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Topic # 151565 29-Aug-2014 08:40
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I have a new custom built greenhouse I was talking about a while back, made with twinwall plastic. It's working well, in winter with ambient temperatures during the day of around 10-15 degrees it gets up to 20-28 degrees, at night I've seen the outside get down to -1 but the greenhouse rarely goes below 3-5 degrees. So that works pretty well.

I'm thinking heat is going to be a pretty big problem in summer, it wouldn't surprise me if it hit 40 degrees, and photosynthesis is impaired over around 28 degrees. I have doors at each end, but I don't want to have to open and close them manually every day, plus that will let pests and birds in. I have self opening windows, but they're not huge. I also have a panel that can be removed and a grill put in for summer. I have no power in there but I could get it in there reasonably easily.

I think I'm going to need mechanical ventilation. I've read you need to change the air in the greenhouse once per minute in summer, which means a decent sized fan. Ideally I'd like to find a shuttered fan, so I can have it there summer and winter. Without shutters it will let a lot of heat out, which isn't great for winter. Initially I thought about solar, but I don't think a solar fan can generate the airflow I need. I'd need a thermostat as well.

I'm having trouble finding anywhere in NZ that sells a suitable fan, my google-fu is failing me. Does anyone know of anywhere good that sells this stuff?




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  Reply # 1117728 29-Aug-2014 09:05
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SOG ? i know they're more indoor, but they'll probably have a clue.... 




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  Reply # 1117740 29-Aug-2014 09:24
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Switched on Gardener has some fans, but none shuttered. Good idea though.




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  Reply # 1117769 29-Aug-2014 10:30
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Could you not attach a backdraft shutter to a fan - like this http://www.electrical.co.nz/vents-150mm-back-draft-shutter





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  Reply # 1117783 29-Aug-2014 10:47
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Not really. That thing is huge, I want a small window/wall mounted fan, not a system that's half a meter long.

I wonder if a bathroom fan would do the job. It's probably not powerful enough.




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  Reply # 1117796 29-Aug-2014 11:04
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If you want to change the air once a minute, you're going to need more than your average kitchen expelaire unit, but you could check the plumbing/electrical places for a standard window mounted fan unit.



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  Reply # 1117805 29-Aug-2014 11:08
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Once a minute sounds like a lot to me, especially if I use shade cloth or something. I'll try AHI Carrier and Fantech, AHI is pretty reasonably priced, Fantech not always. I just wondered what other people use.




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  Reply # 1117812 29-Aug-2014 11:27
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You probably will need to cover the greenhouse in shade cloth during the hotter months.  Depending on the prevailing wind etc you might be able to set up a matched set of of DC fans at either end (incoming air low down, hot air at the apex).  It's been over twenty years since I studied greenhouse design but I seem to recall a borer bomb being used as a smoke bomb for looking at what the air movement does inside the green house.

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  Reply # 1117826 29-Aug-2014 11:31
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Aliexpress has some solar fans here.



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  Reply # 1117832 29-Aug-2014 11:38
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Wow, solar fans are expensive. I think power and standard fans will be cheaper. Shade cloth is no problem, I'll just have to find a good way of attaching it above the greenhouse, no point putting it inside I guess.




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  Reply # 1117838 29-Aug-2014 11:49
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timmmay: Wow, solar fans are expensive. I think power and standard fans will be cheaper. Shade cloth is no problem, I'll just have to find a good way of attaching it above the greenhouse, no point putting it inside I guess.


It isn't too hard.  Dowel at either end to keep it rigid works.

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  Reply # 1119350 1-Sep-2014 09:22
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timmmay: Not really. That thing is huge, I want a small window/wall mounted fan, not a system that's half a meter long.

I wonder if a bathroom fan would do the job. It's probably not powerful enough.


You can get much smaller ones. I bought one from bunnings or mitre 10 a while back for my bathroom extraction fan. It was only a 100mm diameter unit and the outside wall mount plate had a swinging door on it which would shut if the fan wasn't operating to prevent backdrafts while the unit wasn't on. That would be what you're after.

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  Reply # 1119356 1-Sep-2014 09:32
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I'd try to use convection to do the job, it's probably as effective as a fan if you set it up right.

With a reasonable opening at the top and bottom you should get good airflow. Cover the openings with chicken wire to stop birds without restricting air flow.





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  Reply # 1119361 1-Sep-2014 09:36
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Jeeves:
timmmay: Not really. That thing is huge, I want a small window/wall mounted fan, not a system that's half a meter long.

I wonder if a bathroom fan would do the job. It's probably not powerful enough.


You can get much smaller ones. I bought one from bunnings or mitre 10 a while back for my bathroom extraction fan. It was only a 100mm diameter unit and the outside wall mount plate had a swinging door on it which would shut if the fan wasn't operating to prevent backdrafts while the unit wasn't on. That would be what you're after.


Yep, that sort of thing, but I think I need something reasonably powerful. 1 air change per minute is what's apparently necessary for summer.

andrewNZ: I'd try to use convection to do the job, it's probably as effective as a fan if you set it up right.

With a reasonable opening at the top and bottom you should get good airflow. Cover the openings with chicken wire to stop birds without restricting air flow.



It's meant to help a lot, but mechanical ventilation is probably required in summer. The greenhouse is a lean to, so putting windows at the peak is difficult. I have an opening quite high up that's probably half a square meter that will be covered in a mesh or chicken wire. I'll make sure there's somewhere down low open on the other side too, for the draft. There are three self opening windows. The doors are huge but because of security I can't keep them open all the time, the greenhouse is on the border of the property and randoms could get in.




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