In order to achieve the desired ventilation of a greenhouse, you need to move out about one volume per minute in the summer. This can be accomplished by passive roof/side vents or with exhaust fans, or a combination. If using only passive vents, the roof vents should be 10-20% of the floor area. The side vents should be 5-10% of the floor area. The variability depends on temperatures and wind. If using only exhaust fans, for one air exchange per minute, they should exhaust about 10 cubic feet per minute per square foot of floor area. If using a combination, then adjust accordingly. One large fan with enough capacity may produce too much turbulence, so 2 or 3 smaller fans may work better.

The trouble with the fan is that it is very hard to find several small fans that would equal the price of the larger one and then finding auto louvers for them. The ideal fan would be variable speed, but they (the greenhouse people) just want too much money for them and I can not afford them. I can build roof vents as I have allowed for beefing up the support in the roof. I can find affordable roof vents on the net. If you know where I can find them please let me know. One of the reasons I posted the prices was that I was hoping that someone would come along and say "Man, so and so has bla bla bla much cheaperr than that" I am still hoping for that solution.
I will have to sit down again and calculate using the formulas that you have posted.
Thanks
1eyedJack and the Dawg

You want to copy the html tag, the "a href" box, for inserting a pic:
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Is it absolutely necessary to have roof vents? - not with an adequate exhaust fan. I actually keep my roof vents closed during most of the summer. The air movement is much better across the GH with the ridge vent closed. Remember that cuestaroble's formulas are for passive ventilation, not ventilation using an exhaust fan.

Is a fan that moves 1600 cfm to powerful? - no, that is one air exhange about every 40 seconds, and that is OK.

Would it disturb plants too much? - probably not if you use care as to placement. A large-leaved plant like a banana or GBOP probably would be best placed off to the side somewhere.

What temp should I set it at? - depends on what you grow, but I wouldn't set it lower than the full open setting on the intakes. Maybe 75-85F. You can experiment with that.

Is that the correct place to mount it? - sounds good to me.

Is that enough intake for the fan? - my calculations show that your intakes are 2.5 times the area of your exhaust, which should be fine.

You may want to try to increase the "fully closed" temp setting on the intakes from 40F, depending on what you grow.

Will I need another smaller fan for air circulation? - yes, but not when the exhaust fan is on. Your exhuast fan will move plenty of air for HAF purposes, but when the fan is off, you may want the air to continue to move around the GH.

The roof vents seem to interfere with the exhaust fan/shutter operations in my gh. My shutter opens and there is a 3 degree F. differential before the exhaust fan kicks in. (If the shutter were not already open, it is possible for the exhaust fan to prevent it from opening once it starts up.) Because of the vents, it initially brings the temperature down about two degrees, but then it actually rises again. This is what I have observed lately, anyway. Plus the exhaust fan is noisy.

Another thing to consider: my evaporative cooler requires lots of fresh air (i.e., door or window open) to obtain the full benefits. If it doesn't get it, the humidity increase is not considerably noticeable. See if your cooler will require the same, because it will definitely affect your exhaust fan operation if the door is open.

I haven't decided to take the vents out of service for the summer, since I like the idea of passive vents and less noise from the exhaust fan. But it's not optimal.

Regarding additional ventillation, virtually any plants will need it. I have two (one oscillating and one big box fan) running at low speed 7/24; my gh is 10x12. Plus the fan on the swamp cooler which I have running a few hours a day. Without ventillation, the plants suffer.
Regards,
Ann

Thanks you Ann for your comments/advice,

Re: the roof vents/exhaust fan and shutters. My shutters will depend on the fan to open them. At other times they will be held closed by "Rare Earth Magnets". If you are not familiar with them they are extremely strong, but small magnets. For example a 1" by 1" round one or two if allowed to come together at any speed will break your finger. I kit you not. I will use small ones glued to the louvers and have no doubt that the louvers will remain closed until the fan goes on.

re: The noisy exhaust fan. The one I am looking at is made by or call "Whisper cool" and is supposed to be very quite. The noise in not much of a problem for me as I am hearing impared to some extent as a result of years of very heave shooting, like 20,000 rounds a year. Hey, I got the ammo free so what could I do but shoot it.

Re: The swamp cooler. Mine will draw directly from the out the air it cools. I will mount a cowling on the outside to prevent the enterence of rain and when the unit is off that should take care of unwanted air coming in. If not then I will reverse mount another shuttered louver.

The need for extra fans will have to wait for me to see the need.

Thanks again for the comments and good gardening
1eyedJack and the three legged cat AKA The Dawg

I have a custom built, redwood stained wood framed 10 x 12 greenhouse, 12 foot tall. It was laziness that we didnt put in roof vents, thinking we could add later. Once I put in an attic fan, which really pulls air out of the greenhouse, I realized I didnt need roof vents, and have no plans to add. The attic fan is on a thermostat, so it just goes on when needed. I have a storm door in the front, with a screened window that I leave open, and that attic fan just pulls air in through the window. I placed misters in front of the window/door, its mounted on a stand, but I could hang some as well. Those misters work just like a swamp cooler, at a fraction of the cost, and the greenhouse is always cooler than outside air, and the moister stays high for the plants as well. So I cancelled my plans for a swamp cooler, none is needed.

KittyL,
THanks for the comments. I still want to try the Swamp cooler, both in the GH and in the dwelling. I am tired of paying the Power Co.. Although the unit is larger, I live and work in an area basically 12 by 20 and I really don't care if the rest is cool or not. It is empty and the only living thing that goes there is "The Dawg". She likes to sit in a window in the back.
The fan I am looking at is both Temp and Humidity controled and the price is right so I will give it a try.
Misters will have to wait until I have tried the Swamp cooler.
Thanks again for the comments
I am off to bed
1eyedJack and the Dawg