I've decided to go geothermal to replace my electric baseboard system. After receiving three quotes, I selected an installer and will be beginning the installation of a GeoStar Sycamore heat pump in the next couple weeks. It's a 2,100 sq ft house in Lansing, NY, 43,000 BTU/h loss, 21,000 BTU/h gain. The installer has proposed a 4 ton system with five 150ft trenches containing 600ft of pipe in each, buried 6ft deep. I'm not sure what size pipe. My question is related to the loop field design. Instead of using five separate 150ft trenches in a grid layout, is there any reason a single 600ft trench with five pipes shouldn't be used?
Welcome. Good choice to go geo, especially versus electric baseboard. Really not that much difference between them in terms of flow and heat transfer. There is a bit more separation in the grid layout between the circuits, if you measurements are correct, But it becomes installers preference. As long as they use 5 different circuits. You loop field appears appropriate for your 43KBTU heat loss in your area. Who is your installer, if I may ask?
Are you sure the pipe will be out and back straight pipe? 600' of pipe in 150' trench, installer may be using a slinky loop. CJ
Don't know him. But 4 tons, 5x600 ft of straight seems reasonable for your house in your area. How old is your house?
While it may not be significant, one consideration I began to question after my install of straight out and back pipe was the effect of thermal expansion on pipe. It would seem that with a seasonal delta T of 60 degrees, that there can be around 0.7" of contraction/expansion in 150 feet of pipe. That could add some strain to connections. Were I to do it over again, I'd have considered one slinky loop near the connection points to ensure strain relief.
Thanks for the advice. I'm going to proceed with the single, 600ft continuous trench with 5 pipes, so that won't be an issue.
Keep in mind pipe spacing and trench spacing both have effects on the groundloop design. In the second, you don't effectively have any trench spacing.
Correct. I will be maintaining at least 1ft spacing between the 5 pipes in the trench and about 10-15ft spacing for the trench.
There speaks an engineer. Indeed, doing exclusively slinkies in my horizontal installs, I never experienced an strain issues on the connections. Not sure if they would surface, or the plastic would be "stretchable". To Shayne: I think the point Trent was making, that if it is an issue, a single 600ft trench is the lesser preferable option. Also, if you space pipes only 1' apart, they will steal some heat from each other. If you have the room, spread them out more. Now, the Geostar Sycamore is essentially a 7-Series Waterfurnace, which is very efficient, but also pulls more heat out of the ground than lesser efficient models. It all comes down to the question wether 43 KBTU/h heat loss is on target, which I don't think anyone here would know.
Note: before you read my response, I am openly pondering... I am not making a statement. I have no experience with geothermal. I'm responding because I have/am considering the issue in my loop design. Strain on which connections? I assume you mean the header. When I was reading up on HDPE pipe, I got the understanding that if the connections are fused (socket or butt fused) the pipe acts as one piece, as though it has no connections. At the header it is a fused joint, and as long as it is buried the soil pressure should keep it in place. However I wonder, if the loops were straight out and back and the header was inside the building, it might cause the pipe to put expansion and contraction (linearly) against the wall perforation. P&F Distributors site says in their Engineering Considerations for Temperature Change "This is a significant length change compared to other piping materials and should be taken into account in piping system design". The value they give for the DriscoPlex product is .00009 inches per inch length per degree F, but .0001 may be a better value to work with for HDPE pipe in general. Slinky loops probably have no discernable issue even when the header is inside, and if a straight loop were angled from the wall perforation then the expansion and contraction strains should be reduced a little. I imagine that if you had a long straight loop run, that you could do an S jog near the building before the rest of the loop run, and the soil around the S jog should restrain the pipe so that the header or wall perforation were not affected significantly.
I'll call that a theoretical concern, since even in straight pipes in the field, length change due to expansion and contraction has not created issues. Not in slinkies, and not in straight pipe SDR-11 pipe.
