SN EN 257 : 2010

Cooperating organizations
National foreword
Foreword
1. Scope
2. Normative references
3. Definitions
3.1 Mechanical thermostat
3.2 Types of mechanical thermostat
3.3 Closure member
3.4 Breather hole
3.5 Presetting device
3.6 Fixed bypass
3.7 Bypass adjusting device
3.8 Temperature sensor
3.9 Leak-tightness
3.10 Pressures
3.11 Flow rate
3.12 Rated flow rate
3.13 Operating curve
3.14 Backlash
3.15 Maximum cycling frequency
3.16 Mounting position(s)
3.17 Adjusting knob
3.18 Temperature
3.19 Deviation
3.20 Drift
4. Group of thermostat
5. Construction requirements
5.1 General construction requirements
5.2 Materials
5.2.1 General material requirements
5.2.2 Zinc alloys
5.2.3 Housing
5.2.4 Resistance to corrosion
5.2.5 Impregnation
5.3 Connections
5.3.1 General
5.3.2 Threads
5.3.3 Flanges
5.3.4 Compression fittings
5.4 Seals for glands for moving parts
5.5 Pressure test nipples
5.6 Electrical equipment
5.7 Flow characteristics
5.8 Temperature adjustment
5.8.1 Range adjustment
5.8.2 Set-point adjustment
5.8.3 Fixed setting thermostat
6. Performance requirements
6.1 Mounting position
6.2 Leak-tightness
6.3 Calibration temperature set-point
6.4 Backlash
6.5 Opening of a snap-acting thermostat with a closed
       position
6.6 Opening pressure and closing pressure for thermo-
       stats with a closed position
6.7 Rated flow rate and bypass flow rate
6.8 Operating characteristics of the thermostat
6.9 Temperature
6.9.1 Ambient temperature range of the body
6.9.2 Effect of storage and transit temperatures
6.9.3 Thermal overload of the temperature sensor
6.10 Operating torque of the adjusting knob (or spindle)
6.11 Torsion and bending
6.11.1 General
6.11.2 Torsion - group 1 and group 2 thermostats with
       threaded connections
6.11.3 Torsion - group 1 and group 2 thermostats with
       compression fittings
6.11.4 Bending - group 1 and gorup 2 thermostats
6.11.5 Thermostats with inlet and outlet connections not
       of the same nominal size
6.12 Durability
6.12.1 Elastomers
6.12.2 Marking
6.12.3 Performance
6.12.4 Resistance to scratching
6.12.5 Resistance to humidity
7. Test methods
7.1 General
7.1.1 Test samples
7.1.2 Test conditions
7.1.3 Mounting position
7.2 Leak-tightness
7.2.1 General
7.2.2 Closure parts and external leak-tightness
7.2.3 Internal leak-tightness in the closed position
7.2.4 Non-metallic housing parts
7.3 Calibration temperature set-point
7.4 Backlash
7.5 Opening of a snap-acting thermostat with a closed
       position
7.6 Opening pressure and closing pressure for thermo-
       stats with a closed position
7.7 Rated flow rate and rated bypass flow rate
7.8 Operating characteristics of thermostats
7.9 Temperature
7.9.1 Ambient temperature range of the body
7.9.2 Effect of storage and transit temperatures
7.9.3 Thermal overload of the temperature sensor
7.9.4 Check for change in calibration
7.10 Operating torque of the thermostat set-point
       adjuster
7.11 Torsion and bending
7.11.1 General
7.11.2 Ten second torsion test - group 1 and group 2
       thermostats with threaded connections
7.11.3 Ten second torsion test - group 1 and group 2
       thermostats with compression joints
7.11.4 Bending moment tests
7.11.5 Thermostats with inlet and outlet connections
       not of the same nominal size
7.12 Durability
7.12.1 Elastomers
7.12.2 Marking
7.12.3 Performance
7.12.4 Scratch test
7.12.5 Humidity test
8. Marking, installation and operating instructions
8.1 Marking on the body
8.2 Installation and operating instructions
8.3 Warning notice
Annexes
A. Leak-tightness test - volumetric method
A.1 Apparatus
A.2 Test method
B. Leak-tightness test - pressure loss method
B.1 Apparatus
B.2 Test method
C. Use of ISO 7-1: 1982 and ISO 228-1: 1982 threads
       for gas connections
D. Conversion of pressure loss into leakage rate
E. Identification of clauses which meet the
       Essential Requirements of the Gas Appliance
       Directive (90/396/EEC)
Tables
1. Connection sizes
2. Maximum permitted leakage rate
3. Torque and bending moment
4. Test sequence
5. Tightening torque for flange bolts to ISO 7005:
       1988
6. Number of mechanical cycles
Figures
1. Thermostat test apparatus
2. Typical thermostat characteristics over range of
       temperature set-point adjustments
3. Arrangement for torsion test
4. Arrangement for bending moment test
5. Paint scratch test apparatus
A.1 Apparatus for testing leak-tightness (volumetric
       method)
B.1 Apparatus for testing leak-tightness (pressure
       loss method)

Describes the construction and performance requirements for mechanical thermostats for gas applications. Establishes the definitions, test conditions and marking. 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. Applies to thermostats for all gas appliances. Excludes thermostats used with gas appliances installed in the open air.