Ventilation - feel like I'm in over my head!

Okay, I know that I'm going to need to get a vent system going. From what I can tell, my hoop house runs from 65 to 95% at all times, worst in the mornings.

There's a couple of places that sell "packages" - GH mega store and ACF specifically - but I'm not sold on what they give, because I remember stuff that was said here...

I think I got the CFM calculation okay - I have a 16'x24' that is 8' tall. Which gives me 3042 cubic feet - and from what I can tell, I should be doing an 18" fan, but they tend to skip from 16" to 20". Bigger is better? And the 20" fans tend to come with two vents.

The other item in these packs (besides fan and vent) is a thermostat controls to set them off. They're single phase. The double phase ($40 more) states that it can open the vents prior to the fan switching on, which sounds like something someone on here said was needed. Can you do that with a single phase?

This is really turning out to be a lotta cash ... I wish there was a cheaper way to do it! I keep telling myself that it's like a horse - the initial outlay is the worst part....

Any advice / suggestions *greatly* appreciated!

Here's a good site with calculators for heating,cooling and ventilation http://www.littlegreenhouse.com/heat-calc.shtml

I would, without a doubt, go with the 20" fan and definitely two shutters, one on either side of the door-as high as you can put them. I have large grhouses and I dont use thermostats that open the fans first-that barely does anything in getting rid of the hot air. Its the fan coming on that will get the hot air out, and with a 20" fan, it will run less and do a better job than a 18".

Thanks, to the both of you... might have a line into a used set up that might be more practical - I am only afraid his equipment might be for a larger GH than I have. Not sure if that would be a bad thing or no...

I have a 48' x 24' greenhouse. I currently use one 24" fan with two 24" vents. I am planning on adding another 24" fan next year (so the one doesn't have to work so hard, and to have a back-up). That said, during these fall and winter months, the one fan does just fine. I raise mostly tropicals, so I have my fan kick on at 88 degrees F, and the temp never gets above 90 or 92. My vents both open at the same time that the fan kicks on. I mounted my fan high on one end of the greenhouse, the vents low on the opposite end. I am guessing that with a greenhouse the size of yours, one vent should be just fine.

I live in the deep south--it is very hot here in the summer. I built my greenhouse myself, and I have it fixed with greenhouse plastic down one of the 48' sides and as a center panel on one 24' side (the same one with the fan). That way, when it is is very hot in the summer, I raise the greenhouse plastic on these two sides (I have insect screen under the plastic), and I quit using the vent fan at that time, relying instead on two humidity fans inside the greenhouse (they have a water connection that blows a mist of water with the air they blow).

It all sounds complicated, but as you go along, you tend to figure out what works and what doesn't. Hope my experience helps.

Thanks Bob! Really, the big problem I've had (so far) is the incredibly high humidity and I'm told that the fan is the solution, so, here we go! I haven't even dared to take anything out to the hoop house yet (sniff) because it's been so high. Guess it keeps me from paying to heat it for now.

Heat and humidity are a trade-off. Even though I will have condensation on the sides of my greenhouse throughout the winter, my gauges show that as the greenhouse gets hotter in the day, the humidity drops. As it gets cooler, the humidity can go up to 99%. I solve this to some degree by running a small, cheap fan inside the greenhouse, on low setting, when I want to bring the humidity down. Seems to help.

I have a new green house 8' X 12' Easy2build. I need to know how to keep the temp. down, this has 2 vents on top near the back and one on the front bottom. Temp today is in the low 60's and inside it got to 107, I opened the front door but temp. didn't go down very much. I don't want to bake my seedlings. This is my first time to have a greenhouse etc. I need all the help and suggestions you can give me.
Thanks
Vicki
I'm in zone 7b in Ok. it is going to get down in the 20's this week, so weather is crazy.

You need an exhaust fan. You appear to be relying on passive ventilation and that just won't be enough. Pick a fan with a cfm rating at least equal to the volume of your GH. This plus shade cloth will keep you within 5-10 degrees of outside temp. Evap cooling can get you down to outside temp or sometimes lower.

This message was edited Feb 18, 2009 7:43 PM

I have the thermostatically controlled shutters, Pagancat, and I love them! They consistently open @ 70 degrees GH temp. They are at the rear (more shaded)sides of the GH, and unfortunately, one of them is located right under my propogating bench. No problem most of the time, but on a clear, cold (20-40 degrees outside) sunny day, the GH heats up FAST. The vents open as expected @ 70 degrees, but when the exhaust fan comes on I'm sucking frigid air right into my propagating area. Not good. I've found I can prevent this "sudden change in temp" by opening one or both of the roof vents when the vanes of the side vents open. That seems to be enough to keep the temperature change from being less radical, and more gradual. Couldn't do that without the thermostatically controlled shutters.