ISO 23551-9:2022
(Main)Safety and control devices for gas burners and gas-burning appliances — Particular requirements — Part 9: Mechanical gas thermostats
Safety and control devices for gas burners and gas-burning appliances — Particular requirements — Part 9: Mechanical gas thermostats
This document specifies safety, construction, performance and testing requirements for mechanical gas thermostat intended for use with gas burners and gas burning appliances hereafter referred to as “thermostats”. This document applies to mechanical gas thermostats of nominal connection sizes up to and including DN 50 with declared maximum inlet pressures up to and including 50 kPa, for use with natural gas, manufactured gas or liquefied petroleum gas (LPG). It is not applicable to corrosive and waste gases. This document 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; — used on gas appliances where the thermostat is not directly exposed to the outdoor environment; and — which are intended for operating control functions. This document covers type testing only.
Dispositifs de commande et de sécurité pour brûleurs à fioul et pour appareils à fioul — Exigences particulières — Partie 9: Thermostats mécaniques
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INTERNATIONAL ISO
STANDARD 23551-9
Second edition
2022-01
Safety and control devices for gas
burners and gas-burning appliances —
Particular requirements —
Part 9:
Mechanical gas thermostats
Dispositifs de commande et de sécurité pour brûleurs à fioul et pour
appareils à fioul — Exigences particulières —
Partie 9: Thermostats mécaniques
Reference number
ISO 23551-9:2022(E)
© ISO 2022
---------------------- Page: 1 ----------------------
ISO 23551-9:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
© ISO 2022 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 23551-9:2022(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Classification . 3
4.1 Classes of controls . 3
4.2 Groups of controls . 4
4.3 Types of DC supplied controls . 4
4.4 Classes of control functions . 4
5 Test conditions .4
6 Construction . 4
6.1 General . 4
6.2 Construction requirements. 4
6.2.1 Appearance . 4
6.2.2 Holes . 4
6.2.3 Breather holes . 4
6.2.4 Vent limiter . 4
6.2.5 Screwed fastenings . 4
6.2.6 Moving parts . 4
6.2.7 Sealing caps . 5
6.2.8 Dismantling and reassembling for servicing and/or adjustment for controls . 5
6.2.9 Auxiliary channels and orifices . 5
6.2.10 Pre-setting device . 5
6.3 Materials . 5
6.3.1 General material requirements . 5
6.3.2 Housing . 5
6.3.3 Springs providing closing force and sealing force . 5
6.3.4 Resistance to corrosion and surface protection . 5
6.3.5 Impregnation . 5
6.3.6 Seals for glands for moving parts. 5
6.3.7 Jointing . 5
6.4 Connections . 5
6.5 Gas controls employing with electrical components in the gas way . 6
6.6 Flow characteristics . 6
6.7 Temperature adjustment of adjustable thermostats . 6
6.7.1 Range adjustment . 6
6.7.2 Set point adjustment . 6
6.7.3 Fixed setting thermostat . 6
7 Performance . 6
7.1 General . 6
7.2 Leak-tightness . 7
7.2.1 General . 7
7.2.2 Requirements . 7
7.2.3 Test . . . 7
7.3 Torsion and bending . 7
7.4 Rated flow rate . 7
7.4.1 General . 7
7.4.2 Requirements . 7
7.4.3 Test . . . 8
7.5 Durability . 8
7.6 Functional requirements . . 8
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ISO 23551-9:2022(E)
7.6.1 Calibration temperature set-point . 8
7.6.2 Backlash . 8
7.6.3 Opening of a snap-acting thermostat with a closed position . 9
7.6.4 Opening pressure and closing pressure for thermostats with a closed
position . 9
7.6.5 Operating characteristic of the thermostat . 9
7.6.6 Ambient temperature range of the body . 14
7.6.7 Effect of storage and transport temperatures . 14
7.6.8 Thermal overload of the thermal sensing element . 14
7.6.9 Operating torque of the thermostat set-point adjuster .15
7.6.10 Effect of high ambient temperature on performance of thermostats for top
burners and griddles .15
7.7 Endurance . 15
7.7.1 Mechanical cycling . .15
7.7.2 Thermal cycling . 16
8 Electrical equipment .17
9 Electromagnetic compatibility (EMC) .17
10 Marking, installation and operating instructions .17
10.1 Marking . 17
10.2 Installation and operating instructions . 18
10.3 Warning notice . 18
Annex A (informative) Leak-tightness test — Volumetric method .19
Annex B (informative) Leak-tightness test — Pressure-loss method .20
Annex C (normative) Conversion of pressure loss into leakage rate .21
Annex D (normative) Gas quick connector (GQC) .22
Annex E (normative) Elastomers/requirements resistance to lubricants and gas .23
Annex F (normative) Specific regional requirements in European countries .24
Annex G (normative) Specific regional requirements in Canada and USA .25
Annex H (normative) Specific regional requirements in Japan .26
Bibliography .27
iv
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ISO 23551-9:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 161, Controls and protective devices for
gas and/or oil.
This second edition cancels and replaces the first edition (ISO 23551-9:2015), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the Foreword, Introduction and Scope has been made consistent with new principles and rules for
the structure and drafting of ISO and IEC documents;
— the structure and numbering of the clauses have been aligned with ISO 23550:2018;
— specific regional requirements in European countries have been moved to the main document.
A list of all parts in the ISO 23551 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
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ISO 23551-9:2022(E)
Introduction
This document is designed to be used in combination with ISO 23550. Together with ISO 23550, this
document establishes the full requirements as they apply to the product covered by this document.
Where needed, this document adapts ISO 23550 by stating in the corresponding clause:
— “with the following modification”;
— “with the following addition”;
— “is replaced by the following”; or
— “is not applicable”.
In order to identify specific requirements that are particular to this document, that are not already
covered by ISO 23550, this document can contain clauses or subclauses that are additional to the
structure of ISO 23550. These subclauses are indicated by the introductory sentence: “Subclause (or
Annex) specific to this document.”
To ensure global relevance of this document, the differing requirements resulting from practical
experience and installation practices in various regions of the world have been taken into account. The
variations in basic infrastructure associated with gas and/or oil controls and appliances have also been
recognized, some of which are addressed in Annexes F, G and H. This document intends to provide a
basic framework of requirements that recognize these differences.
vi
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INTERNATIONAL STANDARD ISO 23551-9:2022(E)
Safety and control devices for gas burners and gas-burning
appliances — Particular requirements —
Part 9:
Mechanical gas thermostats
1 Scope
This document specifies safety, construction, performance and testing requirements for mechanical
gas thermostat intended for use with gas burners and gas burning appliances hereafter referred to as
“thermostats”.
This document applies to mechanical gas thermostats of nominal connection sizes up to and including
DN 50 with declared maximum inlet pressures up to and including 50 kPa, for use with natural gas,
manufactured gas or liquefied petroleum gas (LPG). It is not applicable to corrosive and waste gases.
This document 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;
— used on gas appliances where the thermostat is not directly exposed to the outdoor environment;
and
— which are intended for operating control functions.
This document covers type testing only.
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.
ISO 23550:2018, Safety and control devices for gas and/or oil burners and appliances — General
requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 23550 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
mechanical thermostat
thermostat which controls the temperature by adjusting the flow rate accordingly to the temperature
of the thermal sensing element (3.11) without any external energy, such that the temperature remains
within defined limits
1
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ISO 23551-9:2022(E)
3.2
adjustable thermostat
mechanical thermostat (3.1) in which the temperature set-point (3.15) can be adjusted by the user to
anywhere between minimum and maximum values
3.3
fixed setting thermostat
mechanical thermostat (3.1) that has a pre-set fixed operating temperature which cannot be adjusted by
the user
3.4
snap-acting thermostat
mechanical thermostat (3.1) with only two positions for the flow rate, i.e. “full on-off”, “full on-reduced
rate” or “reduced rate-off”
3.5
modulating thermostat
mechanical thermostat (3.1) which controls the flow rate in accordance with a predetermined and
continuous function of the temperature of the thermal sensing element (3.11)
3.6
modulating thermostat
mechanical thermostat (3.1) that incorporates an on-off action and which acts as a snap-acting thermostat
(3.4) between the closed and reduced positions and as a modulating thermostat (3.5) between the
reduced and full-on positions
3.7
thermostat closure member
movable part of the thermostat which opens and closes the gas way and/or varies the flow rate
3.8
pre-setting device
device for adjusting an operating condition only by an authorized person
Note 1 to entry: It can be fixed or variable, e.g. when it is the gas flow that is adjustable, either an orifice or an
adjusting screw can be used.
3.9
fixed bypass
non-adjustable pre-setting device (3.8) for fixing the minimum gas flow through a thermostat
3.10
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
3.11
thermal sensing element
part of a thermostat which is directly acted upon by temperature changes of the medium to be controlled
or to be supervised, and which, through physical change thus produced, originates the motion directly
or indirectly controlling the action of the closure member
3.12
operating curve
graphical representation of the flow rate as a function of the thermal sensing element (3.11) temperature
at a given temperature set-point (3.15) and at a constant inlet pressure
3.13
backlash
difference of position of the adjusting knob (3.14) when it is moved in both directions to obtain the same
flow rate at a constant thermal sensing element temperature
2
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ISO 23551-9:2022(E)
3.14
adjusting knob
adjusting spindle
adjusting dial
part of the thermostat used to select the temperature set-point (3.15)
3.15
temperature set-point
any value selected within the temperature range at which the controlled temperature should be
maintained
3.16
temperature set-point range
range between the minimum and maximum adjustable temperature set-points (3.15), by means of the
adjusting knob (3.14)
3.17
calibration flow rate
flow rate declared by the manufacturer for calibration
3.18
calibration temperature set-point
temperature at which the calibration flow rate (3.17) should be obtained with the adjustment set to the
position and in the direction declared by the manufacturer
3.19
temperature differential
difference in temperature necessary to obtain a change in the flow rate, at
a given set-point
3.20
deviation
maximum deviation from the temperature set-point (3.15) declared by the manufacturer
3.21
drift
permanent change in the operating curve (3.12) of the thermostat
3.22
bypass
passage provided in which permits a flow of gas to the main burner(s) independently of the action of
the thermostatic valve
3.23
calibration reference point
dial setting at which a control is calibrated for agreement between dial indication and sensing element
temperature
3.24
maximum operating position
highest dial marking temperature for heating thermostats or the lowest dial marking temperature for
refrigeration thermostats
4 Classification
4.1 Classes of controls
ISO 23550:2018, 4.1, is not applicable.
3
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ISO 23551-9:2022(E)
4.2 Groups of controls
Shall be in accordance with ISO 23550:2018, 4.2.
4.3 Types of DC supplied controls
ISO 23550:2018, 4.3, is not applicable.
4.4 Classes of control functions
ISO 23550:2018 4.4, is not applicable.
5 Test conditions
Shall be in accordance with ISO 23550:2018, Clause 5, with the following addition:
Where applicable, the thermostat shall be mounted in an appropriate test fixture and the tests
conducted with the thermal sensing element immersed in the intended medium (e.g. water, oil or air)
for the application, as specified in the installation and operation instructions.
6 Construction
Specific regional requirements shall be as specified in Annex G.
6.1 General
Shall be in accordance with ISO 23550:2018, 6.1.
6.2 Construction requirements
6.2.1 Appearance
Shall be in accordance ISO 23550:2018, 6.2.1.
6.2.2 Holes
Shall be in accordance with ISO 23550:2018, 6.2.2.
6.2.3 Breather holes
ISO 23550:2018, 6.2.3 is not applicable.
6.2.4 Vent limiter
ISO 23550:2018, 6.2.4 is not applicable.
6.2.5 Screwed fastenings
Shall be in accordance with ISO 23550:2018, 6.2.5.
6.2.6 Moving parts
Shall be in accordance with ISO 23550:2018, 6.2.6.
4
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ISO 23551-9:2022(E)
6.2.7 Sealing caps
Shall be in accordance with ISO 23550:2018, 6.2.7.
6.2.8 Dismantling and reassembling for servicing and/or adjustment for controls
Shall be in accordance with ISO 23550:2018, 6.2.8, with the following addition.
If, in accordance with the manufacturer's instructions the thermostat can be dismantled for servicing,
such action shall not result in a change in temperature calibration exceeding the declared maximum set
point deviation (see 7.6.1.1).
Suitable means for maintaining all adjustments shall be provided. Lock nuts or adjusting nuts held by
springs or compression are considered satisfactory, except where their adjustment can be accidentally
disturbed.
6.2.9 Auxiliary channels and orifices
Shall be in accordance with ISO 23550:2018, 6.2.9.
6.2.10 Pre-setting device
Shall be in accordance with ISO 23550:2018, 6.2.10.
6.3 Materials
6.3.1 General material requirements
Shall be in accordance with ISO 23550:2018, 6.3.1.
6.3.2 Housing
Shall be in accordance with ISO 23550:2018, 6.3.2.
6.3.3 Springs providing closing force and sealing force
Shall be in accordance with ISO 23550:2018, 6.3.3.
6.3.4 Resistance to corrosion and surface protection
Shall be in accordance with ISO 23550:2018, 6.3.4.
6.3.5 Impregnation
Shall be in accordance with ISO 23550:2018, 6.3.5.
6.3.6 Seals for glands for moving parts
Shall be in accordance with ISO 23550:2018, 6.3.6.
6.3.7 Jointing
Shall be in accordance with ISO 23550:2018, 6.3.7.
6.4 Connections
Shall be in accordance with ISO 23550:2018, 6.4.
5
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ISO 23551-9:2022(E)
Further requirements for GQC shall be in accordance with Annex D.
6.5 Gas controls employing with electrical components in the gas way
ISO 23550:2018, 6.5 is not applicable.
6.6 Flow characteristics
Su
...
INTERNATIONAL ISO
STANDARD 23551-9
Second edition
Safety and control devices for gas
burners and gas-burning appliances —
Particular requirements —
Part 9:
Mechanical gas thermostats
Dispositifs de commande et de sécurité pour brûleurs à fioul et pour
appareils à fioul — Exigences particulières —
Partie 9: Thermostats mécaniques
PROOF/ÉPREUVE
Reference number
ISO 23551-9:2021(E)
© ISO 2021
---------------------- Page: 1 ----------------------
ISO 23551-9:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
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---------------------- Page: 2 ----------------------
ISO 23551-9:2021(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Classification . 3
4.1 Classes of controls . 3
4.2 Groups of controls . 4
4.3 Types of DC supplied controls . 4
4.4 Classes of control functions . 4
5 Test conditions .4
6 Construction . 4
6.1 General . 4
6.2 Construction requirements. 4
6.2.1 Appearance . 4
6.2.2 Holes . 4
6.2.3 Breather holes . 4
6.2.4 Vent limiter . 4
6.2.5 Screwed fastenings . 4
6.2.6 Moving parts . 4
6.2.7 Sealing caps . 5
6.2.8 Dismantling and reassembling for servicing and/or adjustment for controls . 5
6.2.9 Auxiliary channels and orifices . 5
6.2.10 Pre-setting device . 5
6.3 Materials . 5
6.3.1 General material requirements . 5
6.3.2 Housing . 5
6.3.3 Springs providing closing force and sealing force . 5
6.3.4 Resistance to corrosion and surface protection . 5
6.3.5 Impregnation . 5
6.3.6 Seals for glands for moving parts. 5
6.3.7 Jointing . 5
6.4 Connections . 5
6.5 Gas controls employing with electrical components in the gas way . 6
6.6 Flow characteristics . 6
6.7 Temperature adjustment of adjustable thermostats . 6
6.7.1 Range adjustment . 6
6.7.2 Set point adjustment . 6
6.7.3 Fixed setting thermostat . 6
7 Performance . 6
7.1 General . 6
7.2 Leak-tightness . 7
7.2.1 General . 7
7.2.2 Requirements . 7
7.2.3 Test . . . 7
7.3 Torsion and bending . 7
7.4 Rated flow rate . 7
7.4.1 General . 7
7.4.2 Requirements . 7
7.4.3 Test . . . 8
7.5 Durability . 8
7.6 Functional requirements . . 8
iii
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ISO 23551-9:2021(E)
7.6.1 Calibration temperature set-point . 8
7.6.2 Backlash . 8
7.6.3 Opening of a snap-acting thermostat with a closed position . 9
7.6.4 Opening pressure and closing pressure for thermostats with a closed
position . 9
7.6.5 Operating characteristic of the thermostat . 9
7.6.6 Ambient temperature range of the body . 14
7.6.7 Effect of storage and transport temperatures . 14
7.6.8 Thermal overload of the thermal sensing element . 14
7.6.9 Operating torque of the thermostat set-point adjuster .15
7.6.10 Effect of high ambient temperature on performance of thermostats for top
burners and griddles .15
7.7 Endurance . 15
7.7.1 Mechanical cycling . .15
7.7.2 Thermal cycling . 16
8 Electrical equipment .17
9 Electromagnetic compatibility (EMC) .17
10 Marking, installation and operating instructions .17
10.1 Marking . 17
10.2 Installation and operating instructions . 18
10.3 Warning notice . 18
Annex A (informative) Leak-tightness test — Volumetric method .19
Annex B (informative) Leak-tightness test — Pressure-loss method .20
Annex C (normative) Conversion of pressure loss into leakage rate .21
Annex D (normative) Gas quick connector (GQC) .22
Annex E (normative) Elastomers/requirements resistance to lubricants and gas .23
Annex F (normative) Specific regional requirements in European countries .24
Annex G (normative) Specific regional requirements in Canada and USA .25
Annex H (normative) Specific regional requirements in Japan .26
Bibliography .27
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ISO 23551-9:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 161, Control and protective devices for gas
and/or oil burners and appliances.
This second edition cancels and replaces the first edition (ISO 23551-9:2015), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the Foreword, Introduction and Scope has been made consistent with new principles and rules for
the structure and drafting of ISO and IEC documents;
— the structure and numbering of the clauses have been aligned with ISO 23550:2018;
— specific regional requirements in European countries have been moved to the main document.
A list of all parts in the ISO 23551 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
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ISO 23551-9:2021(E)
Introduction
This document is designed to be used in combination with ISO 23550. Together with ISO 23550, this
document establishes the full requirements as they apply to the product covered by this document.
Where needed, this document adapts ISO 23550 by stating in the corresponding clause:
— “with the following modification”;
— “with the following addition”;
— “is replaced by the following”; or
— “is not applicable”.
In order to identify specific requirements that are particular to this document, that are not already
covered by ISO 23550, this document can contain clauses or subclauses that are additional to the
structure of ISO 23550. These subclauses are indicated by the introductory sentence: “Subclause (or
Annex) specific to this document.”
To ensure global relevance of this document, the differing requirements resulting from practical
experience and installation practices in various regions of the world have been taken into account. The
variations in basic infrastructure associated with gas and/or oil controls and appliances have also been
recognized, some of which are addressed in Annexes F, G and H. This document intends to provide a
basic framework of requirements that recognize these differences.
vi
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---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 23551-9:2021(E)
Safety and control devices for gas burners and gas-burning
appliances — Particular requirements —
Part 9:
Mechanical gas thermostats
1 Scope
This document specifies safety, construction, performance and testing requirements for mechanical
gas thermostat intended for use with gas burners and gas burning appliances hereafter referred to as
“thermostats”.
This document applies to mechanical gas thermostats of nominal connection sizes up to and including
DN 50 with declared maximum inlet pressures up to and including 50 kPa, for use with natural gas,
manufactured gas or liquefied petroleum gas (LPG). It is not applicable to corrosive and waste gases.
This document 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;
— used on gas appliances where the thermostat is not directly exposed to the outdoor environment;
and
— which are intended for operating control functions.
This document covers type testing only.
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.
ISO 23550:2018, Safety and control devices for gas and/or oil burners and appliances — General
requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 23550 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
mechanical thermostat
thermostat which controls the temperature by adjusting the flow rate accordingly to the temperature
of the thermal sensing element (3.11) without any external energy, such that the temperature remains
within defined limits
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ISO 23551-9:2021(E)
3.2
adjustable thermostat
mechanical thermostat (3.1) in which the temperature set-point (3.15) can be adjusted by the user to
anywhere between minimum and maximum values
3.3
fixed setting thermostat
mechanical thermostat (3.1) that has a pre-set fixed operating temperature which cannot be adjusted by
the user
3.4
snap-acting thermostat
mechanical thermostat (3.1) with only two positions for the flow rate, i.e. “full on-off”, “full on-reduced
rate” or “reduced rate-off”
3.5
modulating thermostat
mechanical thermostat (3.1) which controls the flow rate in accordance with a predetermined and
continuous function of the temperature of the thermal sensing element (3.11)
3.6
modulating thermostat
mechanical thermostat (3.1) that incorporates an on-off action and which acts as a snap-acting thermostat
(3.4) between the closed and reduced positions and as a modulating thermostat (3.5) between the
reduced and full-on positions
3.7
thermostat closure member
movable part of the thermostat which opens and closes the gas way and/or varies the flow rate
3.8
pre-setting device
device for adjusting an operating condition only by an authorized person
Note 1 to entry: It can be fixed or variable, e.g. when it is the gas flow that is adjustable, either an orifice or an
adjusting screw can be used.
3.9
fixed bypass
non-adjustable pre-setting device (3.8) for fixing the minimum gas flow through a thermostat
3.10
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
3.11
thermal sensing element
part of a thermostat which is directly acted upon by temperature changes of the medium to be controlled
or to be supervised, and which, through physical change thus produced, originates the motion directly
or indirectly controlling the action of the closure member
3.12
operating curve
graphical representation of the flow rate as a function of the thermal sensing element (3.11) temperature
at a given temperature set-point (3.15) and at a constant inlet pressure
3.13
backlash
difference of position of the adjusting knob (3.14) when it is moved in both directions to obtain the same
flow rate at a constant thermal sensing element temperature
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ISO 23551-9:2021(E)
3.14
adjusting knob
adjusting spindle
adjusting dial
part of the thermostat used to select the temperature set-point (3.15)
3.15
temperature set-point
any value selected within the temperature range at which the controlled temperature should be
maintained
3.16
temperature set-point range
range between the minimum and maximum adjustable temperature set-points (3.15), by means of the
adjusting knob (3.14)
3.17
calibration flow rate
flow rate declared by the manufacturer for calibration
3.18
calibration temperature set-point
temperature at which the calibration flow rate (3.17) should be obtained with the adjustment set to the
position and in the direction declared by the manufacturer
3.19
temperature differential
difference in temperature necessary to obtain a change in the flow rate, at
a given set-point
3.20
deviation
maximum deviation from the temperature set-point (3.15) declared by the manufacturer
3.21
drift
permanent change in the operating curve (3.12) of the thermostat
3.22
bypass
passage provided in which permits a flow of gas to the main burner(s) independently of the action of
the thermostatic valve
3.23
calibration reference point
dial setting at which a control is calibrated for agreement between dial indication and sensing element
temperature
3.24
maximum operating position
highest dial marking temperature for heating thermostats or the lowest dial marking temperature for
refrigeration thermostats
4 Classification
4.1 Classes of controls
ISO 23550:2018, 4.1, is not applicable.
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ISO 23551-9:2021(E)
4.2 Groups of controls
Shall be in accordance with ISO 23550:2018, 4.2.
4.3 Types of DC supplied controls
ISO 23550:2018, 4.3, is not applicable.
4.4 Classes of control functions
ISO 23550:2018 4.4, is not applicable.
5 Test conditions
Shall be in accordance with ISO 23550:2018, Clause 5, with the following addition:
Where applicable, the thermostat shall be mounted in an appropriate test fixture and the tests
conducted with the thermal sensing element immersed in the intended medium (e.g. water, oil or air)
for the application, as specified in the installation and operation instructions.
6 Construction
Specific regional requirements shall be as specified in Annex G.
6.1 General
Shall be in accordance with ISO 23550:2018, 6.1.
6.2 Construction requirements
6.2.1 Appearance
Shall be in accordance ISO 23550:2018, 6.2.1.
6.2.2 Holes
Shall be in accordance with ISO 23550:2018, 6.2.2.
6.2.3 Breather holes
ISO 23550:2018, 6.2.3 is not applicable.
6.2.4 Vent limiter
ISO 23550:2018, 6.2.4 is not applicable.
6.2.5 Screwed fastenings
Shall be in accordance with ISO 23550:2018, 6.2.5.
6.2.6 Moving parts
Shall be in accordance with ISO 23550:2018, 6.2.6.
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ISO 23551-9:2021(E)
6.2.7 Sealing caps
Shall be in accordance with ISO 23550:2018, 6.2.7.
6.2.8 Dismantling and reassembling for servicing and/or adjustment for controls
Shall be in accordance with ISO 23550:2018, 6.2.8, with the following addition.
If, in accordance with the manufacturer's instructions the thermostat can be dismantled for servicing,
such action shall not result in a change in temperature calibration exceeding the declared maximum set
point deviation (see 7.6.1.1).
Suitable means for maintaining all adjustments shall be provided. Lock nuts or adjusting nuts held by
springs or compression are considered satisfactory, except where their adjustment can be accidentally
disturbed.
6.2.9 Auxiliary channels and orifices
Shall be in accordance with ISO 23550:2018, 6.2.9.
6.2.10 Pre-setting device
Shall be in accordance with ISO 23550:2018, 6.2.10.
6.3 Materials
6.3.1 General material requirements
Shall be in accordance with ISO 23550:2018, 6.3.1.
6.3.2 Housing
Shall be in accordance with ISO 23550:2018, 6.3.2.
6.3.3 Springs providing closing force and sealing force
Shall be in accordance with ISO 23550:2018, 6.3.3.
6.3.4 Resistance to corrosion and surface protection
Shall be in accordance with ISO 23550:2018, 6.3.4.
6.3.5 Impregnation
Shall be in accordance with ISO 23550:2018, 6.3.5.
6.3.6 Seals for glands for moving parts
Shall be in accordance with ISO 23550:2018, 6.3.6.
6.3.7 Jointing
Shall be in accordance with ISO 23550:2018, 6.3.7.
6.4 Connections
Shall be in accordance with ISO 23550:2018, 6.4.
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ISO 23551-9:2021(E)
Further requirements for GQC shall be in ac
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