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Greenhouse Ventilation


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Hi guys,


I got a greenhouse that is 3.1m x 1.85m x 2.5m


Id say it has roughly 14 cubic meters of air, the idea is to vent the air in this greenhouse though insulated ducting out of the roof of this greenhouse, then into the roof of my house. The air will also be filtered.


The location of the greenhouse is wedged in between my house with approximately 1m of awnings and a about a 2.4m fence, the top of this fence has white laser lite which will reflect some light and also let a little though.

The sun passes over the house then over the fence, so the location does not get full sun all the day, but will get a few hours every day and a bit of shade - so the greenhouse will not be pelted with heat constantly all day.


My question is, how many times an hour will i need to exchange the air in this greenhouse to control both humidity, heat while giving the plants plenty of fresh air? A bit of research on this says i should replace the air at least once ever minuite- is this necessary given the location of the greenhouse ?


My calculations work out to be about 734 cubic feet per minuite of air that would need to be exchanged after taking into account 30% of inefficiencies.

A Canfan 10" maxfan with a canfilter 100 on it would move 834 CFM - i am rather concerned about the noise though this thing sounds like a fucking rocket engine - you can get a gauge for just how loud it is by checking out this video here:



Pretty funny video too, dude nearly takes his fingers off multiple times


Any insights on this would be greatly appreciated !

Edited by Bento
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You may be able to augment or even eliminate mechanical vent if the outdoor siting is suitable to passive ventilation. You have put effort into determining the suitability of the siting for light. Get to know the prevailing trade-winds and how they affect the airflow through the area (and structures, can create vacuums, wind tunnels etc) and maybe use this to advantage? If you've all ready considered these factors sorry for ramblin lol I so wanna grow grnhouse/outdoor....just gonna take another 10yrs or so lol


Peace to you Bent one v. :sun:

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If the greenhouse was an equally sized growroom i would be changing the air about 25 times an hour, so then i would need something around 270 CFM which would be a lot more cheaper and quieter - however i have not been able to find any information that supports that 25 times an hour in a outdoor green house is sufficient enough to deal with the heat of a west australian summer.


I would really like to hear from someone who has had first hand experience venting a greenhouse.

Edited by Bento
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u obviously wouldnt need to vent a smuch as an indoor grow room

my intention wa sto give u a rough ideA

wats stopping u running the fan with a temperature and humidity moniitor in a few diff areas

is the green house fully sealed?

i fnot why dont u think it will not be able to cope with standard everyday temps

takle care


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its my experience that a non-vented greenhouse can get in excess of 50 degrees easily on a fairly warm spring day, even in just morning sun.

That is what my thermometer maxes out at - that said in my limited experience with very small greenhouses that mature plants (non-cannabis) seem to handle this pretty fine its the younger plants that really get hammered and usually die off.


If it were you Billo how many times would you aim to exchange air per hour?


The greenhouse is not totally airtight, but its airtight enough that creating a negative pressure system inside the greenhouse should not be too difficult at all with the aid of an extraction fan - this would also help with odor control with a carbon filter also attached.


The reason in the original post that i was using an air exchange rate as 1 exchange per minute it that is pretty much what all the greenhouse CFM calculators \ internet articles on the internet advise, one site in particular even has an optimum airflow thats even higher than this number. I do think this number seems a bit excessive, after looking at the sort of extraction fans this calls for on youtube it seems like id be buying a small extremely noisy jet engine. Normally after reading from multiple sources id accept that as true, but the fan seems totally crazy overkil - im actually starting to lean back towards 25 times an hour being alright, but its hard to just discard the information i have read on these sites that at least once per minute is necessary and even more than this is optimal.


I really dont want to go overboard with totally overkill gear thats too noisy and doesn't do the job but on the same hand i dont want to buy gear only to have it function badly - i have done a fair amount of research on this before posting here and really i still dont know which is the best way to go for my situation.


I appreciate the help by the way :)


Edit: did a bit more searching and a 8" RO canfan + Canfilter 50 would exchange the air about roughly 34 times an hour and come in about $435 + shipping + ducting

It would also end up being a lot quieter and cheaper than attempting to exchange the air once per minute. Keen on opinions on this !

Edited by Bento
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a few reasons:

1) Growing indoors at this present point in time is impossible

2) Smell containment

3) i dont have to pay a power bill for the sun

4) The neighbours are nosey on both sides

5) i can lock a greenhouse up

6) climate control

7) Its combines the best parts of indoor growing with the best parts of outdoor growing

Edited by Bento
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Heya Bento =) , mate I've run greenhouses out back, to be honest, in the middle of summer, its a mission, and ya need all the light ya can get for ya plants, not just a few hours, wont work well, and you'll end up with fluffy buds.

If I run a greenhouse I have all me plants vegged and ready to go by end of June, then into the greenhouse, let em flower and buds by christmas..... after that it gets too hot and you just end up with piles of mould due to the insane humidity.


If ya have nosey neighbours man an outdoor grow may not be for you. I would go inside when ya can......after summer and then have yaself ready for a greenhouse grow next year.

Ya dont need to vent into ya roof, and if ya do, all ya doing is pumping humid air into ya roof space, not a good idea. Also carbon filters and fans dont like too much moisture, carbon filters will fail and fans will rust.


A good greenhouse needs an intake area, roughly 1/3 the size of the greenhouse, covered in shade cloth, this allows the air to cool before entering and keeps bugs out.

Fans go on the opposite end up at roof height.


You can do it mate, but gonna take some planning, and be a good project for when the temps are right. Peace. Nibbler.

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