When using a pressure washer, that is when the trigger is pulled and the pressure washer is spraying water, fresh water is constantly flowing through the pump. This inlet water is cool: typically 60 degrees. This keeps the pump cool.
When you stop spraying, but the engine is still running, the water is re-circulated within the pump; but this re-circulating water is under pressure. This is called by-pass: because the water is by-passing the outlet and re-circulating within the pump head. The high pressure in the pump head makes the water hotter and hotter. Typically after a few minutes the temperature in the pump head has climbed to over 145 degrees. It will keep climbing until you pull the trigger again and fresh inlet water is allowed to come into the pump head, thus cooling the pump.
For this reason you should not let your pressure washer idle, not spraying water for more than a few minutes. Without cooling the pump head the temperature will begin to climb to 200+ degrees. When it gets that hot, water seals and o-rings soften and wear quickly. Valves can be distorted and warp. Cavitation forms in the low pressure side of the pump destroying components. When you do pull the trigger and start spraying water again, the influx of cool inlet water may cause the ceramic plungers to crack through thermal shock. Let’s put it this way: nothing good comes out of leaving a pressure washer too long in by-pass.
You can help prevent damage from leaving your pressure washer too long in by-pass by making sure that your pressure washer has a thermal relief valve installed. This valve opens up when the internal temperature of the pump head reaches 145 degrees. When the valve opens up, spilling water out of the pump, new fresh water can enter the pump inlet and cool the pump down. Just make sure that if you have a thermal relief valve that it resets properly after it opens up and that you don’t have a leak that will cause other problems.
Some pumps are designed to avoid this problem. The Kranzle pump uses stainless steel plungers with a ceramic coating instead of all ceramic plungers. The steel acts as a conductor of the heat back to the pump transmission oil bath to prevent the build up of excessive heat in the pump head. These style pumps can run in by-pass far longer than other types of pumps without significant damage.
But the best way to prevent damage from excessive by-pass is to not to let your pressure washer stay in by-pass for more that a few minutes. If you are not going to be spraying, shut off your pressure washer.