Fans and air movement question

Do-it-yourself projects such as greenhouse or shadehouse builds and related topics.
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Sharpy
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Fans and air movement question

Post by Sharpy »

Hey all, I have done some searching through the threads here and didn't find or "missed" the info I am looking for.

Preparing for this coming summer, I am trying to make some of the last decisions on how my outdoor set-ups are going to go.

I will be using the small cold frame greenhouses, approximately 4' x 2.5' x 1.5', double wall polycarbonate. Planning on adding 6 inch vents in the bottom of the side panels and putting in a 6 inch solar fan in the top part of the back (tallest wall). So should have some air flow when the fan is off, and when on it will pull the air from the bottom, up and out the top. This is the plan anyways :wink:

Now I am looking at a couple different solar "attic" fans. One with a battery backup and one without. Do I need to be concerned with air flow at night? Not sure if I need these guys running 24 hours a day or just when they are getting directly hit with sun?

I am trying to avoid having the lids on the cold frames open during the day. I know they make the automatic openers but the cost is relatively high compared to the fan. And the openers still won't guarantee that the lids will close when a storm rolls through and I am trying to limit the amount of labor involved. Don't want to have to run outside and close all the cold frames every time I think its going to rain.
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Saxicola
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Re: Fans and air movement question

Post by Saxicola »

I'm confused as to why you are doing this setup for next summer. The Southeast, especially Florida is pretty much tropical in the summer. The smaller the greenhouse the harder it is to keep the temperature stable, and your setup is pretty tiny. Are you concerned about the rain? If so I'd just build a rain shelter that is mostly open on the sides so it keeps the rain out but allows good airflow and doesn't cause things to overheat.

Just got thinking... did you really mean summer or did you mean to say winter?
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Sharpy
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Re: Fans and air movement question

Post by Sharpy »

Yep I mean summer. The reason for going this route is because I can not build a greenhouse on my property. And after a lot of comparing of prices etc., it turns out that these small cold frames are actually cheaper per usable square footage then a full size "walk-in" greenhouse.

I really want the thing to be as pest and weather resistant as it can be and also low maintenance. Screened vents, closed lids, etc.

If the initial one works, like I am expecting it to, then I will be putting in a large quantity of these in rows in the middle of my back yard.

Just want to know if a fan really needs to be blowing once the sun goes off the cold frame. And if it is also needed to be running during the night.
Sharpy (Doug)

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iann
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Re: Fans and air movement question

Post by iann »

I too wouldn't want to be running an enclosed frame in a Florida summer. I would think a roof over the plants would be all you need or want. Maybe even a roof with some shade!

About running a fan, it isn't entirely for extracting hot air although that can be a bonus. In an enclosed environment behind glass you need to keep the air moving. Glass magnifies the heating effect of the sun and plants can overheat. Cacti and succulents are very prone to this because they don't transpire so they only way they cool down is into the air. Polycarbonate isn't as bad as this but you still get some of the same problems.

I run a fan in my greenhouse on any sunny day from about late March until September. Only during the day. Some people run a fan 24/7, it can reduce condensation and help to avoid fungal problems.
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Sharpy
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Re: Fans and air movement question

Post by Sharpy »

iann wrote:I too wouldn't want to be running an enclosed frame in a Florida summer. I would think a roof over the plants would be all you need or want. Maybe even a roof with some shade!
One benefit of the enclosed frame, for me at least, is stopping (or trying to, lol) the critters. Squirrels, mice, cats, racoons. All the fun little guys that like to get into things.

The enclosure isn't going to be air tight. I have lots of options for how much natural air flow I can provide. Will be (2) 6 inch screened vents in both ends, and can add another (2) to the front face if it seems to stagnant.

As for the shade part I have considered this also. Pretty sure the poly panels are only 10% shade factor so will be adding 30% shade cloth over the top. Or whatever shade factor is needed for what is going in that particular box. My brother owns a upholstery shop, so he can make me pretty much whatever I need. This is why I had asked Ralf about those plastic fasteners he uses for the bubble wrap. They should work great for holding on the shade cloth panels.

I may have to get one set up and put a hygrometer in there and see what the humidity levels do with the fan on/off, day/night.

Definitely want to avoid possible fungal issues.
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peterb
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Re: Fans and air movement question

Post by peterb »

I think temperatures inside a closed cold frame, even with vents and fans, will be extreme. I vote for an open shelter that is just a roof to protect from rain.

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Sharpy
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Re: Fans and air movement question

Post by Sharpy »

Once again I cannot have any kind of wooden, hand build stuff in my area. If its over about 30" tall I will have trouble from code enforcement. And if I do a 15" tall roof its going to be a tad difficult if not impossible to actually access the cactus unless I did the roof on some kind of hinge, but have not even thought about this before.

I did a little bit of calculations on the fan cfm vs. the cubit feet of the cold frame.

Cold Frame = ~11.2 cubic feet
The fan moves 11.6 cubic feet per minute.

If its exchanging all the interior volume every minute, then the inside temp is not going to be much different from the outside temp. At least I would think so.

I could also not put the poly panels in the ends and make a stiffening frame to fit the channel. Then either screen the ends or use a low shade cloth that's breathable. So the only poly would be the front, back and lids. Those also covered in shade cloth, or at least the top. (took a break during the posting) That may actually not be a bad idea because I still have the poly panels sitting around, and can be attached in the winter time :-k
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iann
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Re: Fans and air movement question

Post by iann »

I'll say again: this is not about removing hot air and replacing it with cold air. That is insufficient. You need to move the air over the cacti., even if it is hot air. It is the plants you need to keep cool, not the air, and plants in the sun will be a lot hotter than the air. The dangerous temperature for plants is 140F or more, not the air temperature but the plant temperature. Still air at exactly the same temperature as outside will allow your plants to burn. Of course you can shade them sufficiently to stop that, but keeping them in a hot dark box is really not the right way to go. A visual aid: air temperature around 30C, plant temperature a lot more:
thermometer5.jpg
thermometer5.jpg (69.26 KiB) Viewed 6362 times
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Sharpy
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Re: Fans and air movement question

Post by Sharpy »

So if I turn the fan around and make it a pusher not a puller, that would be better correct? That much air moving "in" would be like a small whirlwind I would think.

Keep in mind the height of what we are dealing with here also. Its only 16" tall at the high back point. 7 inches of that would be fan, leaving only 9 inches below that. The plants will be in carrying trays on a very short table. So the plants themselves are going to be very much in line with the air coming right out of the fan.

This help at all?

edit: And thank you for the pic. Also you mentioned about a "dark box". I was under the impression that 40% shade factor is a pretty good way to go. Several large cactus folks in the lower US seem to grow under these conditions. That is why I came to the conclusion that was a good value.
Sharpy (Doug)

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EdwardLowe
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Re: Fans and air movement question

Post by EdwardLowe »

Sharpy wrote:Hey all, I have done some searching through the threads here and didn't find or "missed" the info I am looking for.

Preparing for this coming summer, I am trying to make some of the last decisions on how my outdoor set-ups are going to go.

I will be using the small cold frame greenhouses, approximately 4' x 2.5' x 1.5', double wall polycarbonate. Planning on adding 6 inch vents in the bottom of the side panels and putting in a 6 inch
solar panels fan in the top part of the back (tallest wall). So should have some air flow when the fan is off, and when on it will pull the air from the bottom, up and out the top. This is the plan anyways :wink:

Now I am looking at a couple different solar "attic" fans. One with a battery backup and one without. Do I need to be concerned with air flow at night? Not sure if I need these guys running 24 hours a day or just when they are getting directly hit with sun?

I am trying to avoid having the lids on the cold frames open during the day. I know they make the automatic openers but the cost is relatively high compared to the fan. And the openers still won't guarantee that the lids will close when a storm rolls through and I am trying to limit the amount of labor involved. Don't want to
have to run outside and close all the cold frames every time I think its going to rain.
Hello friend were you able to find the solar fans you were searching for? I do have similar requirement of fans so can you help me out? Waiting for quick reply
Last edited by EdwardLowe on Sun Dec 01, 2013 4:38 pm, edited 1 time in total.
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Harriet
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Re: Fans and air movement question

Post by Harriet »

I'm a little late here, but since we share weather that only varies by 50 miles, I want to caution you about a closed cold frame even in the winter in Florida. Even with good circulation you are still running the risk of putting your plants in a convection oven!

I run a fan in my greenhouse (double wall polycarbonate) constantly during the daytime all year round, only heat it when the temperatures drop below 50, and still, on cool sunny days, I have had temperatures of 100 degrees or better during the day. I open the sides of the GH and leave them open for 9 or 10 months out of the year and the temperature seldom goes below 80 during that time; a closed box, even with fans would be much hotter.

I have been trying to kill a rat in my greenhouse for forever, because rats are not welcome, and I have seen signs of rat damage to a couple of gasteria. Someone tried nibbling on a euphorbia a couple of years ago, but that only happened one time! The only plants squirrels have gone after have been my Portulacaria and one other pretty little succulent with red flowers. (I am amazed at how "well" I retain plant names, old age is setting in!) Cats only want to poop in the sand, and armadillos, 'possums and racoons pay no attention to my C&S. I have no tortoses in my area, but I imagine they would be a threat to my opuntia if I did. So, unless you are having a completely different set of problems, keeping critters at bay may not be that difficult.

If you are doing this primarily for summer protection I would suggest screens instead of poly carbonate for the sides... AND the fans! You can always wrap the sides in poly film during the winter. Also, I think both Jens and Ralf have posts with pictures of outdoor beds with designs that might be adaptable to your zoning restrictions.
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