I recall a few posts ago some asking the question about how to descale their hot water pressure washer. While I was tweaking mine for duty tomorrow I read the manual, and it said to put the input hose to the pump in a 5 gallon pail of water with descaler solution (higher than the pump), without the sprayer wand connected, but put the discharge hose back in the pail, and cycle the mix for awhile.

Sounds like a good plan.