I wasn't sure how to post 2 links in my message; so here is the link to the shutters I am considering purchasing.

Here is a link that might be useful: Shutters I am looking at

The power shutter is the better investment. You just need a second thermostat. Nothing fancy. I set mine 10 degrees lower than my 2 stage exhaust fan. That gives me 3 cooling options. Having that power vent open on a cold sunny winter day will avoid your fan from kicking on and off through the day as much, and cooling the house too fast. It will also reduce the temperature fluctuations to the plants. In the summer I cut my vents off, and prop them open, so they are not using electricity all summer. Another idea would be to get a small vent fan to go in the peak of your house to vent that peak air out before it can buld up and make your big fan turn on. Anything you can do to reduce the time that thing runs will be helpful. Having that second fan will also pay off big when you main fan motor bites the dust one day in the middle of the summer. The house will get pretty hot, but things might have a fighting chance to survive. It is a good idea to have a back up motor and belt on had as well.

Greenhouse companies spend some time matching the size of the fan and the vents. It probably makes your fan run much more efficiently. I have a 42 inch Acme fan with 2 4x5(approximate)power vents spaced at plant height. Having that cool air pull over your crop evenly is ideal. One might not be enought to solve your pressure issues, but would be better than nothing. You don't want to have to go prop the door open every morning. Sorry to make so many suggestions of how to spend money. Once you get your house automated it is so much nicer, and you can concentrate on the plants.

You don't need a separate thermostat for the motorized shutter. We are running ours off the same one for the exhaust fan. You just have to run a wire to the shutter to power it up when the fan kicks on. Farmtek has good prices on those shutters too and they have a 5% off coupon code right now too. We got a "scratch and dent" one from their closeout section for almost half price a month ago. I deal with Mike there at phone extension 109. That's a pretty small shutter you're looking at. Don't know what size your greenhouse is, but that's pretty small.

It's pretty important to size your shutters to your fans. If you don't you can put a big drag on our fans and work them to death and shorten their life. I prefer the two stage fans, especially if you suspect you are oversized on them. That way they can operate gently if a lot of ventilation isn't required. I also have my automatic shutters linked to the fans so they come on simultaneously.

There are lots of times you need some ventilation but don't want cold air being sucked over plants. That's why all my g'houses also have large manually operated drop vents. I do winter crops and if it's sunny but freezing out, my g'houses can crank up to eighty or ninety. By dropping open a manual vent, hot air expands and cold air doesn't rush in, but the excess heat can escape.

I never let a fan take over when the temps are too low. I seldom let them operate when the outside temp is below what the heater is set at. If you do, they'll play tag with each other.

I used automatic foundation vents. These are normally used in house crawl spaces. They do not require electricity but rather work on a bi-metal concept. They are fully open at 70 F but the lower the temp goes the more the louvers close until they are fully closed at 40 F.

http://www.lowes.com/lowes/lkn?action=productDetail&productId=17140-228-RABL/LO&lpage=none

http://www.antonline.com/p_A-ELBLACK-GP_398008.htm

If you are handy, you can hook up your shutter with an electric linear actuator, an old car windshield mechanism, or car window motor mechanism.

They come with limit switches and can be very cheap used. Get one that's 12V so you don't have to run conduits.

dcarch

Also, if you don't have a 12vDC supply for the actuator, a used car battery will have plenty leftover power to drive it for months.

dcarch