Posted By Simon Chidwick Hi George, Not sure what context you are using a 'water heater' but this may help you clarify things. If this is a domestic water heater the PSSR's do not apply. The regs only apply: to systems containing a 'relevant fluid' (water is not). to systems that have a pv of >250 bar litres unless it is steam where the regs apply at any pressure. to systems capable of causing reasonably foreseeable danger to personnel in the event of a system failure - e.g. a safety device failing and the temperature rising to over 110 C in the case of a water heater. Simon