Exhaust fan and questions

I am trying to complete my 20’x20’x10’ tall hoop greenhouse and I could use some help sizing the exhaust fan and inlet vent. Do you use the same calculator as normal greenhouses? Would a 3,100 cfm fan be sufficient?
I have also acquired a 75,000 BTU Reznor gas shop heater but I am thinking that it’s much more heater than I really need. It would come on for shorter periods of time but are there any other problems with an oversized heater? I also have a 5,000 BTU 220Volt electric heater.
I live on the Gulf coast so I am more concerned with cooling than heating. Here are my average temperatures. http://pics2.city-data.com/w1/lha9244.png
Thanks for any help you can offer.
Oldude

according to the calculations we use http://www.igcusa.com/Technical/fan-shutter-sizing.html your CFM requirements are 4800. this means two 20" fans rated at 2750 CFM apiece would work, or one 24" fan would work as well. this is going to run between $200-$400. since your mainly concerned with cooling, opting to help the fans with a shade cloth is another option. if you throw up a 50% shadecloth that reduces your need down to one 20" exhaust fan.

as for the heater http://www.igcusa.com/greenhouse-btu-calculator.html, figuring an outside temp of 40 degrees and a desired inside temp of 70 degrees and a covering of inflated double poly, your heat requirements are around 18,000 BTU. this means your reznor is overkill and the electric is underkill. i wouldn't see a problem using the larger heater as long as you had adequate air circulation within the house. a couple circulation fans is certainly going to be cheaper than a new heater.

Thanks igusa
What I did was to figure the volume of a 20’x20’ cylinder and divide in half which would have given me a lower CFM rate, but I guess that this does not apply to greenhouses.
Farmtek agrees with your findings with a formula of floor space in square feet times 12, so now I begin my search for larger fans and inlet shutters.
Thanks again Oldude.

If you haven't found them yet, this is what I'll be using for my 40 x 20 x 8 frame house. It will have 2 x 24" fans and 40" shutters. Nice and easy calculators on the site as well, making things quite easy to understand.

http://www.greenhousemegastore.com/prodinfo.asp?number=FA-VRSG-

Chris

A bit off track, but should I assume that this is a 'must have' for a greenhouse? I have a 24'x16' ft hoop house with double plastic and the humidity is outrageous - whether it's below freezing or not, the condensation is very heavy - and, of course, then it rains all over as soon as it melts.

Yes. An exhaust fan and shutter is essential in something that size for 2 reasons. Cooling in the summer, and reducing fungus, mold, pests, etc during the other months. Good circulation is essential, and you'll appreciate what it will do.

Chris

Thanks, Chris.

Sorry, olddude = I hope you got your info before I switched tracks...

Pagancat
Yes I did get information and installed one 36”vent with the 24” fan. I still had trouble cooling so I added another 36” vent and everything is working very well now. My first tomatoes are almost ripe and all of my wife’s plants are happy. Here’s the link that goes from start to finish with my greenhouse. http://davesgarden.com/community/forums/t/775596/
Thanks Oldude

Pagamcat
How far away is your greenhouse from your house? The reason that I ask is that these 24” fans (guard mounted direct drive) are loud. Yes they move every bit of air that they say that they will but they scream. My patio is 90’ from my 24” (one fan) fan running 75% RPM (variable speed) and I find it irritating with my old ears. These guard-mounted fans deliver a lot of air at high fan tip speed, which makes them very loud. I have found an old belt drive fan that delivers the same volume but is whisper quiet. Next year I plan to use this old belt drive fan as my primary fan with the 24” fan as back up on a separate thermostat. You might want to consider the price of two 24” fans as compared to one slow speed larger diameter fan.
Oldude

Thanks, that's a great suggestion. Where did you find the belt drive fan or do you have a suggestion as to where I might?

My home is only about 30 feet farther from the GH than yours is. And my hearing is pretty good, at least it will be until I get one of those fans, from the sounds of it...>smile< ...