EN 257:2022

SLOVENSKI STANDARD
01-december-2022
Nadomešča:
SIST EN 257:2010
Mehanski termostati za plinske aparate
Mechanical thermostats for gas-burning appliances
Mechanische Temperaturregler für Gasgeräte
Thermostats mécaniques pour appareils à gaz
Ta slovenski standard je istoveten z: EN 257:2022
ICS:
17.200.20 Instrumenti za merjenje Temperature-measuring
temperature instruments
27.060.20 Plinski gorilniki Gas fuel burners
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 257
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2022
EUROPÄISCHE NORM
ICS 27.060.20 Supersedes EN 257:2010
English Version
Mechanical thermostats for gas-burning appliances
Thermostats mécaniques pour appareils à gaz Mechanische Temperaturregler für Gasgeräte
This European Standard was approved by CEN on 8 August 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 257:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Classification . 9
4.1 Classes of control . 9
4.2 Groups of control. 9
4.3 Classes of control functions . 9
4.4 Types of DC supplied controls . 10
5 Test conditions and uncertainty of measurements . 10
6 Design and construction . 10
6.1 General . 10
6.2 Mechanical parts of the control . 10
6.3 Materials. 11
6.4 Gas connections . 11
6.5 Electrical parts of the control . 11
6.6 Protection against internal faults for the purpose of functional safety . 11
6.101 Flow characteristics . 11
6.102 Temperature adjustment . 11
7 Performance . 12
7.1 General . 12
7.2 Leak-tightness . 13
7.3 Torsion and bending . 13
7.4 Rated flow rate . 13
7.5 Durability . 14
7.6 Performance tests for electronic controls . 14
7.7 Long-term performance for electronic controls . 14
7.8 Data exchange . 14
7.101 Calibration temperature set-point . 14
7.102 Backlash . 14
7.103 Opening of a snap-acting thermostat with a closed position . 15
7.104 Opening pressure and closing pressure for thermostats with a closed position . 15
7.105 Operating characteristic of the thermostat . 15
7.106 Ambient temperature range of the body . 19
7.107 Effect of storage and transport temperatures . 19
7.108 Thermal overload of the temperature sensor . 20
7.109 Operating torque of the thermostat set-point adjuster . 20
7.110 Endurance . 20
8 Electrical requirements . 22
9 Electromagnetic compatibility (EMC) . 22
10 Marking, instructions . 22
10.1 Marking . 22
10.2 Instructions. 22
10.3 Warning notice . 23
Annex A (informative) Abbreviations and Symbols . 24
Annex B (informative) Leak-tightness tests for gas controls – volumetric method . 25
Annex C (informative) Leak-tightness tests for gas controls – pressure loss method . 26
Annex D (normative) Conversion of pressure loss into leakage rate . 27
Annex E (normative) Electrical/electronic component fault modes . 28
Annex F (normative) Additional requirements for safety accessories and pressure
accessories as defined in EU Directive 2014/68/EU . 29
Annex G (normative) Materials for pressurized parts . 30
Annex H (normative) Additional materials for pressurized parts . 31
Annex I (normative) Requirements for controls used in DC supplied burners and
appliances burning gaseous or liquid fuels . 32
Annex J (normative) Method for the determination of the Safety Integrity Level (SIL) . 33
Annex K (normative) Method for the determination of a Performance Level (PL) . 34
Annex L (informative) Relationship between Safety Integrity Level (SIL) and
Performance Level (PL) . 35
Annex M (normative) Reset functions . 36
Annex N (informative) Guidance document on Environmental Aspects . 37
Annex O (normative) Seals of elastomers, cork and synthetic fibre mixtures . 38
Annex ZA (informative) Relationship between this European Standard and the essential
requirements of Regulation (EU) 2016/426 aimed to be covered . 39
Bibliography . 42

European foreword
This document (EN 257:2022) has been prepared by Technical Committee CEN/TC 58 “Safety and
control devices for burners and appliances burning gaseous or liquid fuels”, the secretariat of which is
held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2023, and conflicting national standards shall be
withdrawn at the latest by October 2025.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 257:2010.
The following significant changes compared to the previous edition have been incorporated in this
document:
a) alignment with EN 13611:2019;
b) requirements from EU Directive 2014/68/EU were not adopted;
c) updating of Clause 2, normative references;
d) reference to EN 437 removed;
e) information on life time for safe function (designed lifetime) added to instructions.
This document has been prepared under a Standardization Request given to CEN by the European
Commission and the European Free Trade Association, and supports essential requirements of
EU Directive(s) / Regulation(s).
For relationship with EU Directive(s) / Regulation(s), see informative Annex ZA, which is an integral part
of this document.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United
Kingdom.
Introduction
This document is intended to be used in conjunction with EN 13611:2019.
EN 13611:2019 recognizes the safety level specified by CEN/TC 58 and is regarded as a horizontal
standard dealing with the safety, construction, performance and testing of controls for burners and
appliances burning gaseous and/or liquid fuels.
The general requirements for controls are given in EN 13611:2019, and methods for classification and
assessment for new controls and control functions are given in EN 14459:2021 (see Figure 1).
EN 126:2012 (see Figure 1) specifies multifunctional controls combining two or more controls and
Application Control Functions, one of which is a mechanical control function. The requirements for
controls and Application Control Functions are given in the specific control standard (see Figure 1,
control functions).
Figure 1 — Interrelation of control standards
EN 13611:2019 should be used in conjunction with the specific standard for a specific type of control (e.g.
EN 88-1:2022, EN 88-2:2022, EN 88-3:2022, EN 125:2022, EN 126:2012, EN 161:2022, EN 257:2022,
EN 298:2022, EN 1106:2022, EN 1643:2022, EN 1854:— , EN 12067-2:2022, EN 16304:2022,
EN 16340:2014, EN 16678:2022 and EN 16898:2022), or for controls for specific applications.
EN 13611:2019 can also be applied, so far as reasonable, to controls not mentioned in a specific standard
and to controls designed on new principles, in which case additional requirements can be necessary.
EN 14459:2021 provides methods for classification and assessment of new control principles.
Primarily in industrial applications it is common practice to rate the safety of a plant based on values
describing the likelihood of a dangerous failure. These values are being used to determine Safety Integrity
Levels or Performance Levels when the system is being assessed in its entirety.
CEN/TC 58 standards for safety relevant controls do go beyond this approach, because for a certain life
time for which the product is specified, designed and tested a dangerous failure is not allowed at all.
Failure modes are described and assessed in greater detail.

Under preparation. Stage at the time of publication: FprEN 1854:2022.
Measures to prevent from dangerous situations are defined. Field experience over many decades is
reflected in the CEN/TC 58 standards. Requirements of EN 13611:2019 can be considered as proven in
practice.
This document refers to clauses of EN 13611:2019 or adapts clauses by stating “with the following
modification”, “with the following addition”, “is replaced by the following” or “is not applicable” in the
corresponding clause.
This document adds clauses or subclauses to the structure of EN 13611:2019 which are particular to this
document. Subclauses which are additional to those in EN 13611:2019 are numbered starting from 101.
Additional Annexes are designated as Annex AA, Annex BB, Annex CC, etc. It should be noted that these
clauses, subclauses and Annexes are not indicated as an addition.
If by reference to EN 13611:2019 the term “control” is given, this term should be read as “thermostat”.
1 Scope
EN 13611:2019, Clause 1 applies with the following modification and addition:
Modification:
st
The 1 paragraph of EN 13611:2019, Clause 1 is replaced by:
This document specifies the safety, design, construction, and performance requirements and testing for
mechanical thermostats intended for use with gas appliances and similar use, hereafter referred to as
“thermostats”.
This document is applicable to thermostats with declared maximum inlet pressures up to and including
50 kPa and of nominal connection sizes up to and including DN 50 for use with one or more fuel gases.
Addition:
This document is applicable to thermostats controlling the gas flow directly or indirectly through an
integral gas valve. This document 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) controls which use auxiliary energy (e.g. electrical energy supplied externally);
b) an assessment of the control regarding Performance Level (PL) and Safety Integrity Level (SIL).
th
The 4 paragraph of EN 13611:2019, Clause 1 is removed.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 13611:2019 , Safety and control devices for burners and appliances burning gaseous and/or liquid fuels
— General requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 13611:2019 and the following
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.101
mechanical thermostat
thermostat which controls the temperature by adjusting the flow rate accordingly to the temperature of
the sensor without any external energy, such that the temperature remains within defined limits

As impacted by EN 13611:2019/AC:2021.
3.102
adjustable thermostat
mechanical thermostat in which the temperature set-point can be adjusted by the user to anywhere
between minimum and maximum values
3.103
fixed setting thermostat
mechanical thermostat that has a preset fixed operating temperature which cannot be adjusted by the
user
3.104
snap-acting thermostat
mechanical thermostat with only two positions for the flow rate, i.e. “full on-off”, “full on-reduced rate”
or “reduced rate-off”
3.105
modulating thermostat
mechanical thermostat which controls the flow rate in accordance with a predetermined and continuous
function of the temperature of the temperature sensor
3.106
modulating thermostat with additional on-off action
mechanical thermostat which acts as a snap-acting thermostat between the closed and reduced positions
and as a modulating thermostat between the reduced and full-on positions
3.107
thermostat closure member
movable part of the thermostat which opens and closes the gas way and/or varies the flow rate
3.108
presetting device
device for adjusting an operating condition only by an authorized person
Note 1 to entry: It may be fixed or variable, e.g. when it is the gas flow that is adjustable; either an orifice or an
adjusting screw may be used.
3.109
fixed bypass
non-adjustable presetting device for fixing the minimum gas flow through a thermostat
3.110
bypass adjusting device
screw adjustment or an exchangeable orifice, that fixes the minimum gas flow rate through the
thermostat, and which is accessible only by the use of tools
3.111
temperature sensor
device which senses the temperature of the medium to be controlled or to be supervised
3.112
operating curve
graphical representation of the flow rate as a function of the sensor temperature at a given temperature
set-point and at a constant inlet pressure
3.113
backlash
difference of position of the adjusting knob when it is moved in both directions to obtain the same flow
rate at a constant sensor temperature
3.114
adjusting knob
adjustable spindle
part of the thermostat which is used to select the temperature set-point
3.115
temperature set-point
any value selected within the temperature range at which the controlled temperature should be
maintained
3.116
temperature set-point range
range between the minimum and maximum adjustable temperature set-points
Note 1 to entry: The temperature can be adjusted between the set-points by means of the adjusting knob.
3.117
calibration flow rate
flow rate stated in the instructions for calibration
3.118
calibration temperature set-point
temperature at which the calibration flow rate should be obtained with the adjustment set to the position
and in the direction stated in the instructions
3.119
temperature differential for snap-acting thermostats
difference in temperature necessary to obtain a change in the flow rate, at a given set-point
3.120
deviation
maximum deviation from the temperature set-point which is stated in the instructions
3.121
drift
permanent change in the operating curve of the thermostat
4 Classification
4.1 Classes of control
EN 13611:2019, 4.1 is not applicable.
4.2 Groups of control
Shall be according to EN 13611:2019, 4.2.
4.3 Classes of control functions
EN 13611:2019, 4.3 is not applicable.
4.4 Types of DC supplied controls
EN 13611:2019, 4.4 is not applicable.
5 Test conditions and uncertainty of measurements
Shall be according to EN 13611:2019, Clause 5.
6 Design and construction
6.1 General
Shall be according to EN 13611:2019, 6.1 with the following modification:
rd
The 3 paragraph of EN 13611:2019, 6.1 is not applicable.
6.2 Mechanical parts of the control
6.2.1 Appearance
Shall be according to EN 13611:2019, 6.2.1.
6.2.2 Holes
Shall be according to EN 13611:2019, 6.2.2.
6.2.3 Breather holes
Shall be according to EN 13611:2019, 6.2.3.
6.2.4 Screwed fastenings
Shall be according to EN 13611:2019, 6.2.4.
6.2.5 Jointing
Shall be according to EN 13611:2019, 6.2.5.
6.2.6 Moving parts
Shall be according to EN 13611:2019, 6.2.6.
6.2.7 Sealing caps
Shall be according to EN 13611:2019, 6.2.7.
6.2.8 Dismantling and reassembly
Shall be according to EN 13611:2019, 6.2.8 with the following addition:
If, in accordance with the instructions the thermostat can be dismantled for servicing, such action shall
not result in a change in temperature calibration exceeding the stated maximum set-point deviation
(see 7.101.1).
6.2.9 Auxiliary canals and orifices
Shall be according to EN 13611:2019, 6.2.9.
6.2.10 Presetting device
Shall be according to EN 13611:2019, 6.2.10.
6.3 Materials
Shall be according to EN 13611:2019, 6.3.
6.4 Gas connections
Shall be according to EN 13611:2019, 6.4.
6.5 Electrical parts of the control
EN 13611:2019, 6.5 is not applicable.
6.6 Protection against internal faults for the purpose of functional safety
EN 13611:2019, 6.6 is not applicable.
6.101 Flow characteristics
An adjustable bypass shall be set by means of a variable presetting device or shall be adjusted by means
of a fixed presetting device.
It shall be possible to gain access to any fixed bypass or bypass adjusting device for cleaning without
changing the calibration temperature set-point.
The opening and closing of the thermostat closure member with a total shut-off function shall happen by
snap-action between the off position and the reduced flow position.
NOTE Figure 3 shows typical operating curves of a modulating, snap-acting and modulating on-off thermostat.
The flow rate at the moment of snap-action shall not be less than the value as stated in the instructions.
6.102 Temperature adjustment
6.102.1 Range adjustment
The allowed temperature setting range shall be limited by stops. Where applicable, the instructions shall
state the limits in which the temperature setting range may be adjusted using appropriate tools. The
temperature setting range stops shall not change on their own accord.
6.102.2 Set-point adjustment
If the adjusting knob is supplied as part of the thermostat, the marking of its positions shall be easily
recognizable. It shall indicate the direction in which the temperature is raised or lowered. If numbers are
used, higher numbers shall indicate higher temperatures, except for thermostats for refrigerators where
higher numbers shall indicate lower temperatures.
It shall be possible to select any temperature set-point over the whole temperature range by setting the
adjusting knob or spindle between the stops within the maximum and minimum ambient temperatures
as stated in the instructions.
The temperature setting means shall not change on its own accord.
6.102.3 Fixed setting thermostat
If provided, the adjuster of a fixed setting thermostat shall be sealed (e.g. lacquer).
7 Performance
7.1 General
Shall be according to EN 13611:2019, 7.1
...