ISO 23551-9:2015 specifies safety, constructional, and performance requirements and testing of mechanical gas thermostat for gas burners, gas appliances, and similar use hereafter referred to as "thermostats". It applies to mechanical gas thermostats of nominal connection sizes up to and with declared maximum inlet pressures up to and including 50 KPa for burners and gas-burning appliances using fuel gases as natural gas, manufactured gas, and liquefied petroleum gas (LPG). The standard applies to mechanical thermostats controlling the gas flow directly or indirectly through an integral gas valve and which do not require external electrical energy for their operation. It only applies to mechanical thermostats used on gas appliances where the thermostat is not directly exposed to the outdoor environment. It only applies to mechanical thermostats which are intended for operating control functions.