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greenhouse Crappy nighttime temps and greenhouse climates ...

Ya its a shadecloth, I dont have all of it fastened because Im still messing around with the vents. The automatic openers dont seem to work quite right. They open fine in the heat but dont fully close when cold.

I dont think Im going to use them at all to be honest. I think Im going to just close up all the roof vents and get a thermostat controlling an exhaust fan and an intake shutter like we discussed. That way I can keep things controlled much better.
So for the intake shutter, do I need a motorized one or will the negative pressure from the exhaust fan be enough to open the vent? Im guessing Ill need a motorized one but they are freaking pricy! $185 just for an intake vent. If the negative pressure from the exhuast fan was enough to pop a non-motorized shutter open I could get away with a $40-$60 unit.
Personally, I keep my shade cloth on the outside of my greenhouse. It is so humid here through the summer that the inside of my greenhouse is almost always dripping with condensation. Yours being on the inside just struck me as odd.
As for the shutter question... I think the answer is, maybe. From all the research I did before buying mine, the negative pressure will draw the vent open, but it also adds stress to the exhaust fan since it has to work harder to pull them open. I also remember reading that it isn't fool proof. If you have wind blowing at that side of the greenhouse then it might create enough resistance to keep the vent from opening.
You could buy the non-motorized shutter and give it a try, make sure to leave your thermostat electrical circuit accessible, and if it won't open reliably, you can come back later and buy a cheap linear actuator to open the vent for you. The actuator is probably going to run you $60-80 but it will still work out cheaper.
I was anticipating that it may be a hit or miss type of situation. Ill have to put some more thought into that.
Also, the size of the intake vents needed. I found a source that said you need 4 sf of intake space for every 1000 cfm. That sure seems like a lot to me. In growing my other plants in enclosed spaces I always went with a 2x intake to exhaust rule and it worked out well. That was with passive intakes of course but that didnt take into account the cfm of the fan.
I dont think Ill need a HUGE amount of CFM for proper extraction. I roughly figure about 950 cubic feet of space inside. Going with a 12" value fan with shutter it hits 1200 cfm so that should be adequate. I think I may go with 2 sqft of vent space and see how it works out. The other thing is during the summer when large intake is needed Ill just keep the door open which will give more then enough intake. During the winter with the door closed the air coming in will be very cold so I may be able to get away with less intake area ... although I realize it will build more pressure for the fan to pull against but even at .20" of static pressure it still pulls 800 CFM and thats way more then needed during the winter.
All of this is following the rule of 1 complete air replacement per minute during the summer, and 2.5 complete air replacements during the winter. On my 950 cubic feet that comes to of course 950 CFM during the summer and 350 apprx during the winter. So the fan should be more then adequate assuming it can pull against whatever amount of pressure is created with only 2 sf of intake area.
Either way this its been fun! lol ...
Yeah, for sure. I don't think you will even need 2sq ft of intake space really. It depends on how well your greenhouse is sealed. No matter how much I sealed mine there is still a bit of gap in many places. My intake is 1 sq ft and does fine. In the summer, I just prop open the door as well.
My kit came with the motorized intake shutter, so I never had to make the choice. I think you will have great success!
Is going organic an option? Screen the door and Windows and open in the morning...

The more technology you put in there the more £££ and the more will eventually fail.

GH used to be simple - that's why they work for 100s of years ;-)
Ghost pepper - I ended up getting a 12" fan, three speed, with shutter rated at 1100/830/550 CFM. Then I got a single thermostate that I can connect a small 1500w heater. I think it will be sufficient.
Al - Thanks for the suggestions. If you mean organic as in no equipment that is not an option. I wish I could but thats why I had to get the greenhouse in the first place. The greenhouse doesnt have windows. but with the door open and all three roof vents open the temps got to 110 degrees with a 78 degree ambient temp. Floor fans didnt help much at all. Plus, my night time temps are much to low so I need a heater. Along with a heater you gotta have a thermostat. I understand what your saying, though. I wish I could but unfortunately my climate isnt suited to that otherwise I would just grow in the ground. We have a 45-60 day growing period from frost to frost because our night temps get so low, but then the sun comes out and heats everything up QUICK. In the middle of the July/Aug night time highs are barely 50 degrees. However, our daytime temps are 60-80 deg for 6 months out of the year and our sun is STRONG year round with over 300 sunny days per year (I think its around 310-320).
It was $250 for the exhaust fan, thermostate and heater. Not bad.
This set up should allow me to grow year round, or if I want to cut costs I can drop the temps to 40 degrees during the coldest part of the winter and let the plants stall /go semi-dormant for a few months before the next season. I could even do the same thing in summer if need be and just use fall and spring as growing seasons and have a few months dormancy during the peak of winter and peak summer.
Ill just have to see how it goes.