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Martin, Electrical Engineer
Category: Home Improvement
Satisfied Customers: 4956
Experience:  Design, construct, fix and grow stuff around and in the home.
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I built a 30 x 12 x 7 tall greenhouse and want to ke

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I built a 30' x 12' x 7' tall greenhouse and want to keep it below 75F even when the outside temp is 110F-120F.

I installef two 1.5 ton Carrier window air conditioners and fans and am using shade cloth (50 percent light transmission).

Currently the outdoor temperature range is 70-100F and the inside temperature range is 60 - 80F. What seems strange is that the highest outside temp and inside temp don't coincide. Sometimes it's 65F inside when it's 100F outside or 70F inside when it's 80F outside.

Does this have something to do with how the two units cycle relative to each othet?

Would a third AC unit provide the cooling I want?

Electricity costs are irrelevant.



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Customer: replied 4 years ago.

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Thank you for your patience,
Hello and welcome. A big volume of entrapped air sure can cause a delay in inside and outside temperatures. If you want to passively reduce the temperature the most possible, try to have the smallest amount of dark material inside (ideally all white).

Also note that AC tend to produce dryer air, some plants may not like that. A third AC might eventually get the temperature like you want but for a green house i usually prefer geothermal tunnels bringing cool air (without any motor fan, just the a chimney effect) and sprayed water (from a well or buried plastic drum) in the air to control the temperature.

Customer: replied 4 years ago.

I'm afraid ac is my only option. This is where I live. Heat index of 172F!



Getting the humidity too low is not a problem.


I think I need an HVAC technician or mechanical engineer to answer my original question as posted. Thanks.

Don't underestimate the cooling power of deep and long geothermal tunnel (unless you have the greenhouse on a roof or something).

The 30 x 12 surface area (when the sun is on top) mean 360 feet^2 of area or 33m^2. If we take the worst case scenario of every area in the green house filled with lush green, the greenhouse will receive and absorb 33KW of sun power.

If you want to reduce temperature from 40%, you need a perfect AC system of removing 20KW or power from the system or around 68289 BTU of air conditioner in total (3 x 22763 or the closest you can find, 20000 BTU models should be enough as a plant is not pitch black and some light will bounce out).
Martin, Electrical Engineer
Category: Home Improvement
Satisfied Customers: 4956
Experience: Design, construct, fix and grow stuff around and in the home.
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