NEVER USE A WATER SUPPLY SOURCE IN EXCESS OF 120° F
This temperature will destroy the seals and check valve in your high-pressure pump.
The supply source of water is the water coming into the inlet side of the pump on your pressure washer. This water will typically come from a faucet or a water tank. Cold or ambient water temperature is preferable. Hot water, boiler water or any water source in excess of 120° F should be avoided.
All hot water pressure washers, regardless of brand or model, pressurize the water before sending it to the burner for heating. All pressure washer pumps are designed to pump cold or slightly heated incoming water. The seals on the pump have very strict limitations to heat. Typical pump manufacturer's rating of internal pump parts are a maximum temperature of 140° Fahrenheit. Water temperature of 120° F quickly heats up to 140° F within the pump due to frictional heat. When the temperature of the water is 140° F, the temperature in the pump is near the maximum temperature the soft parts of the pump (seals, o-rings, etc) can handle without failing prematurely. When the pump goes into bypass, the temperature quickly climbs much higher. This reduces the life of the seals, o-rings and other components. Use of hot inlet water creates a need for more intensive maintenance on the pressure washer.
Some manufacturer's produce a high temperature seal set that is rated to 170° F. If you must use a water source in excess of 120° F, you should consider using high temperature seals. Check with the manufacturer to see if high temperature seals are available for your brand of pump.