I have had several customers complain to me about their operators wiring the guns open on the wash rack and leaving them running and what the dangers are. There are a couple of reasons that you should not leave the gun on when not using it, wired or not. They are as follows:
Gun control is for the protection of the user. Having a trigger locked in open position with 3,000 psi is incredibly dangerous. Back in the day when I was a contract cleaner using this equipment I had a layer of skin ripped off me with the spray from not being careful and almost lost a toe with a working gun. Pressure washer operators not only need steel toed boots, protective clothing and eye wear but need to have the equipment operating in its intended form. All equipment we sell is certified to UL1776 Specs. If you modify that by wiring the gun open, using a non certified hose without the proper length of hose guard on it, etc. you are opening yourself up to a lawsuit should the operator get hurt.
Maintenance & Wear & Tear
You are going to not only be wasting water you will be having the machine take on unnecessary stress. When the gun is on the off position the pump goes into bypass and the heater shuts off therefore saving fuel as well as wear on the burner motor. The best thing to do if you are going to stop washing for more than 3-4 minutes is to cool the machine down by turning off the burner first, then turning off the machine when the wand is lukewarm to the touch. This prevents thermal expansion in the coils, which are one of the most expensive components on the machine and reduces wear on the unloader, etc. The bottom line is that modifying equipment puts you as an Owner, Manager or Supervisor in a compromised position. You need to make sure that all operators are using as specified with every piece of equipment for yours and their safety.