Piping. To effectively drain (and prevent a vacuum from forming), all piping in the HTF loop must be sloped. A 1/4-inch slope per foot of run is an acceptable minimum, but a greater slope—up to vertical, especially in unconditioned or exterior locations and on the hot return line from the collector—is recommended if possible.
It is also important to adequately support long, horizontal pipe runs. Even if they are correctly sloped at installation, the potential for sagging over time can result in low spots that can impede drainage or allow undrained HTF to freeze. Using one-size-larger-diameter pipe (such as 1-inch, when 3/4-inch could be used on a pressurized loop) and substituting two 45° fittings in place of a single 90° elbow can help ease flow restrictions.
Collectors. The collectors must also be sloped toward the return side, the same 1/4-inch per foot as the piping. This means that on a 4-foot-wide collector, the inlet port should be in a plane that’s 1 inch below the outlet port, which is typically located on the opposite edge of the collector. The collector also needs to be tilted—approximately 25°—and should only be flush-mounted on roofs that have a 5:12 pitch or greater. Collectors should be installed so that the longer, narrow, riser tubes run vertically, with the header tubes horizontal. For most collectors, this equates to a “portrait” orientation. This is to reduce the possibility of the riser tubes sagging over time under the weight of the HTF, eventually creating water-trapping low spots in the collector.