Broken glass. Glass breakage is the problem that most often arises with flat-plate (and evacuated-tube) collectors. While the glass can be replaced by removing the collector trim, finding a source of low-iron tempered (LIT) glass can be difficult. If you can’t purchase it locally, shipping a single piece of glass can be expensive—sometimes more than the glass itself. Because of this, most people opt for common tempered glass. Any metro area will usually have at least one tempering plant that supplies local dealers with tempered glass for commercial windows, and patio and shower doors.
Carefully measure the glass before ordering, since tempered glass cannot be re-cut. Broken glass should be replaced quickly or additional damage to the collector can result from wind and rain. The fragments from a 4- by 8-foot tempered glass pane will fill a 5-gallon plastic bucket; a wet/dry vacuum cleaner is a handy cleanup tool. The gasket around the glass can most often be reused. If it is defective, fill in with a contractor-grade silicone sealant.
Condensate buildup. All collectors will exhibit some condensate on the glass intermittently, but if it is present all the time, standing water may be accumulating in the bottom of the collector. The remedy for this is drilling 1/8-inch weep holes in the bottom corners of the collector. The holes must be placed in the back of the collector and away from the glass front, since a drill will fracture tempered glass, and care should be taken to avoid drilling into any tubes.
Absorber coating deterioration. If copper or aluminum is showing through the absorber’s paint or coating, it should be repainted. The original coating may have been a selective surface like black chrome but that will be impossible to replicate in the field. Semi-selective paints like Solkoat are available online but difficult to apply without experience. Painting absorbers with a spray can of flat black paint won’t have the same properties as a selective or semiselective coating, but is much better than bare or partially bare metal. Use high-heat stove, barbecue, or engine paint to refurbish absorber coatings. Roughing the absorber surface with sand cloth from a soldering kit can help the new paint adhere to the metal.
High collector glass temperature may be due to failed solder joints or a low flow rate through the collector, which could be caused by several things. Some collectors used a lower-temperature solder to bond the copper tubes to the absorber plate. After years of enduring repeated heating and cooling cycles, these joints can separate, reducing efficiency significantly. A collector with a debonded absorber will be very hot, and the temperature difference between the supply and return lines will be small. Much of the heat in a collector with a debonded absorber will be reradiated through the glass and can be detected with an infrared thermometer. Absorbers that have debonded need to be replaced if you can find a manufacturer that will supply them—ask if they make custom absorber plates. If a replacement absorber can’t be found, the entire collector will need to be replaced.
Unlike a debonded absorber, low flow rates will show as a high temperature difference between supply and return piping. Ideally, a collector loop will have a difference between 10°F and 30°F from inlet to outlet. Differences greater than 50°F marginally affect efficiency, but differences above 70°F indicate that the system is operating at a significantly lower efficiency, since heat loss through the glass is increased.
Lower flow rates could be caused by installation design flaws, an undersized pump, a partial pump failure, or a restriction in the piping, heat exchanger, or collector tubes. Partial pump failures can be due to hidden impeller damage. In hard water areas, reduced flow rates may be caused by scale buildup from dissolved minerals in open-loop, direct forced-circulation, and thermosyphon systems. First look for installation design flaws like the tubing being too small or too many collectors for the size of tubing. The pump problems (in particular, impeller damage) may require pump removal and inspection.