4000w in an attic... pro advice needed!!

What's rollin folks?
I would like to get some thoughts about attic growing in general, particularly climate control issues. I am in a unique situation where i have a large finished attic with a 13x7x8 grow area.

I know the most important variable to control will be temperature. I have 2 ecoplus 8in fans w/ a 8in carbonaire filter. I have access to 3-4 aircooled xtrasun hoods, but I really like going barebulb if i can. I am in the process of purchasing an a/c unit soon, most likely portable around 14,000btus to cool 3000-4000w hps

My main issue is ventilation!!!! The only current option i have is a skylight in the attic which is on a 35-40 degree slope and opens about 10-12 in. My climate is VERY rainy come winter. I was thinking of keeping the skylight open 24/7 and nailing up some plywood to exhaust both my ac and lights , but the worry about leaks, mold, and possible property damage has begun to sink in. Would the exhausting heat dry any moisture that may leak into the skylight(there is a 1/2in rubber lip that extends above the roof shingles). Is this a sound idea or am i insane?

My other option is a furnace flue that runs through the attic. It is for the furnace only which will not be used. I would like to utilize that perfect 6 in pipe to get rid of all of that steaming air... especially in winter. I have really limited hvac experience and this house is about 100 years old. I would like to tie a y fitting or something similar (and possibly temporary) to serve as the main vent for the garden heat. The furnace flue is in working condition and seems like the better option. Can a section of it be removed and replaced back to original?

Any advice, even horror stories of crop failures in the attic are welcomed

cheers

plenty of people grow in there loft, the only problem that i can think of is the loft will be naturally hot to begin with, so the a/c is a good move, as for venting into your flue then you should be ok but will probably need some sort of back draft damper to be on the safe side, someone with more knowledge will answer that for you, for the window you could just open it a 1nch or 2 just enough to let the hot air out but no rain in, either way will work but i think the flue option is the better/safest because that's y its there to get rid of heat,

well, if you use the furnace flue, you may have 2 problems: carbon monoxide risk if anyone ever uses the furnace, and/or negative pressure may actually blow your air and odors down the flue and into the house. can't you just cut a hole in the wall and put in an attic vent? attached is a photo where i drilled holes in the brick with a hammer drill, then chiselled out the brick, and inserted an 8" aluminum sleeve. this was for a fireplace though.

Thanks guys. I am planning on using backdraft dampers either way. I'll post some pics tonight to show the actual space.

Rob: do you think keeping the skylight open only a few inches be enough room to vent that much hot air? The skylight is 24in wide. If i open it upward a few inches it would be about a 4inx24in gap. I plan on at least 2 vent holes in the plywood.

Saint T: I am renting this home so creating holes is really not a viable option. I do like your spirit though. I wish i had the balls to start chiseling bricks

Thanks again. Keep the ideas coming.

Cheers

So here is the actual space i plan on using. Notice the furnace flue and skylight. These will be the only exhaust/intake options available. I also included a pic of the basement and furnace. It seems to be a straight shot from the attic to the basement(pic 1 and 2). I would like to take in air from the skylight through my lights the DOWN the furnace flue. I would like my A/C to exhaust up the flue out the roof vent. My only question now is the proper way to tap into the flue. I don't want to mangle anything beyond repair. Any suggestions?? Is this even possible??

2-3 inches x 24" should be fine, i would give it ago and see what happens even if you need to open it another inch or so

Is this just an exhaust vent for your furnace? what are winter temps in your area? If the furnace is not in use then just rig a y connector up side down so the fork of the y is facing the ground and put an inline fan on the other end closest to the roof. Will suck everything connected to the y right out. you may have to construct something to block off the part of the y thats going down to the furnace if its not pulling enough air, but its worth a try. Youre probably going to need some type of inline fan to help vent anyways. Leaving windows open when youre not around is a scary thing.....especially if its in a part of the house that you dont frequent much.

Youll need some serious cooling for 4000w in a loft.scroggsy.

DO NOT FUCK WITH THE FURNACE FLUE PIPE! in my opinion, you are next to the roof so you would be silly if you didnt add your rooms own vent through the roof. Unless you know what you are doing or hire a professional, fucking with the flue pipe can easily kill your plants, you, and your family. I am not going over board, it can be very, very, dangerous

edit: My father is a second generation sheet metal worker in the union. I am not a pro myself but him and my grandfather are. You can do what you are talking about but the flow has to be right, shit has to be sealed well and high end mechanical dampers should be in use timed with your fans and furnace. It is no joke what you are talking about if you want to do it right.

Ive been lookin at those pics and i had a revelation. Why dont you just pull one of the boards off of the wall up there and install your own stack coming out of the roof. When the time comes to leave, cap the stack at both ends and seal it with construction grade sealant and presto, no one is any the wiser. Put the board back up and you cant even tell from the inside. The more i think about routing your exhaust to the furnace flue, the more i really dont like it, chasmtz has some really good points.

Hustle, Couple quicks i think about when i am planning my exhaust... I have a chimney at my house that isnt used at all.. I was gonna blow my exhaust through there but if i was growing in the winter where I live I am pretty sure there would always be steam comin out of my chimney.. My draw some questions on why my "furnance" is always running. I found a different way but its not available in your options..
Mine is.... I have a garage in the back which has a drain that runs from the garage into the basement. Well someone before me cut that 6'' pvc pipe for a hiding spot.. well I was just gonna tie my exhaust vent line right up to the pvc pipe and it will blow the exhaust up and out through my garage.. lmao.. i think its an ingenious idea. But then i am worried that in the winter that exhaust will melt the snow on the top of the garage or melt it away from the garage door .. JUst something to think about i guess. I havnt had a grow yet. Probably wont for a couple years.. But in those couple years I am gonna have everything planned out thats for sure. I am gonna start buying piece by piece so when its time. I have everything already. Keep us posted with the pics man! I think you have a great location.