greenhouse running abs pipes under greenhouse dirt?

I am wondering if anyone has ever tried to warm a greenhouse/coldframe by blowing warmed air through piping ran under the soil and then vented into the greenhouse? My garden is a 8 foot x 32 foot raised bed, that has a pvc coldframe covering it, to make a single layer greenhouse. I have a 6 inch inline fan to pulls warm air from my house into the garden. The way I have it now, the fan blows from only one end only making the other end quite a bit cooler. I am wanting to balance out the temperature inside, so I am going to hook up the fan to 4 inch abs piping and vent it over 3 or 4 smaller vent holes (2 inch). I was thinking of maybe burying the pipe and looping it 3 passes under the garden (one pass under each of my rows) before venting into the greenhouse. Is there any benefit to doing this? I was thinking that maybe this would help to warm the soil faster in the spring and I could continue to pump air through the soil in the summer to help with cooling. The 3 passes would be around 90 feet long and then a 4th pass that would have the 3 or 4 vents coming up to the surface. If I had the space, I could see this working on a very large scale burying pipe below frost level and having several hundred feet of piping, to do a geothermal type application. In my yard, I am limited to not digging down, due to buried lines.
Using geothermal isn't really about warming the air, it's about using the earths temperature at the depth you bury the pipe. Depending on your winters and summers makes a difference in how deep you have to go. It will certainly help you but is it worth the cost to install it?
Check out this video.
Its just me but I think the concept is great....except for using heated air.  A recirculating closed warm water system would give a greater efficiency of heat transfer.  Insulate all exposed pipe, and the water reservoir, and let the soil "insulate" the exposed  buried pipe. :P  Ive been thinking of a similar setup.  I plan on using a 110v hot water heater element or two???  and a digital temperature controller (off amazon) and a water pump.. Should be pretty easy to maintain a reasonably constant temperature in winter....I hope.