I wrote this article this morning for my blog and thought I would share it here, too.

A Basic Understanding of Adequate and Proper Air Circulation in the Hobby Greenhouse

I know when I built my greenhouse, I had very little knowledge or appreciation for what it takes to have a successful greenhouse. Success is not as easy as one would assume, or I assumed! One area I am learning the importance of is adequate and proper air circulation. Good air circulation is crucial to good plant health and growth. There is a lot more to it than opening a vent or throwing in a fan to accomplish these ends. Too little fan capacity or poor spacing and location are common mistakes. Temperature variations, excess moisture, and low available carbon dioxide are three plant hazards that can be alleviated with good air circulation.

Without circulation there can be wide temperature swings between different sections and levels within the greenhouse, as much as 10-15 degrees, especially at night. These different temperature "pockets" affect plant growth and quality.

In addition to having a more uniform temperature, proper circulation reduces the incidence of foliar diseases. Moving air removes moisture from the plant canopy resulting in a drier microclimate. When leaf temperatures cool much below the air temperature, the dew point is reached, and condensation occurs harboring disease organisms. Proper circulation will reduce this.

During the day, photosynthesis depletes the carbon dioxide levels around plants. Adequately moving the greenhouse air will replace the carbon dioxide depleted air around the plants with fresh air having a higher carbon dioxide content and encourage photosynthesis to continue throughout the day.

Conquer these 3 deficits: temperature variations, excess moisture, and low available carbon dioxide, and you are on your way to a much healthier greenhouse environment.

So, what is the optimal flow rate and placement of greenhouse fans? The total fan capacity should be about two times the floor area. If your greenhouse is 10' x 15', your flow rate should be 10 x 15 x 2 for a total of 300 cfm. If you are growing tall plants or have obstructions like hanging baskets, you'll need slightly more. Horizontal positioning of fans is preferable since air layers in horizontal levels. The goal is to break up and disturb these layers.

Placement of fans should be such that you are not just blowing air around but circulating it, breaking up the layers. Using two properly sized fans at opposite ends is best, placing one fan blowing across the length of your greenhouse above the plant canopy and the other blowing back in the opposite direction at a lower level. This will generate the needed circulation. (Note the root of the word "circulation" is "circle".) If you have smaller fans or a wide and long greenhouse, you may need additional fans either in line or spread across, but the placement should be the same to achieve adequate circulation throughout.

Many hobby greenhouse growers are concerned about the high cost of electricity when it comes to maintaining their greenhouses, but you can minimize your cost by using small, low horse power fans (1/10 to 1/15 hp). These fans work well in providing the air movement needed. They move the air with little resistance and at a lower cost than using exhaust fans for this task. Exhaust fans have to "pull" the air and overcome the resistance of louvers or vents. Exhaust fans certainly have their place, but air circulation is not their forte.

Each greenhouse is different, with different plants, conditions and needs. A basic understanding of proper air circulation to reduce temperature variations and excess moisture as well as promote good photosynthesis is key to designing and maintaining an adequate and proper air circulation system for your successful greenhouse growing.

This is a good article, Jim. I know my greenhouse is in need of better circulation. The solar fan provided with the greenhouse kit doesn't begin to be enough.

Great read! I have a box fan in mine but after reading this will be looking to add another.

Thank You for your informative information.

I am planning on a new greenhouse to cover my hydroponic outdoor system for tomatoes, peppers and eggplant. Construction is 1 1/2" PVC with a 10' eave and 14' peak (arched), 20' x 25' , anchored to 2x6 on sidewalls, staked in to ground. I am estimating 5000cfm airflow minimum and planning on 2 2500 cfm fans on one end, and 2 2'x4' louvers on the other end. The plant bases are 2' off ground, so I was planning on the louvers and fans at about 6-7' off ground. I have access to some industrial controls (thermocouples and temperature setpoint switch).

1. Will I also need internal air circulation? 2 small box fans in opposition center?
2. Do I just set the fans to heat level for cooling, or run constantly, or timer control (recommendations helpful)
3. Would I need evaporative cooling on the intake louvers?

Any other suggestions appreciated. Here's a pic of my system

An excellent article in content and writing, MoonShadows. Where might I find you blog?

I'm off grid, so minimizing night time power use is important for my greenhouse design. I wonder about pulsing the fans, vs constant flow at night. Any thoughts on ways to reduce night time ventilation needs?

Sorry, that blog is no longer active.

Great article! My greenhouse is a little larger than a hobby greenhouse at 1730 sq feet. But all of the things you address are true no matter what the size. My greenhouse was built for me by people who mainly build them for agricultural purposes, so mine was considered 'small' to them. Here where I live, you could almost have a shade house like in South FL if not for the fact that we do get significantly colder temps for significantly more nights in our pretty short winter. But a screen house would be almost perfect here for 9+ months out of the year.

Mine was built in 2003 so stuff is starting to get old and that means it may be possible for me to make some changes when new things become necessary. Right now, I have 4 HAF (high air flow) fixed mounted fans at the level of 12 feet (of a 12-20 ft roof slope). These come on automatically depending on when the thermostat thinks they need to, based on what I set it on. They really do move a lot of air in the attic, and you can feel it on the ground as well.

But my major cooling force are two 4ft x4ft exhaust fans in front that pull air through a 36ft vent at the back which is covered with honeycomb. The original purpose of the honeycomb was to create a swamp cooler. Water can be recirculated through the pads and when the air is pulled through it cools it. But, here in this climate, the swamp cooler really did not contribute that much to cooling. What it did was create even more humidity in an environment that, most of the year, is already at 65%-100% humidity to begin with. So I turned off the water to the pad years ago.

I have wondered about the feasibility of replacing the honeycomb with something else. Something like screening that would allow a lot more air flow through. The honeycomb is about 4" thick. Replacing it with a screen, even something like small square chicken wire, would allow a lot more air through. But I wonder about heat retention then in winter. The honeycomb serves as a big insulator when the vent is closed.

In the past, I have also sometimes had additional fans (four inexpensive Home Depot box fans) that I hung up in the rafters at the 12 foot level and positioned to blow air in a circle and slightly downward. I may replace these this summer, I'm not sure. They worked well, but I had a lot of vines that kept growing up in and through them and causing problems. I have removed the vines now and there would be more unobstructed air flow.

@MoonShadows, what do you think of these ideas?

I would say if the honeycomb did little to cool, created more humidity, and you are not using it, I would definitely try the screening. What about replacing with a powered louvre (shutter) if you are concerned about cold times in Florida?

Here's one company that makes good products, but if you go with this idea, shop around...

J&D Manufacturing
https://www.jdmfg.com/dairy/in...

I've also seen, can't remember where, power shutters with rubber on the tip of each section so it seals better when closed.

I know HD Box fans are cheap, but they don't move as much air as a good greenhouse fan or two. Look for low hp/ high efficiency. I have two Hydrofarm ActiveAir wall mounted 3-speed oscillating fans, and I seldom have both of them on at the same time, and when I do, they are both on low. This winter, I kept 1 of them on low, and it took care of the whole greenhouse. I probably should have only bought one.

I forget where I bought them online, but here's a link to see the model I bought. I think I paid $39 a piece. Hydrofarm makes a number of different fans. One thing I like about mine is there are two long cords, so I don't have to climb up on something to set the speed and/or turn on/off the oscillation.

https://hydrofarm.com/p/ACF16

Thanks! Do they make a louvre shutter 36 feet long?

36 feet? I never heard of one that long. I imagine that would have to be fabricated for you...at a huge price ticket. The largest I think they sell is about 6 feet

Just went back and read your previous post...must have missed the "feet". If you are only going to use the louvre to bring in fresh air, why not remove or seal the 36 foot one and put in 1 or 2 smaller intakes? Your internal fans will disperse and circulate the cooler air....won't they?

What is the height dimension of the present one? Wow, 36 feet wide. I can imagine that.

The back vent is 36 feet long and the honeycomb is 4 feet tall I believe. I have fought for many years to keep the aroids out if it. It is the PERFECT substrate for vining plants like Monstera, some Anthuria and climbing Philodendrons to want to root in. But I think they would cause it to decompose rather quickly. Right now there is ONE that I can't remove (I could, but don't want to disturb it), a Monstera deliciosa aurea-variegata.

I think she said it's over 1700 sq feet, Jim. I can't even imagine that, but it sure would be fun to have!

You have that right Karen.

I just met a new friend who has moved to our area. They just built a 1600 sq ft greenhouse! That was for the wife, and the husband has a huge astronomical observatory. They recently moved to NM from Florida.

I built this greenhouse, which is 1730 sq ft, in 2003 with all the equipment (sprinkler system, heater, automated thermostat controller, HAF fans, exhaust fans) for under $20,000. I did it because I was running a 'niche' tropical plant nursery mail order, AND to house my tropical plant collection which at that time was already huge. When I built it, my climate zone was 8B. We have moved to 9A, and just had our first 10A winter.

@plantmanager, Karen happy for you that you have a new friend so close. She like what you do and her husband like what your husband likes. Now you all will have a grand time together.

Txtea said:@plantmanager, Karen happy for you that you have a new friend so close. She like what you do and her husband like what your husband likes. Now you all will have a grand time together.

I was thinking the same thing!

Thanks, I just wish they lived a bit closer. They are about 30 miles from us.

Thumbs up Moonshadows!
I have some fan and ventilation issues that need to be addressed before this years hot weather.
I built my 8' x 12' greenhouse last year. I use it for propagation mainly but also want to keep my in ground greenhouse veggies all the way through the growing season.
I built my greenhouse with 4 close-able base vents at the corners, a storm door with 2 foot square screened window and a solar/piston window at the rear.




Not enough ventilation! I put in a cheap 20" box fan which helped but my tomatoes went to the ceiling and did not do well with temps to 110 degrees.
In the pic below I show 2 additions I made this year. A 16" oscillating fan and a 6" 425 CFM exhaust fan (green in upper right) I picked up at a garage sale. The blue square is the 2'x3' solar/piston window.




Questions:
Is this enough? I am thinking maybe a couple more $9 closesble vents at the midpoints? And maybe another solar/piston window near the peak?
How many square feet is your greenhouse Moonshadows?