quote:
Would it be completely stupid to replace the elbow with a Y coupling and install a flapper valve where it connects to the water heater.

I don't fully understand what you're trying to do, but the answer is likely yes.

There's no cooling "exhaust" for an A/C, so I'm assuming you're talking about exhausting a high efficiency furnace into the high efficiency water heater exhaust? There are some applications where you can combine vents for gas appliance exhausts, but high-efficiency units (hence the PVC exhaust) aren't one. First, they're usually powered exhaust, and you can't assume the flapper will effectively seal off back-flowing exhaust into your other appliance. Secondly, their discharge is very corrosive, and I wouldn't trust a flapper not to freeze up due to corrosion over time.

Exhaust vents aren't something to cheap around with, but especially in this case - This is about as easy as it gets to add another discharge with such a short run and easy access.

ETA - re-reading, I now realize you're talking about a portable AC unit and its discharge, and the answer is definitely "Don't do it". You shouldn't mix combustion exhaust vents with other vents, period. But the concepts above still apply.
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FCBlitz
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1:07p, 5/15/26
If the water heater is a "On-Demand" and electric. I would have no problem putting a Tee somewhere and sharing an exhaust port through the wall. Again if the concept is an elect WH, 12" to 18" of duct work and an atm vent on the other side of wall.

I would also really try to find language in a building code that speak to you can/or can't join exhausts. In your case it seems exhausts from either device will be intermittent at best and is open directly to atmosphere.

I guess one additional caveat. If this set requires a flapper valve of some sort…..I would not do it.
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Dr. Doctor
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1:07p, 5/15/26
AG
First off, you said no mini-split; that's where you're wrong.

If you put a portable unit in the garage, you will cool it down, but you'll work harder because the air to remove the heat (the duct you want to use) takes air FROM the room you're cooling. That will in turn create a negative pressure in the overall room. Which will pull in hot air from outside.

The best system is to keep the hots and colds away from each other. That is done with A) full house HVAC unit (condenser outside, evap inside), B) wall unit where the outside (hot) stays outside and the cold (evap) stays inside or C) mini split (same as A, but smaller).

The other issue is trying to use the water heater's exhaust is that you can overpressure the vent via the portable unit and now you have CO going INTO your garage/house when it is supposed to be going OUTSIDE. You'd also run the risk of pushing other combustion gases and/or the HEAT from the water heater INTO the space you are trying to cool (you know, the whole thing you are trying to avoid).

You are potentially setting yourself to use a lot of energy to not really do a lot. Other than run up your electric bill.

~egon
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FatZilla
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In reply to Dr. Doctor • 1:32p, 5/15/26
AG
Dr. Doctor said:
First off, you said no mini-split; that's where you're wrong.

If you put a portable unit in the garage, you will cool it down, but you'll work harder because the air to remove the heat (the duct you want to use) takes air FROM the room you're cooling. That will in turn create a negative pressure in the overall room. Which will pull in hot air from outside.

The best system is to keep the hots and colds away from each other. That is done with A) full house HVAC unit (condenser outside, evap inside), B) wall unit where the outside (hot) stays outside and the cold (evap) stays inside or C) mini split (same as A, but smaller).

The other issue is trying to use the water heater's exhaust is that you can overpressure the vent via the portable unit and now you have CO going INTO your garage/house when it is supposed to be going OUTSIDE. You'd also run the risk of pushing other combustion gases and/or the HEAT from the water heater INTO the space you are trying to cool (you know, the whole thing you are trying to avoid).

You are potentially setting yourself to use a lot of energy to not really do a lot. Other than run up your electric bill.

~egon


They also make the portable units that take air in from outside via a dual hose port and inject that air after cooling it to prevent the negative pressure. Those would be a much better option for cooling this type of zone.

Example unit: https://a.co/d/05z3FbDK
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will.mcg
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2:35p, 5/16/26
AG
Why not vent the AC exhaust into the attic?
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Skywalker18
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In reply to will.mcg • 5:18p, 5/16/26
AG
I could but doing it this way would prevent cutting holes in anything.
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SharkinAg
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In reply to Skywalker18 • 6:02p, 5/17/26
AG
Skywalker18 said:
I could but doing it this way would prevent cutting holes in anything.


Well, the idiot who installed a mini split in my garage messed up and ended up drilling two holes through my brick wall. Didn't even tell me. Not visible from the inside. I walked around after he left and saw an extra hole filled with foam. After my 6 hour install I knew it would be pointless to call and *****.

On the bright side my garage stays nice and cool whenever I need it to be.
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RoyVal
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In reply to Skywalker18 • 7:08p, 5/17/26
AG
Skywalker18 said:
I could but doing it this way would prevent cutting holes in anything.

really not that big of a deal. I installed a mr cool DIY in my hardi board siding garage wall myself. I helped a buddy install a mr. cool DIY in the brick wall next to his garage. Later, I helped the same buddy install a mr cool DIY in a metal shop wall for his dad's garage shop. Took longer for me to run a new circuit in his garage that it did for him to cut a hole in the brick and install the mini split.
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agrams
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8:35a, 5/18/26
AG
I ran mini splits for a long time, but with the dust from my work, even with building a plenum for extra filtration, the inside units corroded and leaked. I recently replaced it with a portable with dedicated inlet and outlet vents and have been happy with the results, with about 1/3 the cost of replacing a mini split.



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BCOBQ98
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In reply to FatZilla • 12:08p, 5/20/26
AG
I'm using this exact model in my garage. It works….ok. I plan on a mini split in the future. It's loud, condensate overwhelms its built in handling when it's extremely humid and I added a pump to get rid of it, and not that powerful.

The walls of my garage are spray foamed and I have insulation in the attic. I built a new garage so there is no longer a garage door, just a man door. Insulation isn't the issue.
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maverick2076
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8:33a, 5/21/26
Mini splits are not difficult to DIY. Plenty of how to videos. You don't even have to go with Mr. Cool. A $100 vacuum pump and gauges can save you $1000 on your unit.
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Skywalker18
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8:38a, 5/21/26
AG
I appreciate yalls advice. Not going with a mini split right now. Brand new home and the wife already has funds allocated to plenty of other projects!

Just looking to do something cheap and easy for this summer. That's why I am looking into doing it the way I am. Anyone else want to chime in about using the water heater exhaust? I mean hell, it should affect anything over a single summer use as long as I remember to cover the open port after stopping A/C use.
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RoyVal
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In reply to Skywalker18 • 11:33a, 5/21/26
AG
Skywalker18 said:
I appreciate yalls advice. Not going with a mini split right now. Brand new home and the wife already has funds allocated to plenty of other projects!

Just looking to do something cheap and easy for this summer. That's why I am looking into doing it the way I am. Anyone else want to chime in about using the water heater exhaust? I mean hell, it should affect anything over a single summer use as long as I remember to cover the open port after stopping A/C use.

I don't know much about tankless water heaters. What is exhausted? just hot air? natural gas exhaust? carbon monoxide? that's what I'd find out first if I were you.
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Shelton98
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12:02p, 5/21/26
AG
If that water heater is gas..... I damn sure wouldn't mess with the exhaust.
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sts7049
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In reply to Skywalker18 • 4:46a, 5/22/26
AG
i admire your desire to do things cheap, easy, and simple. but you really need to recognize when a situation might require just a bit more thought or care.
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agrams
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In reply to Skywalker18 • 8:33a, 5/22/26
AG
it all depends on backpressure and flow from the combined units. if significantly different then one will overpower and restrict the other. if the heater is stronger, the ac will be less efficient. if the ac is stronger, the heater may not be able to vent properly, which could be an issue with exhaust gas and combustion systems.

also, exhaust lines from ac's use that to help remove moisture. so when your ac is running and your heater isn't, its going to introduce moist air into your heater, which could introduce its own problems.
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Skywalker18
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10:49a, 5/22/26
AG
First off, some of yall must have grew up rich or have too much money on your hands! Seems like nobody has a set of DIY on the fly balls anymore! I joke I joke....

Secondly, I did a bit more digging into my water heater situation and it looks like that exhaust pipe has more to it than its appearance shows. Ill post the picture below. While I was confident I wouldnt cause noxious gases to enter my garage if I made a switch, I didnt plan on this pipe being set up like this. If i made the switch, my exhaust from the A/C would now be providing the inlet air to my WH. That wouldn't be a good idea!

I'm going to go the route of agrams setup. Drill a vent hole in the siding and roll! If anyone has advice on that, please share!


Stainless pipe if for exhaust and outer ring brings fresh air to the unit.
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Skywalker18
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In reply to agrams • 10:50a, 5/22/26
AG
Appreciate your advice and the pictures above of your setup!
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agrams
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In reply to Skywalker18 • 12:47p, 5/22/26
AG
i ordered this bit thinking it would do the job.. it did not

https://www.amazon.com/dp/B0DK1ZV1RL?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

It did make a nice template to drill and hammer out small hole pattern, but it generally sucked.

Here are the vent hoods i ordered:
amazon.com/dp/B0FPFNZP7Q?ref=ppx_yo2ov_dt_b_fed_asin_title

You will need to pry the inlet one open. It has double mesh/netting, so i am not too concerned about stuff getting inside.

be gentle with the brick veneer. Mine was surprisingly fragile/brittle when hammering out the holes.

to connect the inside wall to the vood pipe from the outisde, I bought a few of these:
https://www.homedepot.com/p/Master-Flow-6-in-Air-Tite-Take-Off-ATTO6/100143514?g_store=1978&source=shoppingads&locale=en-US&fp=ggl&pla&mtc=SHOPPING-BF-CDP-GGL-D26P-026_002_AIR_CIRC_ACC-NA-NA-NA-PMAX-NA-NA-NA-NA-NBR-NA-NA-NEW&cm_mmc=SHOPPING-BF-CDP-GGL-D26P-026_002_AIR_CIRC_ACC-NA-NA-NA-PMAX-NA-NA-NA-NA-NBR-NA-NA-NEW-17697203498--&gclsrc=aw.ds&gad_source=1&gad_campaignid=17687898604&gbraid=0AAAAADq61Ud3E1AF3s9Xebm5C_q4ec49U&gclid=Cj0KCQjw_b_QBhCSARIsAP6hR4cL5e028VhTvMCFLk7ZmodGIDLvh1-KSj6NVegs6A2AHBZ0iqIbQhcaAviKEALw_wcB
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FatZilla
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In reply to Skywalker18 • 1:24p, 5/22/26
AG
Skywalker18 said:
First off, some of yall must have grew up rich or have too much money on your hands! Seems like nobody has a set of DIY on the fly balls anymore! I joke I joke....

Secondly, I did a bit more digging into my water heater situation and it looks like that exhaust pipe has more to it than its appearance shows. Ill post the picture below. While I was confident I wouldnt cause noxious gases to enter my garage if I made a switch, I didnt plan on this pipe being set up like this. If i made the switch, my exhaust from the A/C would now be providing the inlet air to my WH. That wouldn't be a good idea!

I'm going to go the route of agrams setup. Drill a vent hole in the siding and roll! If anyone has advice on that, please share!


Stainless pipe if for exhaust and outer ring brings fresh air to the unit.


I used this bit below from inside to outside. Used small but long masonry bit to punch through drywall and then brick and get my centering (and then checked outside after using small bit to make sure i liked the spot) and then drilled going outward (this was for a dryer vent) using linked hole saw bit below . Make sure you go in the middle of your studs and check for water pipes before doing your main drilling. You may hit insulation if you have a double wall like i do in garage. Worked like a charm for me. Bit was basically destroyed by the end because of the brick but i didn't mine it, it was dirt cheap. Can likely get a cheap bit at harbor freight if you have one nearby. Use it, abuse it, then toss it.

https://a.co/d/0bNw7aFh
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RoyVal
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In reply to Skywalker18 • 1:55p, 5/22/26
AG
Skywalker18 said:
First off, some of yall must have grew up rich or have too much money on your hands! Seems like nobody has a set of DIY on the fly balls anymore! I joke I joke....


buy this unit. DIY. done. happy to help you out if you're in the Houston area....
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Skywalker18
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In reply to RoyVal • 2:26p, 5/22/26
AG
RoyVal said:
Skywalker18 said:
First off, some of yall must have grew up rich or have too much money on your hands! Seems like nobody has a set of DIY on the fly balls anymore! I joke I joke....


buy this unit. DIY. done. happy to help you out if you're in the Houston area....

My budget is 300 bucks dude! Facebook marketplace for a used portable and 50 dollars in supplies! Ill hit my target temp of 75-80 degrees for a less than a quarter of that! I will say i have a brand new home thats pretty energy efficient with insulated walls and an insulated garage door so i have that in my corner.
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Silvy
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9:21a
AG
FB Marketplace is littered with 12k mini splits in the $300-400 range. You'll eventually grow out of doing everything on a barebones budget because you'll get tired of doing the job / paying the cost twice when spending a little bit more yields much better results (speaking from experience).
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RoyVal
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In reply to Silvy • 10:30a
AG
Silvy said:
FB Marketplace is littered with 12k mini splits in the $300-400 range. You'll eventually grow out of doing everything on a barebones budget because you'll get tired of doing the job / paying the cost twice when spending a little bit more yields much better results (speaking from experience).

exactly....buy once...cry once.....

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FatZilla
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4:30p
AG
I wouldn't trust a used mini splint without having to pay for a full vacuum and recharging.
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