Man, I'm getting impatient about not being able to look up old threads! If you're asking about HAF fans (for horizontal air flow) and not exhaust fans, I am using two oscillating fans for air flow, and both run 24/7. One is an 18" wall mount oscillating fan (SMC brand) I bought at Kmart for $40. We mounted it high over the doors. I don't know how long it will last (so far six months.) Several of the greenhouse supply sites have high quality hanging or wall mount HAF fans designed for GH use, but they cost a lot, so I thought I'd try this first. It has a remote control to turn it on and off, or to change the speeds, which is really handy since it's up high. It has three speeds. I also have a small 10" oscillating table top fan sitting on a shelf at the other end of the GH. This one is annoying since I can't position it so it doesn't blow right on the plants sitting next to it on the shelf. None of my plants like the windy spot next to the fan, they all get very dried out. I may try eliminating that fan to see what happens. I can see why hanging or wall mount fans are better. For cooling, I have a 16" exhaust fan on a thermostat. It only kicks on when it reaches the temp I set the thermostat to, so it only runs during the hottest part of the day (and not at all this time of year.) I bought the thermostat at kkontrols.com, and I bought the fan at ACF Greenhouses. I haven't bought the shutter vents that are normally used with an exhaust fan because I planned to use screen doors instead for the air intake. Right now I just open the doors (my screen doors are still on the "to do" list.)

I looked up my area's average temperatures and found this: Daily temperatures rise above 85' on an average of only one day per year and fall below 23' on an average of only 2 nights per year. During the winter, the average daily high and low temperatures are 47' and 38', respectively. The summer months are equally mild, with an average high temperature of 68' and low of 53'. With this info, do I still need a complete air exchange once per minute? I've read both of your responses and now wonder if I have an exhaust fan in the upper north gable end set to come on at temperatures above 80', and have 4 automatic vents on the lower opposing side windows, as well as the screen door opened in the south side shouldn't that be sufficient airflow in my moderate climate? I'd like to stay away from the automatic roof vents because they seem to fail so quickly, much faster than the ones in the side windows. And, what temperature am I aiming for? Is about 80' good for my peppers and tomatoes and for my seedlings? I am still eating lettuce from last fall's outdoor sowing, so I don't expect to need much extra heat in the greenhouse in winter. (Sorry, I just love to brag about my winter lettuce!) Again, thank you for your advice. I just want to do this right the first time.

It could be automated. All it takes is the right equipment. Obviously you have to avoid the loop. I've seen that in person, open doors right next to the exhaust fans. That assures that nether the doors nor the fans do much good. The trouble with a fan in the roof vent are at least a couple. Fans will be much smaller than what is needed for effective natural ventillation. My 1700 sq ft greenhouse has three 36 inch exhaust fans. Mostly I run only two. That area of roof vents would be totally inadequate. You'd need 10-20 times that area of passive roof venting. Also a fan in the vent area would impede air movement when the fan wasn't operating. To me roof vents are too expensive for what they do. With roll up sides I'd rather have a means to uncover the ends all the way to the peak. This would allow air into the sides and out the peak at both ends. Neither roof vents nor open ends sound cheap or easy. But I'd rather jury rig the ends than the top for venting. Roll up sides by themselves will work during cool weather. Then during the freeze free period, uncover the ends and replace with bird netting if critters might be an issue.

You want to copy the html tag, the "a href" box, for inserting a pic: 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.