Greenhousesalsa
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Location: Apache County, Arizona

Exhaust fan in a greenhouse

Last summer was my first attempt at greenhouse gardening.
We had a tough time with heat build up. Some days the temperature would go over 105. We added a fan on a timer, and also misters.
Eventually we also put shade screen on the south end, where the sun beat in.
This year, we want to put in an exhaust fan. The greenhouse is about 1600 cubic feet. Reading on the fan websites, they say we need a fan that will replace the air in the house in one minute. Without spending an arm and a leg, we can get a 14" fan, that moves 1400 cfm.
We can get a smaller fan with just under 900 cfm.
1400 seems a bit strong.
Does anyone have any experience with exhaust fans and is it really necessary to have one as big as 1400 cfm?
Thanks!

imafan26
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Location: Hawaii, zone 12a 587 ft elev.

It depends on the size of the greenhouse and the placement of the fans. If you have side and roof vents you should open them up to let the warm air out. As the warm air rises to leave it will pull in cool air from the side vents that will help with cooling as well. The best placement for fans is two fans at near the apex on either end of the greenhouse and additional fans placed about mid height so that the air blows through the leaves not at the pots or over the plants. To get the most out of the fans, from the side with the prevailing winds have the fans blow into the green house, have the fans on the opposite side run in reverse. It maximizes air flow and the fans will work to pull air through the house better if they are not blowing at each other. If you have fans only at one end it needs to be able to reach across to the other side otherwise you will have pests building up where the air flow ends. You can use smaller fans if they can reach but you would be better off still placing fans on both ends to help pull the air through. Greenhouses were designed for solar gain which makes is valuable as a season extender, but it is a liability in the heat of summer.

CanadianLori
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Joined: Sun Apr 24, 2016 6:19 am
Location: Oakville, Ontario 6a

One thing that I found helps dramatically with heat is that I put a "screen door" on my small greenhouse. Yes, open vents can help but they need to suck the air from somewhere so if you have a screen door, the circulation boost is immense.
Hope this was of some help.
Lori

PingoBags
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Joined: Thu Apr 07, 2016 6:31 pm

CanadianLori wrote:One thing that I found helps dramatically with heat is that I put a "screen door" on my small greenhouse. Yes, open vents can help but they need to suck the air from somewhere so if you have a screen door, the circulation boost is immense.
Hope this was of some help.
Lori
Can't believe I never thought of that. I built my first greenhouse 2 months ago while we had a week of only -5 weather and not once did I take heat into consideration. Now that it's getting warmer, I added a fan and some shade cloth.
I will definitely be adding screen to my door. Not the type that goes on windows, something more raccoon proof.

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MoonShadows
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Location: Stroudsburg, PA - Zone 6a

I have automatic roof vents and two doors two Dutch doors in my greenhouse. Even with the roof vents open and the tops of both doors open, my greenhouse can reach well over 100F. I want to get an exhaust fan for next season, but I rigged up another fan this past season and it really helped. I was able to attach a blower fan near one of the roof vents. It really helped to move the air out at a time when I couldn't afford a regular greenhouse exhaust fan. Another solution if you don't want to or can't buy an exhaust fan would be one of those accordion window fans. They can be easily attached to the inside of vents, and with a hot thermocube, they can automatically come on at 78 and go off at 70.

This is the fan I used.
61td8SgTd-L._AC_UL115_.jpg
61td8SgTd-L._AC_UL115_.jpg (4.78 KiB) Viewed 11121 times
This is a thermocube if you are not familiar with them.
41-LGE8gISL._AC_UL115_.jpg
41-LGE8gISL._AC_UL115_.jpg (2.81 KiB) Viewed 11121 times

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applestar
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Location: Zone 6, NJ (3/M)4/E ~ 10/M(11/B)

I don’t have a greenhouse (yet), but have been making lists of what I think I would “need” when and if. And thermocubes —both hot and cold— have been on my list for some time. It’s good to be validated that it’s a worthwhile item. :D

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MoonShadows
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Joined: Mon Dec 11, 2017 4:50 am
Location: Stroudsburg, PA - Zone 6a

@applestar, they are also much cheaper than thermostats and work well.

imafan26
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Joined: Tue Jan 01, 2013 8:32 am
Location: Hawaii, zone 12a 587 ft elev.

Our "green houses" serve a different purpose. It still has high humidity and usually 10-15 degrees warmer inside the hot house than outside. Our side walls are usually made of screen fabric so air can pass through them. We need the fans to lower the temperature in summer which is already 90 degrees outside. A solid opaque roof cover is preferred since we don't want a lot of sun and for flowering plants, we don't want rain to get on them either since it damages the fruit and flowers. The fabric walls are for bug control. Some people find the opaque roof still lets in too much light and heat and they will put an additional screen cloth or even tarp under the roof to block more light. an offset roof with screens are more common than a vent you open and close. Most of the time the vents are open. They have to have wire mesh screens to keep the birds out.



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