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The piping in a solar loop is subjected to a wide range of temperatures, varying from 300°F to–30°F (or lower). Copper tubing is the best kind of pipe to use for the solar loop. It can withstand this temperature range and is very durable and easy to install. Copper tubing comes in various grades and is classified by wall thickness and rigidity. Soft copper (annealed) can be bent; hard copper (drawn) is very rigid. The heavier the wall thickness, the more rigid it is. Type M copper tube is thin walled; type L is medium walled; and type K is heavy-walled. We suggest using type L for the solar-loop piping. Use type K for underground piping runs. We suggest using hard copper in all instances except underground piping runs. All types of copper pipe have the same exterior dimensions, so all fittings are made the same and come in one standard size for use with all types. However, the exte-rior diameter is actually ⅛” larger than its nominal designation.

Figure 4.7: DC Pump

It is important to use only copper pipe, or in some cases stainless steel, for the hot supply pipe in the solar loop. People often want to use a flexible product such as Pex tubing or rubber hose. These products will not last very long and will deteriorate well before the system wears out. In his repair business, Bob had to replace a lot of hoses that were used in the 1980s. Not a single installation from those days that used rubber hose is still operating in Wisconsin (to our knowledge). Pex tubing will also fail in a very short time. We know of several attempts to use Pex tubing for the hot supply line, and the failures were always within the first year. It is possible to use Pex tubing for the return line back to the collectors, but it is important to terminate the Pex at least 10 feet from the collectors. The Pex must never be used for an outside pipe run unless it is buried. We suggest that you consider Pex only for use in the solar loop when burying the return line out to a ground-mounted array.

There is one alternative to copper for use in the solar loop. In the past few years several solar companies have developed a corrugated stainless steel piping that is both flexible and durable enough to handle the high temperatures of a solar system. Its flexibility simplifies installation and significantly reduces labor costs. It typically comes in a long roll with both the supply and return lines encased in high temperature insulation. Often a sensor wire is also sandwiched in the center of the insulation.

Though the product has now been around long enough to have a proven track record, it has a couple of drawbacks. The corrugation that makes it flexible creates increased surface area inside the tubing and results in significantly more resistance to flow, or friction head. In some cases this may require the use of a larger pump and cause increased electricity operating costs. Be sure to factor in the friction losses when selecting and sizing the pump. At this point it is also more expensive, although much of this is offset with savings in labor. Overall, it is still a good product and a welcome addition to the solar professional’s toolkit.

Because of its flexibility, corrugated stainless tubing should never be used on a drainback system. The dips between the stainless links and sagging between pipe hangers will create pockets where solar fluid can accumulate. In a pressurized system, it may be necessary to flush or blow out the solar loop when replacing the fluid to ensure that all of the old liquid has been removed from the pipes.