SHW circulation pumps have no suction lift and must be primed (be below the source water) at all times to function. They normally have life spans of 10 to 20 years and are made to withstand fluid temperatures up to 240°F.
Alternating current (AC) solar pumps are lubricated only by the circulating fluid. Many direct current (DC) pumps powered by PV modules and a few AC pumps use a magnetic drive. The magnet is an interface between the motor and the spinning impeller. These also usually lack provisions for periodic lubrication.
Always replace pumps with ones made of the same materials and close to the same head and flow specifications. Replacement pumps don’t need to be from the same manufacturer.
Pump stations are factory-assembled with most of the components needed for an antifreeze system and save installation time. A caution with the stations is that some have pumps with a check valve in the pump impeller housing. Pumps used in a drainback system cannot include a check valve, so be sure that you use the correct one for your system.
Bearing failure. The most common problem with pumps is bearing failure, which becomes more likely to occur if the pump is installed incorrectly. The shaft of the pump should be oriented horizontally. A pump with the impeller housing on the top or bottom has the shaft in a vertical orientation, which can increase lateral pressure on the bearings.
A continuous screeching noise is a sign of bearings that are about to fail. If the pump impeller is locked up (not movable by hand), the bearings are frozen and the pump needs to be replaced. Pumps mounted with the shaft vertical can also have air trapped in the housing, stopping the circulation.
Damaged motor windings. Although less common, motor windings can also fail. If the impeller spins and the capacitor checks out OK, the windings are probably the problem. Winding failure is usually accompanied by a strong burned smell, so you should test this only after you’ve ruled out bearing failure. AC-powered pumps often use a capacitor to start the pump. The housing on AC pumps where the wire or conduit enters the pump contains the capacitor. A capacitor must be removed to be tested, and the wire connections should be marked to put it back together correctly. A multimeter on a low resistance measuring scale is used to charge and discharge the capacitor by alternating the meter leads on the two capacitor wires. If the meter exhibits a burst of resistance when charged and then falls back to open (discharged), the capacitor is good. If the capacitor doesn’t cycle through this simple charge/discharge test, replace it. A new capacitor costs less than $20; a new bronze high-head pump can cost a few hundred, so it is worth the time to rule out the capacitor first.
Chuck Marken is a Home Power contributing editor, and a licensed electrician, plumber/gasfitter, and HVAC contractor. He has been installing, repairing, and servicing SHW and pool systems since 1979. He has taught SHW classes and workshops throughout the United States for Sandia National Laboratories, Solar Energy International, and many other schools and nonprofit organizations.