Fresh air intake + Air Con cooling = no need for exhaust fan?

My grow closet is 50 cubic feet, and its inside a room that has a window AC unit.

Too cool the grow closet I just run my window AC unit and use a fan to blow cool air into my 50 cubic feet closet. Fresh outdoor air goes into my closet via an intake fan and some ducting. My closet door is wide open with cool AC air and fresh outdoor air going into my grow closet. Iam leaving the door of my 50 cubic feet closet wide open to prevent stagnant air from sitting in my 50 cubic feet closet.

If I have fresh air intake and AC cooling my small 50 cubic feet grow closet do I really need a exhaust?

well that depends on what kind of light/hood you have and how close it is to your plants etc. Is smell not a worry for you because if it is the air still goes out but instead of pulling it into an exhaust to be cleaned the air is just going out wherever it can and your house will reek of bud. If you just set up the closet first and run your lights for a couple of days you will know if you need to exhaust or not.

nickbbad said:

well that depends on what kind of light/hood you have and how close it is to your plants etc. Is smell not a worry for you because if it is the air still goes out but instead of pulling it into an exhaust to be cleaned the air is just going out wherever it can and your house will reek of bud. If you just set up the closet first and run your lights for a couple of days you will know if you need to exhaust or not.

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Well I notice that whatever temperature the room is at the closet is also at that temperature, or very close to it. So as long as my window AC unit can cool the room then it can cool the closet while the lights are running.

As for smell I can always use a dionizer or just close the door of the room to prevent the odor from spreading all over the house.

haha its not that easy i was wondering this same thing. and i was thinking a can fan would take care of this? the smell that is.....

Yea dude no way just closing the door is gonna keep in the smell
Also not sure but will the air exuasting from the a/c smell also ?

bam bam said:

Well I notice that whatever temperature the room is at the closet is also at that temperature, or very close to it. So as long as my window AC unit can cool the room then it can cool the closet while the lights are running.

As for smell I can always use a dionizer or just close the door of the room to prevent the odor from spreading all over the house.

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You'll run into problems with odour/temp control if you only have an intake fan. As you blow fresh air into the room, it will displace the air already present, through any cracks and holes your growroom may have, and there will be plenty, houses aren't usually built to be sealed. This means that you will have odour leaks all over the place, and it'll take a lot to eradicate them. Closing the door isn't really a solution, if the door fits well enough to keep the smell in, it will raise the pressure inside the growroom and make the fan work harder, and also increase the chance of stale air building up in the room. (or if the leaks are big enough, just force the air out there instead)
IMHO, your best bet is to turn your intake fan around so it's an exhaust, and put a carbon filter on it. That way, you control where the air leaves the room, and you stop it smelling on it's way out. Do a dry run with all the gear up and running, without plants and see what sort of temps you get. It may be that you have to add a small passive intake hole on the opposite side to your exhaust to keep air flowing through the room, but generally if you move air out of the room, more will find it's own way in.
Hope this helps

whats up hairy bob? just found this post, and you and bam bam are having like the same convo we were.

Bam Bam I def. agree with hair bob, especially after he coached me t hrough my own lack of knowledge. set up an exhaust before you do intake...you might not need the intake...and it sounds like more than likely you won't. plus you can filter the smell...and ya there is no way it won't leak... if you are in a duplex...or apartments...someone is going to call and say it smells like skunk...and if they are pesty enough...someone will start knocking on doors...

But i got question though... so lets assume me and this guy bam bam set up our exhaust and we keep our window mounted hvacs working... we set up our exhausts on timers so that they kick on often enough to ventillate the rooms adequately... and we either do or don't put in passive intake holes...

How will we know if the room is getting enough fresh air... what if my room is sealed really well... which I am pretty sure it might be... I mean I have a couple cracks here and there... how will i know the room is seaking out enough air throughout the rest of the house and bringing it in through the cracks? will my intake fan sound like its struggling to find more air to push out the amount cfm's it's supposed to be displacing.

My first go is going to be hoping that the cracks are sufficient... if they aren't, then im' going to do the passive intake hole... No matter what will that hole be bringing in enough air...do the laws of pressure work so that the room will always be balancing itself out?

If it's not...THEN i guess i will get an intake fan... but i hear you should do half as many cfms on the intake as the exhaust as to ensure there will not be that build up of pressure which will force your air out of areas that you didn't want it going.

Thanks everyone.

I wouldn't assume that the cracks will be enough, depends on how well your house is built really. The problem with that is you have no control over where the air comes from. The air leaking in would be going through the walls, warming up slightly as it goes, making it less efficient than having some way of ducting in cool air. Why not just get an air con with a fresh air intake? That's the best way, cooling the air as it enters the room rather than letting it seep in and then get cooled. I thought that was what you meant with the switchable panels before tbh.
Don't get an intake fan, if it is smaller than the exhaust it will limit the amount of air coming in to whatever the fan can blow + whatever gets through the cracks. If you move air out, more will come in if it has a way to get in in sufficient quantities. How leaky your room is compared to how powerful your fan is will determine how much the pressure is lowered.
If the pressure drops significantly then the air becomes thinner, just as if you go high up a mountain. So it contains less carbon dioxide and oxygen atoms per cubic foot. I haven't come across this situation because everyone I have seen growing either has a way for air to come in, or they have an overheating room, but I would speculate if you were able to move enough of the hot air out to keep temps in check then you could conceivably approach a point where the plants begin to suffer as a result of the lower co2 levels. What that point would be, I wouldn't like to guess and I'm to wrecked to do the maths right now.
I could be wrong, it could be that by lowering the pressure in the room the air coming in would simply travel at greater velocity and at least come close to equalizing the pressure, keeping it at a constant, workable level. I have a friend who's last grow, before he moved, had such a big fan in it (bloody great steel industrial unit, 12" ductwork) that when you opened the door, inwards, you had to make sure to hold on tightly to the handle as you turned it, otherwise the door would fly open if the fan happened to be running, and that was with a decent amount of passive intake. That fan was the very definition of overkill though, I doubt you'll be extracting quite so much air.
I just re-read your q and if the fan is only coming on at intervals then the pressure would equalise in the time the fan was off, meaning most of this post is irrelavent. I'll leave my thought process there for you to see, I've typed it now...

Why not just get an air con with a fresh air intake? That's the best way, cooling the air as it enters the room rather than letting it seep in and then get cooled. I thought that was what you meant with the switchable panels before tbh.


100%...but where to find one that does this...window mounted hvacs don't bring in fresh air...they just recycle the air that is already in the room and blow the hot air outside...(at least that is my understanding)

Now I'm thinking I'd rather run my exhaust 24/7...
And I won't assume the cracks are enough and I'll do the passive intake hole... Any idea about size ratio... i should probably keep the intake hole smaller then the exhaust to ensure negative pressure right? I know that it will make my fan work harder then if the intake hole was like 5x bigger then the exhaust.

So if I run my exhaust 24/7 and my exhaust squirrel cage or something similar is working harder because of the negative pressure...any idea how long it will last...it's not going to go out on me mid grow is it?

If you create negative pressure the fan is not working harder, the air it is moving is slightly thinner and so creates less resistance on the fan blades. If the fan were blowing into a higher pressure, that would work the fan harder.
The passive intake only needs to be slightly smaller than the exhaust to maintain negative pressure, say if the exhaust is 4", the intake should be around 3 1/2".
A decent fan will be designed to run pretty much constantly for a long time, I've had mine running more or less full time since March with no problems, and it's been up there for the best part of a year. If you want to run it 24/7/365 it might be an idea to run it through a fan speed controller to prolong it's useful life. Even running it at 9/10ths will significantly reduce the wear on the bearings, and the noise level will drop noticeably.