Jack, The boiler laws for Washington State are here:
http://www.lni.wa.gov/TradesLicensing/Rules/files/boilers/BoilerLawBook.pdfThe section of interest is "70.79.080 Exemptions from chapter."
Your boiler meets the definition of a miniature hobby boiler, but this wording says that it has to be certified by an inspector to be exempt from the chapter.
"(14)(a)(i) Miniature hobby boilers that have been certified by an inspector as of June 7, 2018; or
(ii) Miniature hobby boilers that have not been certified
by an inspector as of June 7, 2018, but that receive certification from the chief inspector prior to being placed in service"
I am not a legal expert I am an engineer, but that is how I read it.
Dan