In a previous article we talked about parts of a pressure washer, in this article we talk about specifics, namely pressure washer unloader valves.
Pressure washer unloader valves are the main “safety” in your power washer system. The most commonly used is the “pressure actuated” type valve. When the pressure gun trigger is released the valve senses the pressure build up and like a “traffic cop” it diverts the water flow from the pump outlet back into the inlet side of the pump this is called “bypass mode.” Without this critical component this closed-gun situation would build pressure to the point where it would stall the power source, rupture the gun, hose or even the pump itself.
The most basic function of the unloader is to act as a “traffic cop” to the water flow in your system. Depending on the type of valve it will either sense “pressure build” or “reduction in flow”. Depending on the type, when one of these conditions is met the unloader will then actuate to divert the water flow back to the inlet side of the pressure pump, where it will circulate through the pump back to the inlet side of the valve to the inlet side of the pump again in what is called a bypass loop when this is occurring the pump is said to be in “bypass mode” and the unloader valve is said to be “cycling”. This scenario ends when the trigger is squeezed alerting the unloader valve to redirect the flow to the high pressure nozzle once again. While this may seem to be problem solved it is not without risks. When the water flow is in bypass new cool water is not entering the system. The moving parts in the pump are creating friction which produces heat that is transferred to the water flow in bypass. Since a limited amount of water is in bypass this transfer of heat can occur quickly.
Most pumps are designed to handle water temperatures of 140 F. When the water in bypass reaches 155 deg Fahrenheit damage to the pump will begin. Damage can occur to the pump packings, plungers and seals and even to the short bypass hose in external bypass setups. It is a good idea to have a pump with a thermal relief valve to offer some protection against excessive heat buildup. It is a better idea not to leave a pump in bypass mode for more than 2-3 minutes, by simply squeezing the trigger gun you will introduce new cool water into the system.
Please see the next article for details of types of pressure washer unloader valves.