N.Y. Comp. Codes R. & Regs. tit. 12, § 14-2.2 – Setting and insulating.

(a) Boilers which require access to permit of proper care and inspection shall have a minimum clearance of 24 inches from the building walls or partitions.
Exception:
Boilers other than horizontal tubular or watertube type or not exceeding 30 inches in diameter may be set with a minimum clearance of not less than 16 inches from building walls or partitions in the clear.
(b) Where more than one firebox or economic type of boiler are installed, a space of at least 24 inches shall be provided between the boilers for inspection and repair.
(c) Wet-bottom boilers shall be set not less than 12 inches above the floor.
(d) The setting walls of a water tube boiler shall be so constructed that all drum heads will be accessible for external inspection.
(e) Where it is necessary to go on top of boiler or boilers for maintenance or operating purposes, a clearance of at least seven feet shall be maintained from the working platform to the underside of any overhead structure.
Exception:
Where boilers are to be installed in an existing building where it is necessary to go on top, there shall be a clearance of at least three feet. There shall be not less than six inches of clearance space above the highest point of any valve or other fitting when such valve or fitting is at maximum opening.
(f) The clearance above a vertical boiler (except of the water tube type) shall be sufficient to permit of inspection and necessary repairs of the upper tube sheet.
(g) When boilers have riveted nozzles attached, the flanges of nozzles where attached shall not be covered permanently with insulation. Provision shall be made by using removable sections of insulating material to expose the flanges for inspection purposes.
(h) Where boilers are of riveted construction and the dimensions of the riveted joint cannot be determined for computation purposes on the inside of the shell, the insulating material covering the exterior of the shell shall be so attached that a sufficient amount of same may be readily removed for examination of the joint to determine the size and pitch of rivets and such other data as may be required by the inspector.

All boilers hereafter installed shall comply with the following requirements: