All gas-fired boilers must be provided with a properly installed and operating low water cut-off.
The low water cut-off may be installed in or attached to the boiler at the level recommended by the boiler manufacturer, but in no case shall the low water cut-off be installed below the crown sheet. The low water cut-off, when not installed directly in the boiler, may be installed either in the main supply line (vertical riser) as close to the boiler as possible or in a water column of continuous piping attached directly to the boiler.
The low water cut-off must be designed and approved for the medium used (steam or water).
No valves or other obstructive devices shall be installed between the boiler and safety controls.
02-658 C.M.R. ch. 13, § 5