New York Codes, Rules and Regulations
Title 12 - DEPARTMENT OF LABOR
Chapter I - Industrial Board Of Appeals
Subchapter A - The Industrial Code
Part 4 - Low Pressure Boilers
Subpart 4-5 - Specific Requirements For Low Pressure Heating System Boilers
Section 4-5.8 - Stop valves and check valves

If a boiler may be closed off from the heat distributing system by closing a steam stop valve, there shall be a check valve in the condensate return line between the boiler and the system; and if any part of a heat distributing system may be closed off from the remainder of the system by closing a steam stop valve, there shall be a check valve in the condensate return line from that part of the system. Such check valve shall be placed to prevent boiler contents from entering the distributing system. In lieu of a check valve in the condensate return line, a proper return loop may be provided.