These low pressure gas-fired hot water boilers are design certified by CSA International, for use with natural and propane gases. Boilers are constructed and hydro statically tested for maximum working pressure of 50 psi (pounds per square inch) in accordance with A.S.M.E. (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code, Section IV Standards for heating boilers.Boilers are certified in accordance with ANSI (American National Standards Institute) Z21.13 standards as gas-fired, direct vent, condensing, hot water boilers.Heating Capacity indicates amount of heat available after subtracting losses up the stack. Most of this heat is available to heat water. Small portion is heat loss from jacket and surfaces of boiler, it is assumed this heat stays in the structure. Net AHRI Rating represents portion of remaining heat applied to heat radiation or terminal units (i.e. Finned tube baseboard, cast iron radiators, radiant floor, etc.). Difference between Heating Capacity and Net AHRI Rating, called piping and pickup allowance, establishes reserve for heating volume of water in the system and offsetting heat losses from piping. Net AHRI Ratings shown are based on piping and pickup factor of 1.15. Net AHRI Rating of boiler selected should be greater than or equal to calculated peak heating load (heat loss) for building or area(s) served by boiler and associated hot water heating systems. Manufacturer should be consulted before selecting boiler for installations having unusual piping and pickup requirements. Utica is a subsidiary of ECR International Inc.