Leaks in collectors are mostly due to freezing—usually the result of gizmo freeze-protection like freeze valves. Freeze breaks in the absorber tubing should be repaired by brazing or silver soldering. The silver solder process is easier than normal soft soldering used on pipe joints and the copper tubes don’t require a bright fluxed finish to seal the leak. Soft soldering inside collectors is not suitable due to the lower melting point of the solder.
Automotive radiator additives, like Alum-A-Seal, a powder that accumulates to plug leaks, can be used to stop pinhole leaks caused by slightly corrosive fluids like overheated antifreeze, but this is a temporary repair. As it does in car radiators, the conditioner can give an old collector a few extra years of life. However, this strategy should only be used in systems with double-wall heat exchangers due to the uncertainty of the chemical contents in additives. This is definitely not considered a “best practice.”
Evacuated-tube collectors are most often used in antifreeze systems. Although the tubes have been in the marketplace for more than three decades, due to higher costs than flat plates they have been popular mostly in cold, cloudy climates like northern Europe and Canada. The superior insulation of the vacuum in the tubes makes them most applicable in harsher climates with reduced solar resources.
Broken glass. Evacuated-tube manufactures use borosilicate glass or soda lime glass in the collectors. The glass is not tempered, so breaks into shards that can be more dangerous than the small pieces from tempered glass breakage. Consequently, evacuated-tube collectors are more fragile than flat plates, as glass strength is proportional to its thickness. Broken glass in a twin-tube-type collector can often be replaced separately from the absorber/heat pipe components. The vacuum in twin-tube collectors is between the two pieces of glass much like a thermos bottle. The heat pipe in twin-tube models slips into the tube on installation and can be removed and reinserted into a new glass tube. In single-glass collectors, the heat pipe is factory-installed.
Vacuum loss. Evacuated tubes have life spans of about 15 years but this varies depending on the type of tube and manufacturer. Vacuum loss, through glass breakage or through the connections, is the most common failure with tubes. Once the vacuum is gone, there is little insulation left to retain the collected heat. The collectors can lose their vacuum in other ways (seals and connections) depending on the type of tube, but exactly how the loss occurs isn’t well understood, since it normally occurs over a few years. There is little evidence left as to how the vacuum was lost in most cases unless glass breakage is evident.
If they’re not abused, most swimming pool collectors, made of UV-inhibited polypropylene, have life spans of 20 years or more. However, polypropylene is a high-temperature plastic (polymer) to which normal sealants and glues will not adhere. This makes fixing pinhole leaks that may appear in the collector’s later life difficult unless factory plugs are used. The repair process requires cutting the leaking tube at both ends near the large header pipes and inserting the correct-size rubber plugs into the tube. This repair isolates the leaking tube but is often a stopgap repair, since these types of leaks indicate that plastic tubes are deteriorating and the collector is likely near the end of its useful life.