Wash. Rev. Code § 70.79.095
A county, city, or other political subdivision of the state may not enforce any law specifically regulating the manufacture, installation, operation, maintenance, or inspection of any electric boiler exempt from this chapter by RCW 70.79.080(10).
RCW 70.79.095
Finding-Intent-1994 c 64: "The legislature finds that small low-pressure boilers are found in devices such as espresso coffee machines and cleaning equipment common throughout Washington state. Such systems present little threat to public health and safety. Government regulation of such systems could impose a substantial burden on many small businesses and provide minimal public benefit. It is therefore the intent of the legislature to exempt these boilers from regulation under chapter 70.79 RCW and similar laws adopted by local governments." [ 1994 c 64 s 1.]