I have read that there is heat lost if I put two layers of lines in one trench. (I layer at 51/2ft and 3ft + aware of OSHA 5 ft rule already). The Rational being the top line is closer to surface seasonal temperature, therefore changes in ground temperature will affect more. Also that both lines will "fight" for the same ground source heat and not function as well as single loop trenches. On the other hand, slinky coils do not need as much space, but I would think that they also would be less effective because there is more coil "fighting" for the same heat source as well. I don't think I can run 4 trench single lines at 300ft. I could if I double. I may be able to run 4 slinky lines if the distance is about 125ea (not sure depends on how wide the trenches will have to be) OR I could do a combination of straight and slinky. Is this common? Just wondering if I would really lose alot if I go with stacking two lines as stated above. Lisa

Loop design is based on the soils ability to convey heat. In pond loops you can virtually throw a 600' coil in without un rolling it while with dry sand you need significantly more footprint.
We shoot for a 5' average depth when doing our multi pipe trenches so you would have to be at 6 and 4.
Our Slinkies work well at ~110' by 30" in damp sand/gravel in MI.
Look up IGSHPA certified installers and commission one to design your loop system. Then you know it will work.
Good Luck,
Joe

Lisa, you are on the right track. The top loop at 3 feet would be fairly strongly impacted by the monthly climate. If your deep ground temperature is 60 F then the 3 foot deep coil would probably be in the 70-75 degree range in summer and 45-50 degree range in the winter.

You can stack loops but you they are less effective for the same length of pipe. How much efficiency is lost depends upon the ground's thermal conductivity (how fast heat moves through it). Also, if the top layer dries out it will be less efficient.

Joe, any chance you could run a model and tell us how much pipe length would be required for a stacked loop vice a straight loop? I know it varies based upon how far the loops are speparated but it would be nice to see how long a loop is required if buried at 6 feet or if stacked at 6 feet and 3 feet.

We can run models for lots of things, but what is the frostline at lisa's?
j

My frostline is 18" in central VA. I have clay soil. I do have some rocks (shale) in part of the area 2"-6" but plan to replace it with dirt (bring in if need be). Lisa

a 6 pipe horizontal installed much as you have described, goes about 150'/ton in MI.
j

Lisa asked for advice on sizing loops in a PM. Plan of action at this point is to see what heat pump dealer asks for so that we can't be accused of underdelivering loops if something goes awry.
J

Hi Lisa,

So you are going with Geo.  Great!  I am over here in Madison and we have talked before.

With the system running, the temp difference between entering and leaving water temperature is only 5 to 10 degrees.   The ground is not highly thermal conductive and BTU's move slowly through the soil.   You are not really not 'losing' a lot of BTU's you just have to compensate with some more pipe. 
 
Geo design software is engineered to take in account all these variables.

Yes a single loop spaced trench is more effective but excavation costs are higher.   It is more cost effective in my opinion to excavate wider trenches and lay multi-loops or slinkies.  The trade off is that more feet of pipe needs to be buried.    

When you said "some rock" at 2"-6"  Is this some thing that a backhoe or excavator can break through?   I had run into granite over here.  My last trench has a section that was only 3.5 feet deep.   If you have ground  were the trench is to shallow, you could add more soil on top of this area to raise the elevation. 

For comparison, I have damp clay and designed for 6 foot average depth.   I have four trenches 100 feet in lenght spaced 11 feet on center.   Each trench has 500 feet of 3/4" HDPE pipe coiled in a 36" diameter slinky (36" pitch, 5 feet of pipe per foot of trench).

Bob G.

Hi again Lisa,

Keep in mind; in DIY you can overbuild on the cheap, if you have the space. Sure, you pay in pumping penalty but you get some of that back, in greater efficiency deep in the heating/cooling seasons.

Still waiting to hear what Lisa's geo contractor suggests for piping.
j