The wet-rotor circulator is also used in small- to medium-size residential and light-commercial hydronic systems (see Figure 10-14). This is a small, close-coupled pump with an integral 40–125-watt motor. It is a sealed pump that does not require lubrication. The wet-rotor pump combines the motor, the shaft, and the impeller in a single assembly housed in a chamber filled with system fluid. In other words, it is both cooled and lubricated by the system fluid, hence the name wet-rotor pump. These circulators do not have a mechanical seal, as is the case with booster pumps. Consequently, seal replacement is not a problem.
Wet-rotor circulators are small, require no maintenance, and provide a long service life before they have to be replaced. A principal disadvantage using the wet-rotor pump is that it cannot be serviced or repaired while connected to the piping; the system must be drained before it can be removed. They are sometimes called throw-away pumps, because it is less expensive to replace them than it is to repair them.
The problem of having to drain the system in order to remove the pump can be avoided if shutoff (check) valves are installed in the return line on either side of the unit.