Greenhouse window opener

Hi All,

As we approach late winter, I’m wondering about keeping greenhouse temps from getting too high. The automatic openers for the vents I’ve seen all kick in between 55F and 60F. Anyone know of an automated opener that can be calibrated to open up around 42F instead?

I had one that was adjustable years ago, but I cannot find it. I now use a louvered fan and a digital line voltage controller.

Out of curiosity what is the magic number for 42, other than Jackie Robinson? Is it to protect, but remain dormant?

Exactly - below 42 degrees maintains dormancy. Non electric automated greenhouse window openers are trying to prevent summer heat damage and use a wax which phase changes to liquid to trigger the opener. They are calibrated for warmer temperatures.

I use an ITC 308 Wifi to control my garage tree enclosure. It can turn on heating or cooling based on the temperature settings you provide. You could hook up the louver vent motors to the Cooling outlet and it would open them when the temp got too high.

I don’t know if it would close them though. Just a thought.

I use a similar ITC model to control the heating and cooling of my greenhouse. The louvers close when the fan shuts down. I am thinking of getting separate controllers for heating and cooling. The problem with the dual control is that it wants to drive the temperature to a single set point so once the heater or cooler turn on it does not shut off until that set point is reached. I use a set point of a 35F (1.7C) and the heater kicks in at 33F (0.9C) and the fan for colling kicks in at 39F (3.9C). As a result, the fan stays on a long time even if the nighttime temperature is going to drop below freezing.

I have collected a fair bit of data on the thermal performance (taught thermo, heat transfer, and lab analysis) and I think I want the heater to kick on at an air temp of 32/33F and shut down at 34/35F while the cooling fan should kick in at about 40F and shut down at 38F. This will allow the greenhouse to use a bit of the heat from the day to keep the heater off for a couple of more hours in the evening on a freezing night.

Got it - so I’m going to get an exhaust fan with it’s own thermostatic control. When venting, how do you prevent the cold air being drawn in from shocking the plants. For example, if it’s 15 degrees outside, but sunny. Solar heat gain inside the greenhouse can easily create temperatures over 40 degrees, so the fan comes on. Won’t it be pulling 15 degree air into the greenhouse? Do you screen the intake so that it doesn’t draw across your trees?

My greenhouse is not glass with shade cloth over the top so it is rarely more than 5F (3.9C) warmer than the outside air. I rely on leakage through various gaps so the incoming air is spread out. If it were to be much warmer than the outside air, I would put a louvered vent at the other end of the greenhouse with some baffles (a couple of spaced burlap or similar open weave cloth) to disperse the cold air.