Running an AC unit inside an attic

Don't splice into the chimney. That has to be done right. Back flows are common, and has to be done to building codes.

And how big is the grow room? 24K BTUs is a bit.

I agree.. whats the need to dump it into the chimney? Just run it back into the attic. No box needed. Just stick it into the room just like you would a window and let tehe rest just work in the attic space. Not quite as efficent as the attic air is hot but it is what it is.

24k btu seems like a bit too. How big was that room? I'm planning on using a 14k btu in mine and its a pretty big set up..

bubblegumgreen said:

I agree.. whats the need to dump it into the chimney? Just run it back into the attic. No box needed. Just stick it into the room just like you would a window and let tehe rest just work in the attic space. ..

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....not........

Not? What you mean? What's need to vent it out the chimney?

I've done this before. Here are some simple instructions...

1. Cover the back of the AC with sheet metal,
2. Cut a hole in sheet metal for your exhaust
3. Hook up exhaust duct
4. Run exhaust duct to wherever u will vent
5. The unit will produce water and drain it through a hole on the bpottom
6. Locate the hole and hook up a hose to it (use neccessary fittings)
7. Run the hose to the stack and drain there (remember gravity applies)... U could also drain to a bucket and empty the bucket frequently, just don't forget.

U won't need a squirrel cage fan, the AC has a fan of it's own for exhaust, that will be plenty.

bubblegumgreen said:

I agree.. whats the need to dump it into the chimney? Just run it back into the attic. No box needed. Just stick it into the room just like you would a window and let tehe rest just work in the attic space. Not quite as efficent as the attic air is hot but it is what it is.

24k btu seems like a bit too. How big was that room? I'm planning on using a 14k btu in mine and its a pretty big set up..

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Yep u could do this too, a little less efficient, but I don't thin u are worried about efficiency running a 24000 BTU machine. Thats like central air...lol

bubblegumgreen said:

Not? What you mean? What's need to vent it out the chimney?

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the exhaust from the ac produces more heat than the cooling can handle. its one of the laws of thermodynamics, i forget which one. if you do it the way you;re saying it will slightly heat the room.

that's why window ac's have the exhaust on the outside and portable ac
's come with an exhaust hose to vent the air out of the conditioned room.

Thats what I'm saying.. the window a/c should work fine in the attic. Just make sure you set up a drain pan. I prefer the ones that resuse the condensate to help cool the coil as it helps it run more efficient and lessens the amount of water you need to drain. Many have a garden hose attachment or the ability to accept one to run the drain. I'd be more concerned about the drain than the exhaust!!

desertrat said:

the exhaust from the ac produces more heat than the cooling can handle. its one of the laws of thermodynamics, i forget which one. if you do it the way you;re saying it will slightly heat the room.

that's why window ac's have the exhaust on the outside and portable ac
's come with an exhaust hose to vent the air out of the conditioned room.

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I'm assuming that the front of the unit is sticking in the grow room and the back where the intake and exhaust for the coil is outside the grow room. Then you can run the a/c in a closed loop mode so it does not intake outside air ino the room or back out. Like running your car a/c on recirculation mode. The back of the unit(outside the grow room) has and intake typically on the sides and exhausts heat out the back...

bubblegumgreen said:

I'm assuming that the front of the unit is sticking in the grow room and the back where the intake and exhaust for the coil is outside the grow room....

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if you go back to the beginning of the thread it sounds like he has no windows in the attic and has just placed it in the room. he wants to build an exhaust loop from the back of the ac to his chimney.

a pic would help but i think your assumption is wrong.

Yes a pic would make it much easier. I hope he has no window in his grow area. I'm assuming he has a room built in the attic and he is not just running wide open in an uninsulated attic as there woudl be no point in trying to cool that.

What I was saying is:

Cut the correct size hole in the wall of the grow room.

Stick the window a/c thru the hole so the back of the a/c is outside the grow room and the front is inside. Put the a/c on closed loop or recirc mode. The front intakes air cools it and dumps it back out into the room. the back intakes air from the sides over the coils and exhausts out the back.

Does that not seem like the best option for him?

i think he's concerned with smell escaping so he's planning: ac exhaust>home made box>ducting>filter>chimney

no reason it wouldn't work if he can get the line airtight.

Odor Right.. Thats why I'm saying use a window unit that has a closed loop or recirc mode its 2 seperate systems in one. 1 Draws air in the front cools and exhausts out the front. The other draws air in the sides of teh back and then exhausts out the back so no odor as the air exhaustd out of the back was intaked from outside the room..

Following me?

I've set up a few like that before and thats how i'm doing my new one..

I was plannig on using a portable but that exhausts air from inside the GR to the outside and that's a no no. Thats unless you add a inline booster and carbon scrubber to the exhaust.

bubblegumgreen said:

Odor Right.. Thats why I'm saying use a window unit that has a closed loop or recirc mode its 2 seperate systems in one. 1 Draws air in the front cools and exhausts out the front. The other draws air in the sides of teh back and then exhausts out the back so no odor as the air exhaustd out of the back was intaked from outside the room..

Following me?

I've set up a few like that before and thats how i'm doing my new one..

I was plannig on using a portable but that exhausts air from inside the GR to the outside and that's a no no. Thats unless you add a inline booster and carbon scrubber to the exhaust.

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and i was assuming he doesn't have a closed loop option on the ac - he already owns it so he can' buy -or he woudn't be thinking about the box - and was going to add an inline booster and carbon scrubber.

Ahhh. So, would it not be better if he just puts a box over it with a passive intake and just run a small can/filter combo and dump that air into the attic? Thats if he does not have closed loop option. I'm not sure why he wants to use the chimney.. I just don't see the need...

I'm not an expert at anything but I did do hvac for a number of years so I know how all that works. each system is different so if he is trying to use what he has I think a box as i stated above would be best. It also helps muffle the sound of the a/c...especially if you are using an older unit..

I don't see a problem vewnting out of a chimney. You just need a flu (it only opens up one way, and stops air if it tries to go the other way)to make sure no air/smoke from the chimney backs up into the attic when the AC is not running.
If u don't like that idea, vent out the roof. Cut a hole and insert a flashing, and a vent cap and viola. $35 bux with 25 feet of flex duct, sheetmetal screws and a tube of caulk for sealing included. U will need to get on top of the roof to put the cap on though. Keep that in mind... When u are done using the vent for growing u just stuff it with insulation. .

Thank you guys for all the replies. The chimney is not in use at this time so I don’t have to worry about smoke and what not. I have already bought the AC unit. I know 24000 is big but I wanted to be safer than sorry. The attic is going to get hot in the summer, with 3 - 600W HPS, 1 – 400 MH, CO2 generator and other equipment. I did consider running my lights at night when it is cooler out. I have decided agents this. I don’t want the risk of light leaks and thermal imaging I would have running the lights at night.
So What I have decided to do is. I will build my box around only the very back of my AC unit. From that box I will run duct to the chimney (probably only 2 or 3 feet). I will have a small inline fan to help pull this air out.

Now I need to decide what to do about the intake the AC needs from each side of the unit. Should I get this from outside? Will I have a problem in the winter? Will my AC unit freeze up if exposed to the MN winter? Can I get this intake air from my grow room itself (probably not my best option as I will be running a co2 generator)? Can I get this from the attic (the part that is not my grow room) (this air will get pretty hot in the summer)? What is my best solution?

The easiest solution would be to get the intake air from the grow room itself. How much CO2 would this waste? My room is about 700 sq feet. Would I have to change my 20lb propane tank more than once a month running it this way?

GrowinONup said:

Ok I understand now. Thank you all for the help. I am going to need to seal off the entire back side of my AC unit and duct it out the chimney. What can I use to seal around the back of the unit to gather it all into my 6” duct?

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looks like you've got it solved. i would go to home depot and get a "register connector" for whatever sized duct you want. "register connector" is not the official name, but it is a piece of sheet metal that has one end that's round for the duct and the other side is rectangular to fit the heating register. use duct tape to seal the rectangular end to the exhaut vent of the ac.

GrowinONup said:

I will definitely get a “register connector.” Thanks that will make it easy to connect the duct to my box. What about the intake on the sides of the AC. Can I get the intake from the grow room itself? How much CO2 would this waste. I’m not so worried about the waste, I just don’t want to drag a 20lb tank in there every week.

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the only issue about co2 is if your plants are using more than can be replaced by air entering the attic. since attics are far from air tight there should be plenty of co2.