http://www.greenpowerscience.com/ CPVC is used for hot water transfer in homes and business. Solar rooftop projects require heated water or heat exchange liquid to sometimes travel a long distance through piping exposed to cool or very cold temperatures. A lot of solar thermal heat can be lost from collector to water heater tank , radiator, or radiant floor heaters. This works under water and in subterranean installation prevent wet ground from leaching heat. Also prevents freeze overs for up to 10 hours in -10c with no circulation.
This vacuum design costing about $1.30 a linear foot can be easily made. After doing one unit you can average about 30 minutes per section. This is fine for subterrain