Earth Air Heat Transfer, fan power?

Hi! I am new to this forum. I hope you can help me.
I buried 115 feet of 4 inch perforated tubing 5 feet underground, I intend to heat/cool my greenhouse with circulating air through it. The greenhouse will will have volume of around 3500 cubic feet.
Can you recommend me how powerful the fan should be? Should I put maybe two (weaker) fans on both sides of the tube?
Any suggestions will be greatly appreciated.

Thanks!

Bor
Slovenia

A double fan design is safer. If you depend on only one fan and that fan fails the time to repair or replace it could allow extreme temperatures to ruin some plants. The resistance to airflow depends on not only the diameter and length of your earth  tubing but the number and angle of any bends. Fluid dynamics is complicated enough that substituting rules of thumb is common. It wouldn't be difficult to measure the resistance to airflow by comparing the power input of a known fan and comparing the resulting airflow returning out of the system. The overall volume of the greenhouse is only one factor. Others include details on weather: Lowest expected temp is not the whole story.  How long an extended period of extreme cold is another, direction and speed of prevailing winds, latitude and seasonal sun angles changes, R factor and shape of greenhouse walls all play into your heat loss. In other words a two week span of 20 degree F temps may slam your crops worse than a single night of below zero. Here in the overcast Great Lakes region the USDA Zone is 6 But  the sun, clouds and winter temps combine to make the period from Thanksgiving to Valentine's Day a period of low yield even with high supplemental energy demands. The additional cost and effort to upsize a system to maintain 50 degrees year round is questionable for Ohio. Stick with just spinach and kale for those 10 weeks when sunlight is scarce. Ultimately farming is solar powered. Other crops can be grown in January but it makes for a big investment with crummy returns.

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/my-earth-tube-story

http://www.builditsolar.com/Projects/Cooling/EarthtubeNotes.htm

I have buried some 4" perforated line in the soil beneath my chicken/greenhouse.
It's not deep enough or insulated at all.
But I plan on using a fan to push rocket stove/solar heat into the soil for storage. I might even boil water in a "open" vessel and push the steam into the ground.
Of note is use of corrugated perforated pipes vs. standard pipe.
Subterranean heating and cooling system (shcs) is one term of art for the systems using the black perforated tubes.
SHCS or SHC system might be the best way to refer to such a system while distinguishing it from earth tubes.
SHC systems using perforated pipe actually take advantage of the condensation that occurs inside the pipe.
Because this condensation is a phase change that occurs when the heat in the water vapor transfers to the surface it passes,it's great for a system which is trying to store heat in the surround soil.
For a system that is trying to warm incoming air,it maybe at cross purposes.
Another feature is the exposure to soil life. Proponents of the perforated pipe say the natural soil organisms defeat the molds and mildew.
I am concerned that I might kill off these organisms if I introduce steam or even just very hot air.
The other pertinent difference between the SHC and earthtube systems is intake air.Earth tubes draw from outside of the building .A SHC system generally pulls heated air from the highest point in the greenhouse.