I have a solar heater in my back field about 80' from the poolhouse and equipment. I ran a couple 1.25" pipes underground to it. Only a part of the flow goes through this heater. The flow is split 3 ways. Returns, a 4x20' mat heater, and the field. Both the mat and field dump into the deep end via hoses. Check this thread and let me know if the pic doesn't appear;

http://www.poolforum.com/pf2/showthread.php?t=943

The heater is 12 100' sections of black poly pipe layed out on black rolled roofing. The pic was taken while it was on tarpaper and I found that didn't last long. The roofing is a lot more rugged. Note the PVC manifolds. Running the pipes in parallel greatly decreases flow resistance. The pipes are held apart and in position by stiff wire wrapped around each pipe along the way and tied at top and bottom to steel rebar driven into the ground. I think I get about 20,000 BTU/Hr from the thing if my memory is right. I'd have to dig through my records but I remember a 4.5 deg F rise and 'I Think' something around 10 GPM or less. If I did the math rright those numbers work out to around 20K/Hr.
The only problem I've had with the rolled roofing are a few spots that have holes and the weeds want to grow through. Holes were punched in by Hooved Rodents (deer) that walked across it when it was snow covered. Seems to be holding up well and has been in place for maybe 4 years now.

I have a series of build pics that I can probable dig up showing how I fabricated this thing before hauling it to the field.
Al