I have never subjected a wall mounted hot water boiler to pressure testing I believe your second coment is correct Cheers Alan

RB, They are only ever at atmospheric pressure as they either have a not too well fitting lid or a vent/overfill line so PS regs do not apply. Pressurising them would result in more heat energy being required to raise the temp and when you try to draw off a nice cuppa the hot water at more than 100C will flash off due to the sudden drop in pressure both as it leaves the tap and inside the vessel resulting in v. hot water splashing around, Hope this helps (and trying to explain without sounding like I'm lecturing, just being helpful) Cheers, Jim