Safety and control devices for gas burners and gas-burning appliances -- Particular requirements

Dispositifs de commande et de sécurité pour brûleurs à fioul et pour appareils à fioul -- Exigences particulières

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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
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ISO 23551-9:2021(E)
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© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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PROOF/ÉPREUVE © ISO 2021 – All rights reserved
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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.
© ISO 2021 – All rights reserved PROOF/ÉPREUVE
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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.
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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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