Greenhouse ventilation

Polytunnels, cold frames, greenhouses, propagators & more. How to get the best out of yours...

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vivienz
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Greenhouse ventilation

Postby vivienz » Sun Apr 26, 2020 3:30 pm

The weather is set to cool to somewhere near normal (whatever that is, these days) next week and we may even get a bit of the wet stuff. This week, however, saw the temperature in the greenhouse peak at 46C which is pretty close to cooking the plants in there.

As the greenhouse is of cheap and cheerful construction, I can't go cutting holes in it to put in side louvres or wall mounted fans, so I'm looking at alternative means of pushing some air through it. I've looked at the solar panel powered fans that you can get but I'm not keen. Plonking an opaque panel on the roof of the greenhouse rather defeats the object of the greenhouse itself, plus I have low confidence that it would remain in situ over the winter with the fierce winds we get here.

My current thinking is that the simplest set up for me would be to have a 12V portable oscillating fan (the sort that can run from a car plug in socket, cigarette lighter socket in the old days) and a deep cycle leisure battery. We have surplus PV generation from the house array in summer and so rather than exporting it, I can charge up the batteries and they can sit in the greenhouse, powering the fan. How long they last depends on the power rating of the fan, but I'm after more of a continual gentle breeze than an all-day howling gale, so I imagine I should get a few days out of one charge based on a 7 hour operating day.

Other factors in favour of the battery/fan set up are that it can all be removed from the greenhouse as soon as temperatures fall in the autumn and it's no longer needed, and it comes in at about half the price of most of the solar set ups that I've seen.

Has anyone tried this system? I'll report back on any progress with the project, but I hope to have something going over the next couple of weeks for when the sun starts shining again after our long overdue rainy spell.
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Stephen
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Re: Greenhouse ventilation

Postby Stephen » Sun Apr 26, 2020 8:27 pm

Vivien, I can not answer directly but an oblique view is that I use a "Carcoon" air bubble over my car stored in a council garage which has no power.
So I use two leisure batteries and alternate them between garage and being charged at home. The air bubble uses two 25W 12V fans (probably off a desktop computer power supply), so a 110AH battery will last for 8 days.
It's an adequate solution.

P.S. 110AH batteries are heavy!

It is also worth remembering that you do want some cool air to come in/hot air to go out. You don't want to just be pushing warm air about! That's a fan oven!
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vivienz
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Re: Greenhouse ventilation

Postby vivienz » Sun Apr 26, 2020 8:57 pm

Good point, Stephen. An oscillating fan would just turn it into a fan oven. I think I may have found a lower priced solar solution, but I'm still not keen on attaching anything to the roof as it's only polycarbonate. Also, the batteries could be used for other purposes, such as small amount of heating to keep the greenhouse frost free, as long as I also added sufficient insulation. It would be more adaptable than a dedicated solar panel. I shall keep researching for the moment, but thanks for the information.
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Geoff
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Re: Greenhouse ventilation

Postby Geoff » Sun Apr 26, 2020 11:19 pm

I've gone back to your post with photograph viewtopic.php?f=24&t=14306&p=155024#p155024, if that structure only has a door and no other openings you are on a loser from the start. Is the one behind it going to be the same?
If there is no option from the manufacturers (shame on them selling a product that is not fit for purpose, name and shame them) all I can think of is making a secondary door that hinges inwards, this could perhaps be wire mesh with half of it having a removable panel so you could have two levels of ventilation. Even then you wouldn't get sufficient ventilation without some means to move the air like the solar fan you are considering, perhaps it could be built into the secondary door. No wonder you asked about shading after burning your tomatoes.
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retropants
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Re: Greenhouse ventilation

Postby retropants » Mon Apr 27, 2020 8:28 am

Hi Vivienz, I only have one door on mine, and it has been over 40 already this year too. I throw dense green mesh over mine and peg it on, on the sunny side, it really does help.
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tigerburnie
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Re: Greenhouse ventilation

Postby tigerburnie » Mon Apr 27, 2020 9:48 am

As we all know heat rises, so you need to get some sort of roof exit for it, how are the top panels held in place and are they glass or polycarbonate, I can't quite tell from the photo.
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vivienz
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Re: Greenhouse ventilation

Postby vivienz » Mon Apr 27, 2020 2:45 pm

The greenhouse does have an opening window at the opposite end to the door so it is possible to get a throughput of air, though not enough if the wind us in the wrong direction. I'm going to have a look at bathroom fans as another option as they can move a reasonable volume of air.
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peter
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Re: Greenhouse ventilation

Postby peter » Mon Apr 27, 2020 9:06 pm

Take a look at Two West's and Elliot online, they do all sorts of greenhouse spares and accessories.
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