I don't know codes for boilers, but some I have been around have disconnect within sight of equipment - typical NEC requirement for most anything, and usually have E-stop switches outside entrances to the boiler room - some I have added because they were not there and that is what was wanted. The idea is if the thing blows open you don't have to go into the room to kill power to the boiler. The E-stops I have wired up just kill control voltage and not main power. I'm talking industrial steam boiler here and not a domestic heating boiler.
I think a shunt trip breaker on main power would be better yet then just interrupting control power - but that is what was wanted and apparently acceptable.