Running an AC unit inside an attic

that will work if you seal the lines well. also, think about insulated ducting for your heat vent.

i may not have understood your design. you have to keep the hot exhuast air completely separate from the input air. you want a box around the exhaust side with ducting to the chimney. you can get your input air from either outside or recirculating air from the attic.

Ahh, refrigeration and growing
I think, like the experienced gowers here, they like pictures so they can advise, I'm not a Grow Pro, but I am a refrigeration fella, A picture of the layout with ac would help me loads. I read your first post and only understand you venting the hot air out, the rest is a bit fuzzy for me, if you got a picture or some sort of the layout on paper or something I'm sure that, I / The forum, could advise better.

If it's a window unit, the back has to be able to get fresh air, the condensor needs to have air to draw away heat, hope you're considering this.

If the attic isn't well insulated, or has low ceilings, you may want to rethink your strategy

Yeah, putting an A/C in a hot attic, with the cooling fins in the hot air, isn't going to be very efficient.

GrowinONup said:

The AC unit is going to be set up so the cool air is blowing into my garden. I will build a “box” around the back of the unit. The back side of my “box” is the outside wall of my attic. That outside wall has a vent in it (going directly outside). This is the vent I plan on passively drawing in air. Then I plan on blowing (with my fan) air from the box out the chimney. So I will have air constantly circulating inside the “box” pulling the hot air out. I hope this makes since. I have insolated my attic fairly well.

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if i understand what you;re doing, i have the same comment about mixing input and exhaust air - it will not work. you do not need to mix air with your exhaust - just have it vented to the chimney. you don;t really need the box, just a sealed duct on the exhaust of the ac

I am going to be running into the same dillema next spring/summer. I am currently in construction of my new rooms in my attic. I will have 2 4x4 flower rooms, 1 2.5x6 veg room and 1 2.5x6 dry room. The attic is above my garage which is heated and insulated. The floor(garage ceiling) was already insulated from when I finished the garage. Due to much conduit in the way I had to make a raised floor with 2x8's. I insulated agin the floor perpendicular to the existing insulation mainly to insulate any sound heard from below as i host many poker games etc. The floor is osb board. The ceiling and walls of the room in the attic are insulated with 6" rolled insulation and then covered with osb board. All walls and floors were covered with think plastic sheeting to help with odor control before the addition of the osb board.


My attic is typically 85-125 in the summer months so it will take quite a bunch or a/c to cool it down. I will be using a 15k window air conditioner for the room running in closed loop mode so it does not bring in or exhaust outside air for smell or co2 reasons. In the winter months it also gets really cold. I have the current setup set so during the winter months the winter months the air cooled hoods will intake and exhaust into/from the main room/work area in the attic and am hoping that the 2 1000 w hps and 2 Tek lights will keep the area warm enough. If not I will add a portable heater in needed. In the summer monhs I fip the damper and will intake and exhaust the hoods to the outside air.

My total room contains the 4 rooms mentioned above and is 8x6 and 13x6 with a peaked ceiling in the center. So about 126 total square feet to cool. the actual rooms are 16 sq/ft for each of the 2 flower rooms and 15 sq/ft for each the veg and dry room. I am using a fuzzy logic co2contoller to keep the main room stable @ 1250ppm and will be using passive intakes from all 3 rooms that will draw air from that main room and dump the clean scrubbed air back in. Using a 265 can fan for each flower room and a 6" duct booster fan for the veg room.

Now that summer is over I have not had a chance to really test it but a 15k btu window unit running in closed loop mode for the well insulated 126 sq/ft room although in the attic should be fine as I am looking to keep teh temp @ 78...

"I will be using a 15k window air conditioner for the room running in closed loop mode so it does not bring in or exhaust outside air for smell or co2 reasons."

What will you do with the hot air the AC spits out?

GrowinONup said:

"I will be using a 15k window air conditioner for the room running in closed loop mode so it does not bring in or exhaust outside air for smell or co2 reasons."

What will you do with the hot air the AC spits out?

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I think that means the cold air side is closed loop.

the hot air side (exhaust) doesn't need much of a "box". it just needs something to gather the exhaust air together and funnel it into the ducting. just duct the exhaust air from the back into the chimney. the ac unit will breath in from the sides, this can be passive intake from the outside.

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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you could use sheet metal to build a funnel type thing. maybe a small wooden box would be perfect for the job. something like 4 inches deep, and the size of the back of the ac unit. fit it right up against and foil tape it together. I've seen other people do something like this, there are pics around online. I'd do it, but I can't have the noise.

Or you can get a portable a/c unit and just vent it out the grow room and use an inline booster to vent it out..

Maybe something like this...

WA-1410H Sunpentown Portable Air Conditioner

lorenzo08 said:

I think that means the cold air side is closed loop.

the hot air side (exhaust) doesn't need much of a "box". it just needs something to gather the exhaust air together and funnel it into the ducting. just duct the exhaust air from the back into the chimney. the ac unit will breath in from the sides, this can be passive intake from the outside.

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If you are doing it in an attic and running it in a closed loop mode meaning just the tempered air side.. why do you even need a box?? It will just intake and exhaust air from the attic for the condensor(back) side..

Ideally what you and I need is a mini split system.. It has a blower system that goes in the room and a consor that goes outside..

Amcor-Toshiba Compressor 12000 BTU Mini Split AC

bubblegumgreen said:

If you are doing it in an attic and running it in a closed loop mode meaning just the tempered air side.. why do you even need a box?? It will just intake and exhaust air from the attic for the condensor(back) side..

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a small box to help collect the hot exhaust air from the back of the ac unit. connect the ducting to that small box and route it to the chimney. try to have fresh air coming in near or directed towards the side intakes to help it run more efficient.