Pressure washers have positive displacement pumps which must continue to pump water as long as they are operating. When the trigger gun is released, the water no longer discharges from the nozzle and must be redirected elsewhere. Most pressure washer pumps send this water back to the suction side of the pump where it continues to recirculate until the trigger is depressed and water once again discharges from the nozzle. This recirculation produces heat and excessive heat will damage the pump.

A pump will either have a relief valve or unloader valve. A relief valve bypasses water at full pressure which will cause heat to build up rapidly during bypass. An unloader valve reduces the pressure on the pump while the water is recirculating, which not only reduces heat buildup but reduces the demand on the motor, thus saving energy.

Automatic shutdown systems are often used and will normally be a pressure/flow activated switch to immediately shut the motor off when the trigger is released or have a time delay for motor shutdown. These devices protect the pump from excessive heat build-up and also save energy.