Thanks to all. So far I believe I have worked out the wiring and fan installation/glass replacement. Now I'm sort of stuck on the air-flow / roof vents issue so I apologize for the long read. Thanks Bruce. I'm in zone 4-5 and we're new to this GH. The GH faces south and the home that we're attached to is on the north side...so we get LOTS of sun. We installed the GH late enough last summer to only catch a small portion of our hottest season so we're still learning/testing. We monitored temps and even with two roof & two front wall vents fully opened (and a screen door)...we easily pegged the thermometer in the 130F on hot sunny days. As planned, we added a misting setup that runs for short periods of time every 5-10 minutes through the hot part of the day. On MOST days this dropped the temps down to acceptable levels, but still over 100F on some days and still not the hottest part of the season....but this may be acceptable for most of our plants. We plan to add shade cloth to help, but our goal with the exhaust fan was to have an additional safety net. We can't be home all the time so if the misting system & auto vents still aren't enough, it was my plan to have the exhaust kick on and act as a safety net. Sort of a third mechnism of protection should the others fail or not be sufficient. We are lucky enough to live up high overlooking a lake, so the GH does get a great cool breeze most of the time...but the sun still seems to win out. Our roof vents are up high enough that a ladder is required to get to them. They are also behind screens, so adjusting them is not a quick process. Even assuming we were home to close them it would be very hard; this is why I say closing them is pretty much not reasonable for us. Our GH manufacturer recommended a 975cfm fan, or larger if the roof vents are open. This seems undersized to me for a 10x16 with such a high peak. They also expressed concern for the roof vents negating some of the cooling efffeiciency of the exhaust fan and recommend shade cloth & air-circulation fans. I certainly understand how the air will be pulled from the path of least resistence, which given their proximity will be the roof vents. I also understand that with enough power, some air will be pulled from other sources. I think what I'm hearing is that there should still be some benefit/safety net effect, especially if a larger cfm fan is used. My current plan is for a 20-inch with a speed-controller that will take it from about 1250cfm to 4300cfm. This will allow me to find an acceptable level of cooling. There are also 16-inch fans that go from 525cfm to 2000 cfm if anyone thinks even 1250cfm will rattle the house. I guess I envision setting the thermostat pretty high so the fan isn't running all day, and only runs on very hot days to supplement the auto-vents, misting & shade cloth. OPTION # 2---We originally considered putting two window fans just under each roof vent. They make twin-fan units that have reversible motors, thermostat control, remote control and are water resistent. The plan was to put them directly below the actual roof vent. In summer they'd pull air and push it out the roof vents, and in winter we could reverse them to move hot air off the ceiling and back into the gh. Our main concerns with this option: that it may be less-reliable, that we'd have to fabricate an attachment bracket, and that the fan units themselves may actually BLOCK airflow from going out the roof vents if they weren't on. Now I'm wondering if Option 1 (Exhaust fan) may be a waste of money/time, Option 2 may cause more problems than it solves and perhaps we should be looking at option 3...air circulation fans/shade cloth or some other option. Getting confusing now....but thank you...hopefully there are some suggestions out there.

It's been about 6 years now, and I don't see any manufacturer name on my fan (just checked.) But we bought the 16" three speed fan on the ACF Greenhouses website, 2950 / 2050 / 1200 cfm, for our 10x12 HFGH. I think that is the slightly larger version of the 12" size you are looking at. It has worked fine, no problems. During the coldest part of the winter (when I don't use the exhaust fan) we cover the outside louvers with a wooden box my husband built to fit, to prevent losing warm air through those small gaps in the louvers. I have not cranked through the numbers in the calculator for the 6x8 size greenhouse, but I can tell you I hardly ever operate my fan at anything but the highest speed. I thought I would want to use the lower speeds (a little less noisy than the highest speed) but I have found I need all the help I can get during our hot summers (we have some days over 100F here each year) so I keep it set on high. I bought a plug-in thermostat from www.kkontrols.com to operate the exhaust fan, and that has worked fine too for six years. I have gotten used to the sound, pretty much, but it is a bit of a relief in the evenings when the fan finally kicks off (silence is good.) I am sure my neighbors can hear the hum from a distance, but nobody has ever complained, thank goodness. I am not a very good person to ask for advice about intake shutters, on the opposite side of the greenhouse. My (crude) method is simply to keep my sliding doors slightly open, with an aluminet shadecloth curtain clipped in place over the opening, as an air intake. Also, in the summer, I have screens in place on the south wall of my greenhouse, so air can get in that way, too. Other folks in this forum will likely have better info for you about ideas for air intake needs. I decided to start out simple, and never got any fancier with my set up.

I'm sorry if I confused matters by mentioning the intake fan I use. I actually used the Lasko 20" box fan for my exhaust for three years, then stepped things higher this year after adding on an air chiller. In theory, all you need is a good exhaust fan. But I've found that the addition of a relatively equally-powered intake fan (placed where the air enters your gh) nearly doubles your turnover. This is only my theory, but within a closed airspace, I don't believe the actual output of an exhaust fan reaches its rated CFM because of the weight/friction/vacuum of the volume of air it's trying to move. I didn't set out to "test" this theory with the addition of my second fan -- it was to operate the air chiller I constructed to cool incoming air. Basically it's a 2x2 frame covered in gh fabric, with a dozen fogging heads and spun plastic furnace filters inside. However, I did note that my turnover is much better "pushing" the column of air from the intake side as well as "pulling" from the exhaust. Because of compromises during construction (my gh is a Rube Goldberg affair, and definitely wouldn't win any beauty contests!), my exhaust fan is placed directly over my door -- which is in the middle of a long side. But my door is pretty airtight. It's a shower curtain turned sideways, such that the grommets snap over exposed nails in the door frame. Because of the UV I need to replace the door every few months, but the design and function is tested and true. I can seal things up tight during the cooler winter months, and get the proper air flow as well. My application is an effort to make a "cool wall" for my intermediate orchids, and isn't necessary for the types of plants most people cultivate. Actually, I need to build the chiller larger because it doesn't have quite enough "horsepower" yet (except when humidity is below 50% -- not very often during the summer). It works, but not well enough...yet. -Bruce

LEDs are a joke. They work, but you might as well use spiral florescents, which give you more light for about 1/10th the cost if you build them yourself. Your tent is 4x4x6= 96 cubic feet. Your exhaust blower moves that much air in 24 seconds. You'd be fine with a tiny computer fan instead of your big blower. I would just add more light, which would make more heat, and then you could put that blower to use. And you don't need both an intake and an exhaust fan. One of them can be passive, which means just a hole. Air pressure will make fresh air find a way inside the tent if you are actively exhausting. For most people, if you have access to fresh air at all, CO2 will be a waste of time. That CO2 generating bag thing would be most useful if you had everything sealed up in a closet and were not able to ventilate with fresh air. But it's tricky, because you don't know the CO2 level of the air without a meter ($500 US last I checked), and too much is bad. If I were you, I would lose the CO2 bag, add more light, and run a smaller blower constantly to regulate temperature. Stanley makes a variable speed floor blower and sells it at Wal-Mart for $40. I think the 3 cfm settings are 90,120, and 250. Also, 4" ducting would be plenty, rather than the 6".