[stevehull]

How about a compromise (copper pipe vs plastic)? Use plastic for the runs out to and from the pond. Then only use the amount of copper at the bottom of the pond for the true exchange loop.

Seems a waste to use expensive copper for the long runs to and from pond bottom.

One thing is certain, you need to measure the temp at the bottom of this lake. I suspect it is 65-70 F even though the surface water is 95 or so. This allows you to calculate the delta T across your heat exchanger and then work backwards to figure out how much copper pipe to put in. It may be cheaper to buy thinner (1/4 inch) copper and do a few parallel loops to get your total surface area.

The ratio of surface area to volume gets much higher in smaller diameter pipe. That maximizes your BTU transfer.

I second AC's warning about propane. This gas has a density higher than air and when (not if) you have a leak, it could get . . . . . interesting. Well know situations in basements of a propane bottle slowly leaking, collecting in a "pool" on the floor, and then someone turns on a light switch.

In the absence of a dielectric contact, copper does not corrode per se, but develops a surface green copper oxide. In the trade, this is known as "patina". Acid water will corrode copper, but that is a different issue as a pond will not have a pH of < 5!

This sounds great, but look carefully at the costs of this large amount (800 feet, 250 meters) of copper tubing.

I have consulted on several projects that have used closed loop pond loops for geothermal units. If I recall, all of them were HDPE plastic and about 300 feet per ton. Inexpensive to purchase, but would take a LOT of gas to fill the entire volume. I sure would use inexpensive propane to test this out, then use a higher priced gas.

How is the water heater going? What temp can it get the tank water up to?


Steve