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Simple and budget friendly GAHT greenhouse climate control system idea

 
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Hi everyone I posted another similar post a few days ago but I came up with another idea. I was thinking of how to make a GAHT system for a dual wall poly greenhouse I will be building for my sustainable nursery to raise the temperatures in the winter. I am in zone 8 so i’m hoping that with the dual wall insulation that the GAHT system will keep it from freezing even when the outside temps drop to 15-20F which is a rare occurrence here. I’m also hoping that when its the average low in January of 35F that it can raise the temps to the mid 40’s to low 50’s to where some plants may even grow a little. One of my main goals was to come up with a system that was more budget friendly than using tons of perforated pipe and it actually seems simpler as well.

It is basically creating an underground chamber filled with round river rock so air exchange can still happen and wrapped in filter fabric to prevent it from clogging with soil. 12” inline fans will circulate air into the rock air exchange chamber. And exhaust pipes spread throughout the greenhouse floor will allow the air to escape the chamber back into the greenhouse. I think something similar to this may have been done before as during my web research I was able to find some info on someone using rock before but they didn’t go into much detail. Other than that i have not had much luck looking for something similar in my web searches. What are all your thoughts on this alternative to the more common (and expensive) perforated corrugated pipe design and has anyone done anything like this? I have attached a diagram that shows the details of my GAHT system design.                            
gaht_greenhouse_with_rock.png
[Thumbnail for gaht_greenhouse_with_rock.png]
 
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Hi A, I've seen something similar myself.  I believe the University of Minnesota has what you're looking for.  Do a search for Deep Winter Greenhouse and their design should come up.  It's a 4' deep gravel basement under a well built greenhouse.  
 
A Booth
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Mike Jay Haasl wrote:Hi A, I've seen something similar myself.  I believe the University of Minnesota has what you're looking for.  Do a search for Deep Winter Greenhouse and their design should come up.  It's a 4' deep gravel basement under a well built greenhouse.  



Thanks, I looked into it a little more, they didn’t give a lot of specifics but gave me the confidence that it had been done successfully before to try it. I had enough materials on hand leftover from my landscaping company’s jobs and home projects to build the underground river rock air chamber without any investment. It’s a smaller system but I figure if the 2 hours I put into it raise the temps of the 240 sqft dual wall poly hoop house I will put over it a few degrees in the winter it was worth it to get a head start on the spring for the perennials I plan to grow in it. I am in zone 8 with temperatures at night above freezing more often than they are below so a few degrees may get it to the point where some winter growth will occur.

I have a 24” auger for my mini skidsteer that I use for my landscaping company so I used that to drill a few holes about 5’ deep and I finished it off by hand after the bulk of the digging was done with the machine. I lined the hole with landscape fabric to filter out soil. I put about 8” of river rock in the bottom and then installed two 6” air duct pipes I pulled from my crawl space since we have a ductless heat pump in the house now. I used a bunch of scrap ABS pipes for the exhaust that I had left over from drainage work my landscape company did.

Now comes the part I am not sure of. I plan on purchasing two 6” inline fans for the 6” air duct inlets. What I am not sure of is how to use both fans at the same time for both heating and cooling. To keep this project cost to a minimum I was hoping to use a basic $30 dual stage thermostat from amazon, the type with a temperature probe and an outlet for heating and outlet for cooling. What I then realized is that those thermostats are designed for a heater on the heating side and an AC or fan on the cooling side but not using the same fans for both because it has two separate plugs for heating and cooling. I thought about doing some sort of spliced wire to plug into both with the same fans but after taking with my brother who knows a lot about electricity it sounds like that is potentially hazardous. Does anyone have any ideas of how to utilize the same fans for both heating and cooling? I am open to another type of thermostat (even if it costs more).

My other idea is that I may just have to have it plugged into heating only in the winter and then have another fan to vent greenhouse in the winter if it gets too hot. Then in the spring when I may not need the system working a full capacity I can plug one fan into cooling and one fan into heating. Then in summer I would switch to cooling only.

Any thoughts or suggestions on how to set up the thermostats would be greatly appreciated. Thanks.
 
Mike Jay Haasl
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I'll have the same issue when I hook up my system.  One fan and two thermostats.  My electrical engineering buddy said we need a "relay".  It's basically a switch that our thermostats can turn on.  They both are wired to the relay and if either one comes on, the relay closes the switch.  In doing that, it sends power through the switch (a different power than that which is going through the thermostats) to the fan.

That way you can use a low power pair of thermostats (even DC) to turn on and off as big a fan as you want since it's getting its power from its own source (through the relay's on/off switching function)

That's about all I know about it, I still have to find/pick a relay and thermostats for my own system.

 
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