This part of ISO 23551 specifies the safety, construction and performance requirements for gas/air ratio controls, pneumatic type (hereafter referred to as “ratio control”), for inlet pressures up to and including 50 kPa, of a nominal connection size up to and including DN 250, intended for use with gas appliances using fuel gases as natural gas, manufactured gas or liquefied petroleum gas (LPG). It also describes the test procedures for evaluating these requirements and specifies information necessary for installation and use. This part of ISO 23551 is applicable to gas/air ratio controls for gas-burning appliances that can be tested independently of the appliance. It is applicable to gas/air ratio controls which function by controlling a gas pressure (or differential pressure) output in response to an air pressure (or differential pressure) and to a furnace back pressure signal input; but gas/air ratio controls which change the air pressure in response to the gas pressure are not excluded. This part of ISO 23551 is not applicable to mechanically linked valves and electronic systems. NOTE Sections of this part of ISO 23551 can be applied to the construction and performance of the ratio control function of multi-functional controls.