(1) Boilers and unfired pressure vessels owned or operated by the Federal Government.

(2) Boilers and unfired pressure vessels regulated under the Atomic Energy Act of 1954 (68 Stat. 921, 42 U.S.C. § 2011 et seq.).

(3) Pressure vessels used for the transportation of compressed gases if constructed and operated in compliance with specifications and regulations of the United States Department of Transportation. This paragraph includes the stationary use of these vessels.

(4) Air tanks located on vehicles operating under the rules of other Commonwealth agencies and used for carrying passengers or freight.

(5) Air tanks installed on the right-of-way of railroads and used directly in the operation of switcher and signals and under Federal or other Commonwealth agency jurisdiction.

(6) Boilers located in single-family dwellings, multiunit dwellings with four or fewer units.

(7) Boilers located in farms, except in sales areas which are accessible to the public.

(8) Storage water heaters and instantaneous water heaters if all of the following limitations are not exceeded:

(9) Unfired pressure vessels having an internal or external operating pressure of no more than 15 psi with no limitation on size when equipped with approved safety devices.

(10) Unfired pressure vessels with a nominal water-containing capacity of up to 120 gallons containing liquid under pressure and not exceeding 100 psi at ambient temperature. These vessels include unfired pressure vessels that contain air, which is trapped in the system and where the compression air serves only as a cushion.

(11) Filters and softeners with a nominal water-containing capacity of 120 gallons or less and pressures not exceeding 100 psi at ambient temperature.

(12) Air conditioner heat exchangers or chillers and refrigeration unit heat exchangers with a design pressure not more than 300 psi and a water temperature not more than 210°F.

(13) Any other type of boiler or unfired pressure vessel that the department may identify by regulation.