prEN 257

This document specifies the safety, construction and performance requirements for mechanical thermostats intended for use with gas appliances and similar use, hereafter referred to as ‘thermostats’. This document applies to thermostats with declared maximum inlet pressures up to and including 50 kPa (500 mbar) of nominal connection sizes up to and including DN 50 for use with one or more fuel gases in accordance with EN 437. This document applies to thermostats controlling the gas flow directly or indirectly through an integral gas valve. This document only applies to thermostats used with gas appliances which are not installed in the open air. Thermostats dealt with in this document are intended for control functions. This document is not applicable to: a) the thermocouple; b) controls which use auxiliary energy (e.g. electrical energy supplied externally); c) an assessment of the control regarding Performance Level (PL) and Safety Integrity Level (SIL).