If your greenhouse is fairly tight the non motorized dampers will work fine. If you have a lot of leaks it will drop the static pressure in the greenhouse and they will have troble opening.

Thanks for the information. I am thinking that with a variable speed fan I can set the speed at a rate which will open the vents easily. I think the GH is air tight but I will check it out. I built it with 2x6's with plywood on inside and exterior siding on the outside but it is a good idea to make sure it is air tight. thanks again.

Just a word of caution: the CFM rating of most exhaust fans are notoriously under-rated. The problem is that they are measured with no load, under perfect conditions. An exhaust fan in a gh operates under load (vacuum), which dramatically decreases the output. If you're looking to turn over the air inside the gh in a certain amount of time, you most likely will need a larger/more powerful fan than you've figured.

Either a single larger fan, or an additional 12" fan on the intake, should get you the results you're looking for. The additional horsepower from a second fan can get you back up near to the CFM you hoped for from the first fan.

I have an 8x14 gh for my orchids, and run two 20" fans. This gets me an exchange of approximately once per minute. If I don't want or need that amount of airflow (like in the winter when the ambient air is cooler), I only run the exhaust.

-Bruce

Correction: I meant to say, "...the CFM rating of most exhaust fans are nototiously OVER-rated."

-Bruce

Thanks Bruce. I will sit down and try to re-engineer this a bit. If I understand your comments, I could / should (as one option) have a 12" exhaust, a 12" intake fan and an additional intake with louvers - of 12"? Or perhaps one larger exhaust fan and keep the two 12" intakes. If I go with an intake fan I assume I will need to take care to adjust the variable speed for each to match somewhat and make sure the controls are set to have them start up at the same time. Could I use the same (one) thermostat for both the intake and exhaust fan? That would simplify things a bit. Thanks again.

If you pressurize the intake side with another fan, you won't need an additional intake vent. And, you are correct -- one larger fan with two intake vents would be optimal with that configuration.

In my gh, I set (and leave) both fans on high, then, depending upon my cooling needs, either run the exhaust only if needs are low, or both if needs are high. (My intake fan also forces air through a chiller.) You could run both off a single thermostat, or two thermostats set in stages, i.e., exhaust on at 75F, intake on at 80F.

However, what would be easiest would be to simply unplug the intake fan in late fall when ambient temps are cooling off and the sun is lower. Then, plug it back in when spring rolls around. There is plenty of "slop" in the air exchange, such that you don't have to match fan speeds precisely, etc. I'm using a 20" Lasko 3300 Wind Machine on my exhaust, rated at 4750CFM, and a 20" Lasko box fan on the intake, rated at 3226CFM. The actual outputs are nowhere near those ratings. Granted, the horsepower on these cheap fans is far less than what you would find with a $200 high-quality fan, but they are extremely cost-effective. As I mentioned before, even one of those $200 fans won't perform with the specified CFM under load.

Another alternative is to use a timer instead of a thermostat if you're trying to save a little money. A waterproof timer is all I've ever used to run my fans, although, admittedly, it requires attention during the colder months when you must evaluate ambient vs. inside temps on a day-to-day basis. In my experience, wintertime requires the most attention between sealing vents at night, opening in the morning, deciding whether to run or not run fans, judging whether to water or not based upon upcoming nighttime lows, etc., etc.

Hope this helps.

-Bruce

Thanks again. Here in the foothills of California, we most often have sunny days in the winter and this winter I have found my gh temps as high as 115. The gh I built has glazing only on the south side which is angled to be 90 degrees to the sun at winter solstice. The north slope and all other walls are 2x6 and insulated. I think the plan I used was for Canada - clearly for a cold climate. I have 21 black water filled and sealed barrels inside under the north side benches - each holds approximately 2.5 cubic feet of water (my heat sink). I just measured everything, taking into consideration the odd roof angles and so on and roughly have about 880 cubic feet of interior space without deducting the cubic feet taken by the water barrels. My modified plan (after your initial post) was to have one 16" exhaust fan with variable speed control (rated at 2000 to 525 cfm within the variable speed ranges), to be installed at one peak and one 12" intake and one 16" intake on the opposite wall. And then, if I needed more, I would add a 12" intake fan under the bench on the north side as always shaded there on the outside and always cooler - and run that intake fan with some duct work to the center of the gh. However, I think you are saying that it is better to have an intake fan and an exhaust fan (opposite sides) plus one intake shutter rather than one exhaust fan with two intake shutters. I think I can do that (my initial budget is already not working out) and can wire those to one or two thermostats. I have been looking at the equipment offered by AFC site - can't tell what brands they are offering - but prices seem competitive.
I had a spare 6" fan in a section of duct pipe left over when I put in a new furnace, so I have wired that to a temp control switch and ran duct work to opposite corners under the bench on one side and set up to blow air from the bottom (where it is typically quite a bit cooler) to the center of the gh for some circulation, and will add one more circulation system fan. I will also be adding a misting system.
Bruce, thanks for taking the time to help with this. I recently retired and have always had a garden but want to add the gh to expand my hobby and get an earlier start on my tomatoes and other garden items. So far everything I have put in the gh has really taken off - even without any fans - just leaving the door open - but I have to watch it carefully as it gets very hot in there even in January. In the summer I will mostly use raised beds as have plenty of room. I am glad I found this forum. Hopefully, some day I will be able to help out with some advice.

I can certainly empathize with your difficulties trying to moderate temps by propping the door open, etc! I've done that too myself, before I rigged up a better system for sealing/opening vents so I could run the exhaust. Winter is the WORST for keeping the temps inside a gh hospitable for tender plants!

There is a clear benefit to having your airflow only moving a single direction, instead of multiple ones if you add extra vents, etc. I'm not sure I made this clear, but if you decide to go with an intake fan, mount it just inside your vent. You may have to construct a bit of a hood around it to shelter it from getting wet when you water your collection. Use a piece of plastic spun furnace filter material in the vent to keep bugs and pieces of organic material from getting sucked inside.

Once you get a suitable air exchange going, you'll find that your "cool spots" inside won't be quite so cool any longer because outside air is overwhelming their ability to remain cool. If you're still not getting your temps low enough, you may have to look into a method of chilling incoming air. For comparison's sake, I have approximately 890-900 cubic feet.

The best prices I've found for gh fans are FarmTek and ACF.

-Bruce

If you get a lot of wind when it's cold outside, then the motorized shutters are better because they won't blow open and lose all your heat in the middle of the night when it's windy and -20. But it sounds like that's not your problem at all. Lucky you!