ISO 23551-9:2015

Safety and control devices for gas burners and gas-burning appliances — Particular requirements — Part 9: Mechanical gas thermostats

ISO 23551-9:2015 specifies safety, constructional, and performance requirements and testing of mechanical gas thermostat for gas burners, gas appliances, and similar use hereafter referred to as "thermostats". ISO 23551-9:2015 covers type testing only. ISO 23551-9:2015 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 burners and gas-burning appliances using fuel gases as natural gas, manufactured gas, and liquefied petroleum gas (LPG). ISO 23551-9:2015 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. ISO 23551-9:2015 only applies to mechanical thermostats used on gas appliances where the thermostat is not directly exposed to the outdoor environment. ISO 23551-9:2015 only applies to mechanical thermostats which are intended for operating control functions.

Dispositifs de commande et de sécurité pour brûleurs à fioul et pour appareils à fioul - Exigences particulières — Partie 9: Thermostats mécaniques

General Information

• Status: Withdrawn
• Publication Date: 13-Aug-2015
• ICS: 27.060.20 - Gas fuel burners
• Technical Committee: ISO/TC 161 - Controls and protective devices for gas and/or oil
• Drafting Committee: ISO/TC 161 - Controls and protective devices for gas and/or oil
• Current Stage: 9599 - Withdrawal of International Standard
• Completion Date: 17-Jan-2022

Relations

• Revised: ISO 23551-9:2022 - Safety and control devices for gas burners and gas-burning appliances — Particular requirements — Part 9: Mechanical gas thermostats
• Effective Date: 23-Apr-2020
• Consolidated By: ISO 17072-2:2019 - Leather — Chemical determination of metal content — Part 2: Total metal content
• Effective Date: 06-Jun-2022

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23551-9
First edition
2015-08-15
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 23551-9:2015(E)
©
ISO 2015

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ISO 23551-9:2015(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2015, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ISO 23551-9:2015(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 . 3
5 Test conditions . 3
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 Screwed fastenings . 4
6.2.5 Jointing . 4
6.2.6 Moving parts . 4
6.2.7 Sealing caps . 4
6.2.8 Dismantling and reassembling for servicing and/or adjustment . 4
6.2.9 Auxiliary channels . 4
6.3 Materials . 5
6.3.1 General material requirements . 5
6.3.2 Housing . 5
6.3.3 Springs . 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
6.4 Gas connections . 5
6.4.1 Making connections . 5
6.4.2 Connection sizes . 5
6.4.3 Threads . 6
6.4.4 Union joints . 6
6.4.5 Flanges . 6
6.4.6 Compression fittings . . . 6
6.4.7 Nipples for pressure tests . 6
6.4.8 Strainers . 6
7 Performance . 7
7.1 General . 7
7.2 Leak-tightness. 7
7.2.1 Criteria . 7
7.2.2 Test for leak-tightness . 7
7.3 Torsion and bending . 7
7.4 Rated flow rate . 7
7.4.1 Criterion . 7
7.4.2 Test for rated flow rate . 8
7.5 Durability . 8
7.6 Functional requirements . 8
7.7 Endurance .14
7.7.1 Mechanical cycling .14
7.7.2 Thermal cycling.15
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ISO 23551-9:2015(E)

8 EMC/electrical requirements .16
9 Marking, installation, and operating instructions .16
9.1 Marking .16
9.2 Installation and operating instructions .17
9.3 Warning notice .17
Annex A (informative) Leak-tightness test — Volumetric method .18
Annex B (informative) Leak-tightness test — Pressure-loss method .19
Annex C (normative) Conversion of pressure loss into leakage rate .20
Annex D (normative) Test for immunity to power-frequency magnetic fields.21
Annex E (normative) Specific regional requirements in European countries .22
Annex F (normative) Specific regional requirements in Canada and USA .23
Annex G (normative) Specific regional requirements in Japan .25
Bibliography .26
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ISO 23551-9:2015(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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 161, Control and protective devices for gas
and/or oil burners and appliances.
ISO 23551 consists of the following parts, under the general title Safety and control devices for gas
burners and gas-burning appliances — Particular requirements:
— Part 1: Automatic and semi-automatic valves
— Part 2: Pressure regulators
— Part 3: Gas/air ratio controls, pneumatic type
— Part 4: Valve-proving systems for automatic shut-off valves
— Part 5: Manual gas valves
— Part 6: Thermoelectric flame supervision controls
— Part 8: Multifunctional controls
— Part 9: Mechanical gas thermostats
An additional part dealing with vent valves is planned.
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ISO 23551-9:2015(E)

Introduction
This part of ISO 23551 is designed to be used in combination with ISO 23550. This part of ISO 23551,
together with ISO 23550, establishes the full requirements as they apply to the product covered by
this International Standard. This part of ISO 23551 adapts ISO 23550, where needed, by stating “with
the following modification”, “with the following addition”, “is replaced by the following”, or “is not
applicable” in the corresponding clause. In order to identify specific requirements that are particular
to this part of ISO 23551 that are not already covered by ISO 23550, this part of ISO 23551 may contain
clauses or subclauses that are additional to the structure of ISO 23550. These clauses are numbered
starting from 101 or, in the case of an Annex, are designated AA, BB, CC, etc.
In an attempt to develop an International Standard, it has been necessary to take into consideration
the differing requirements resulting from practical experience and installation practices in various
regions of the world and to recognize the variation in basic infrastructure associated with gas and/or
oil controls and appliances, some of which are addressed in Annex E to Annex G. This part of ISO 23551
intends to provide a basic framework of requirements that recognize these differences.
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INTERNATIONAL STANDARD ISO 23551-9:2015(E)
Safety and control devices for gas burners and gas-burning
appliances — Particular requirements —
Part 9:
Mechanical gas thermostats
1 Scope
This part of ISO 23551 specifies safety, constructional, and performance requirements and testing of
mechanical gas thermostat for gas burners, gas appliances, and similar use hereafter referred to as
“thermostats”.
This part of ISO 23551 covers type testing only.
This part of ISO 23551 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 burners and gas-
burning appliances using fuel gases as natural gas, manufactured gas, and liquefied petroleum gas (LPG).
This part of ISO 23551 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.
This part of ISO 23551 only applies to mechanical thermostats used on gas appliances where the
thermostat is not directly exposed to the outdoor environment.
This part of ISO 23551 only applies to mechanical thermostats which are intended for operating
control functions.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 23550:2011, Safety and control devices for gas burners and gas-burning appliances — General
requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 23550:2011 and the
following apply.
3.101
mechanical thermostat
thermostat which controls the temperature by adjusting the flow rate accordingly to the temperature
of the thermal sensing element (3.111) without any external energy such that the temperature remains
within defined limits
3.102
adjustable thermostat
mechanical thermostat (3.101) in which the temperature set-point (3.115) can be adjusted by the user to
anywhere between minimum and maximum values
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ISO 23551-9:2015(E)

3.103
fixed setting thermostat
mechanical thermostat (3.101) that has a preset fixed operating temperature which cannot be
adjusted by the user
3.104
snap-acting thermostat
mechanical thermostat (3.101) 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 (3.101) which controls the flow rate in accordance with a predetermined and
continuous function of the temperature of the thermal sensing element (3.111)
3.106
modulating thermostat with additional on-off action
mechanical thermostat (3.101) which acts as a snap-acting thermostat (3.104) between the closed and
reduced positions and as a modulating thermostat (3.105) 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 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).
3.109
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
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
3.112
operating curve
graphical representation of the flow rate as a function of the thermal sensing element temperature at a
given temperature set-point (3.115) and at a constant inlet pressure
3.113
backlash
difference of position of the adjusting knob (3.114) when it is moved in both directions to obtain the
same flow rate at a constant thermal sensing element temperature
3.114
adjusting knob
spindle
dial
part of the thermostat which is used to select the temperature set-point (3.115)
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ISO 23551-9:2015(E)

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 (3.115) (by means of the
adjusting knob (3.114))
3.117
calibration flow rate
flow rate declared by the manufacturer for calibration
3.118
calibration temperature set-point
temperature at which the calibration flow rate (3.117) should be obtained with the adjustment set to
the position and in the direction declared by the manufacturer
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 (3.115) which is declared by the manufacturer
3.121
drift
permanent change in the operating curve (3.112) of the thermostat
3.122
bypass
passage provided in which permits a flow of gas to the main burner(s) independently of the action of
the thermostatic valve
3.123
calibration reference point
dial setting at which a control is calibrated for agreement between dial indication and sensing
element temperature
4 Classification
4.1 Classes of controls
ISO 23550:2011, 4.1 is not applicable.
4.2 Groups of controls
Shall be according to ISO 23550:2011, 4.2.
5 Test conditions
Shall be according to ISO 23550:2011, 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 operation and installation instructions.
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ISO 23551-9:2015(E)

6 Construction
NOTE Regional specific requirements are given in Annex F.
6.1 General
Shall be according to ISO 23550:2011, 6.1.
6.2 Construction requirements
6.2.1 Appearance
Shall be according to ISO 23550:2011, 6.2.1.
6.2.2 Holes
Shall be according to ISO 23550:2011, 6.2.2.
6.2.3 Breather holes
Shall be according to ISO 23550:2011, 6.2.3.
6.2.4 Screwed fastenings
Shall be according to ISO 23550:2011, 6.2.4.
6.2.5 Jointing
Shall be according to ISO 23550:2011, 6.2.5.
6.2.6 Moving parts
Shall be according to ISO 23550:2011, 6.2.6.
6.2.7 Sealing caps
Shall be according to ISO 23550:2011, 6.2.7.
6.2.8 Dismantling and reassembling for servicing and/or adjustment
Shall be according to ISO 23550:2011, 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 (7.6.101.1).
Suitable means for maintaining all adjustments shall be provided. Lock nuts or adjusting nuts held
by springs or compression will be considered satisfactory except where their adjustment can be
accidentally disturbed.
6.2.9 Auxiliary channels
Shall be according to ISO 23550:2011, 6.2.9.
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ISO 23551-9:2015(E)

6.2.101 Presetting devices
A presetting device shall only be adjustable by use of a tool. The means of adjustment shall be easily
accessible and shall not change of its own accord, but shall be protected against unauthorized
interference, e.g. use of a sealing (lacquer).
A presetting device which connects a gas-carrying part to atmosphere shall provide leak tightness by
means which shall not seal on the thread, e.g. use of an O-ring seal.
The presetting device shall not be able to fall into the gas ways of the thermostat. If an O-ring or
equivalent gasket provides a seal against the atmosphere, then when the presetting device is completely
unscrewed, it shall not be able to be pushed out by gas pressure and shall remain tight at the maximum
pressure specified in 7.2.2.
If a presetting device is used for different gas families, it shall have a fixed minimum orifice.
A cover of any presetting device shall require a tool for removal and replacement and it shall not
interfere with the adjustment of the temperature range.
6.3 Materials
Shall be according to ISO 23550:2011, 6.3.
6.3.1 General material requirements
Shall be according to ISO 23550:2011, 6.3.1.
NOTE Regional specific requirements are given in Annex F.
6.3.2 Housing
Shall be according to ISO 23550:2011, 6.3.2.
6.3.3 Springs
Shall be according to ISO 23550:2011, 6.3.3.
6.3.4 Resistance to corrosion and surface protection
Shall be according to ISO 23550:2011, 6.3.4.
6.3.5 Impregnation
Shall be according to ISO 23550:2011, 6.3.5.
6.3.6 Seals for glands for moving parts
Shall be according to ISO 23550:2011, 6.3.6.
6.4 Gas connections
6.4.1 Making connections
Shall be according to ISO 23550:2011, 6.4.1.
6.4.2 Connection sizes
Shall be according to ISO 23550:2011, 6.4.2.
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ISO 23551-9:2015(E)

6.4.3 Threads
Shall be according to ISO 23550:2011, 6.4.3.
6.4.4 Union joints
Shall be according to ISO 23550:2011, 6.4.4.
6.4.5 Flanges
Shall be according to ISO 23550:2011, 6.4.5.
6.4.6 Compression fittings
Shall be according to ISO 23550:2011, 6.4.6.
6.4.7 Nipples for pressure tests
Shall be according to ISO 23550:2011, 6.4.7.
6.4.8 Strainers
Shall be according to ISO 23550:2011, 6.4.8.
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. Bypass rate adjustments shall be independent of pilot rate adjustments.
When specified by the manufacturer, 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.
Figure 2 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 specified in the operation
and installation instructions.
6.102 Temperature adjustment
6.102.1 Range adjustment
The allowed temperature setting range shall be limited by stops. Where applicable, the operating
instructions shall state the limits in which the temperature setting range can 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 operating instructions.
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ISO 23551-9:2015(E)

The temperature set-point adjustment means shall not change on its own accord.
6.102.3 Fixed setting thermostat
If provided, the adjustment means of a fixed setting thermostat shall be s
...