34 Pa. Code § 3a.145 – Steam stop valves.

(a) A boiler equipped with a steam stop valve must contain a check valve in the condensate return line between the boiler and the system.
(b) A heating system equipped with a steam stop valve must have a check valve in the condensate return pipe from the part of the system equipped with the steam stop valve.

This section cited in 34 Pa. Code § 3a.3 (relating to scope).