exhaust fan

Hi Everyone,

We recently purchased a 6 X 12 attached GH from Charlies. I was wondering if it would be necesary to install a exhaust fan? Or would I be able to contol the heat with fans and the two roof vents? We do live in Arizona and even though it is 30 outside right now the GH is already at 94 and has been over 100 since we put it up. Any and all information would be greatly appreciated.
Tami

Tami, I don't believe that you will be able to control the temps without an exhaust fan.

As a very general rule, passive ventilation alone will get you down to within 20-30*F of the outside temp. Adding shade cloth may cut another 5-10*F off of that. In your case, at an altitude of 5400 feet, you may get a somewhat greater than average temp reduction with the shade cloth. I still doubt that it will be enough.

Consider an exhaust fan of about 500cfm, which should roughly be the volume in cubic feet of your greenhouse. Shade cloth, exhaust fan, and intake shutters on the opposite side will generally get you down to within 3-4*F of the outside temp. For more, you need some type of evaporative cooling.

I know a greenhouse grower near Denver who seems to get a great advantage in his greenhouse growing with his altitude...that or he has a remarkably green thumb.

I had a florist friend in Arizona who had a small greenhouse. She made the entire thing into a swamp cooler by installing an exhaust fan on one end and a blanket and hose in the window on the other end. Set the hose trickling at the top of the blanket, the exhaust fan pulled air into the greenhouse through the wet blanket, the temps stayed quite workable.

Hi Tami, I live 50 miles north of Denver and our growing conditions are about the same as yours. My greenhouse is the upside down U shaped so it is harder to control the heat as I have no top vents. Only one small end vent and the door. I bought two 20x20 in. used window fans really cheap and installed them overhead. I did buy a real good thermostst either from Charley's or Growers Supply. With the door and end vent opened and fans running, my greenhouse cools down to a temperature below 80 degrees even at 100 to 103 degrees outside. The roof vents you have are really a big plus. If you have some fans, you may want to experiment before going to the trouble of an exhaust fan.

Thank you all for your responses. I am just really up in the air at what to do. I was trying to get away with just fans going, but the temps in the house now are above 100, with it being in the 50's outside. I do plan on running a swamp cooler in the summer time. I think while the electrician is here I will just make sure he pre-wires for the exhaust fan, because I just really think that I am going to end up using it.

I like the idea of the wet blanket, my only concern there is it is going to block air circulation.

Blue, are your fans laying flat fan facing down, or are they just placed sitting up at oposite ends of the GH? Habitat for Humanity always has fans for sale, cheap. I can try that now befor the electrician gets here.

Thanks Again

Bluejack, are you saying that you are able to achieve >20*F below outside temps? Without evaporative cooling like a swamp cooler or misters? If so, I'd be interested in details of your setup too!

SB

Blue-how big is your grhouse and is it shaded at all with trees or buildings?

Tami, There are two braces running length ways of the GH near the top which my fans are attached to. My fans face down and are only a couple of inches from the top of the GH. When my door and vent are opened, the air really circulates on the inside of the GH. I have the fans connected to a rheostat so I can control their speed. They are also connected to the thermostat and turn on at about 80 degrees. With your vents, you could experiment with some different combinations to see what works best for you.

tigerlily, My GH is 8x10 ft and 8 ft. high and is not shaded.

stressbaby, First of all the 20 degrees drop may not be accurate as I cited this because my thermostat is set to start the fans at about 80 degrees and so often when I go into the GH, they are off. I never pay any attention to how hot it is inside as this is never a problem with the door and vent opened and fans availabe. The temperature I pay very close attention to is the low temp as I want to maintain it above 55 degrees. The GH "skin" is doubled wall and I think 6mm. You can not see thru it and it defuses the light so this might be one reason. The lower 4 ft. has 3/4 insulation panels all the way around plus 4 ft. aluminum type insulation. So the sun has little or no effect on the lower 4 feet. We have above average wind and quite low humidity. Sometimes in the summer it is 10 to 14 per cent. Our nights are cool, 65 is a warm night and 70 is hot. Our high temperature this past June was very hot for us and set a couple of records. July and August were also hot. We have a lot of commercial GH in our area and they use huge exhaust fans and I have never seen a cooling system in one. To sum it up, I think it is a combination of reasons. I just thought Tami might try some different ways before going to too much expense. I always have several experiments going with my gardening and one I am about to start is making my own solar panels to lower my GH electric use. Will it work? I have no clue. My GH is not a show place but serves me well. My plants are always great and I will be starting right at 600 plants next year, mostly tomatoes.

I am going to visit a huge greenhouse complex next week that grows only tomatoes (SunBlest.) I am going uninvited and unannounced. I am making a list of questions to ask them and if you have question you want me to ask, please let me know. Best to you.

Bluejack, it sounds to me as if you know precisely the questions to ask...fire away, and let us know what they say! I'd also be interested in seeing details of your solar setup. If you have time, I would invite you to start a new thread with some pictures.

The wet blanket did reduce air circulation, took a really strong fan at the other end to make it work.

Interesting to hear what other people are doing, useful stuff!

Stressbaby do you think I would have to cut in another intake shutter if the door to the greenhouse window would be open?

Tami, with a 6 x 12 GH, I doubt it. I suppose it would depend on how things were positioned, how the GH was oriented, how strong the fan was, and how big the roof vents are.

It is not unacceptable to use the fan on the intake side...that is, put the fan by the door and push the hot air out through the roof vents rather than drawing the hot air out through an exhaust fan in or near the roof.

Tami - we use an exhaust fan all year round, as much for cooling in the summer and in the winter to keep air movement to stop botrytis. The green house has background heat to keep it frost free in winter, and partial shading for the summer. The fan has been a great investment.

To create the swamp cooler effect - we laid a loose fitting brick floor in the greenhouse directly on the dirt. This has the same effect as a wet blanket, but doesn't stop air circulation. The added moisture helps prevent red spider mite from building up - they love dry, hate moist! We do have both top and side ventilation, and we are a larger gh than yours (in feet we measure 14x22') - but I do find that one fan, hung from the rafter does do the work.

Thanks Stressbaby, I think I will give that a try, the window in the door is big enough that I think it will work good.

Laurie, I have heard alot about those spider mites, I plan on trying to keep it as moist in there as I possibly can. I never thought about using the exhaust for winter air ciruclation, hmmmm thanks for the great idea.

Which brings me to another question, what are some of the tricks you all use to keep the humidity up?

Here is the GH

Tami,

Though somewhat technical, this is the best link, or set of links, that I have come across regarding humidity in the GH: http://www.hortnet.co.nz/publications/science/n/neder/humid07.htm#top

SB