[cmroseberry]

Actually for the tubes on the roof surface, there is usually not much of a temperature difference to drive heat transfer. The roof surface is about 120 F and the dark gray surfaces of the painted PVC tubes are about this temperature too. However, I imagine I am losing some significant heat from the tube for the length that it runs underground since the soil stays around 65 F. When I get in on a new pool installation (as opposed to a retrofit) I will specify that the heat supply tube be insulated before it is buried.

In regard to preheating domestic hot water, a WS heater would, at best, be only effective only during March to November; the period which an attic space gets above room temperature. Objectively speaking, I think that a better approach to generating domestic hot water would be to use a glazed 2-D nonimaginging solar concentrator. Such a collector could produce high temperatures practically anytime the sun is up. There is an article about these nonimagining concentrators in the March 1991 Scientific American, in addition to the book by Joseph O'Gallagher I mentioned in one of my other posts.