SIST EN ISO 11855-1:2021

Building environment design - Embedded radiant heating and cooling systems - Part 1: Definitions, symbols, and comfort criteria (ISO 11855-1:2021)

This document specifies the basic definitions, symbols, and comfort criteria for embedded radiant heating and cooling systems.

Umweltgerechte Gebäudeplanung - Flächenintegrierte Strahlheizungs- und -kühlsysteme - Teil 1: Begriffe, Symbole und Komfortkriterien (ISO 11855-1:2021)

Dieses Dokument legt grundlegende Definitionen, Symbole und Behaglichkeitskriterien für flächenintegrierte Strahlungsheiz- und  kühlsysteme fest.

Conception de l'environnement des bâtiments - Systèmes intégrés de chauffage et de refroidissement par rayonnement - Partie 1: Définitions, symboles et critères de confort (ISO 11855-1:2021)

Le présent document spécifie les définitions de base, les symboles et les critères de confort relatifs aux systèmes intégrés de chauffage et de refroidissement par rayonnement.

Načrtovanje notranjega okolja v stavbah - Vgrajeni sevalni ogrevalni in hladilni sistemi - 1. del: Definicije, simboli in merila za ugodje (ISO 11855-1:2021)

Ta dokument določa osnovne opredelitve, simbole in merila toplotnega udobja za vgrajene sevalne
ogrevalne in hladilne sisteme.

General Information

• Status: Published
• Public Enquiry End Date: 02-Jun-2020
• Publication Date: 06-Oct-2021
• ICS: 91.140.10 - Central heating systems
• ICS: 91.140.30 - Ventilation and air-conditioning systems
• Technical Committee: OGS - Heating for buildings
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 02-Sep-2021
• Due Date: 07-Nov-2021
• Completion Date: 07-Oct-2021
• Ref Project: EN ISO 11855-1:2021 - Building environment design - Embedded radiant heating and cooling systems - Part 1: Definitions, symbols, and comfort criteria (ISO 11855-1:2021)

Relations

• Replaces: SIST EN ISO 11855-1:2015 - Building environment design - Design, dimensioning, installation and control of embedded radiant heating and cooling systems - Part 1: Definition, symbols, and comfort criteria (ISO 11855-1:2012)
• Effective Date: 01-Nov-2021
• Amended By: SIST EN ISO 11855-1:2021/A1:2023 - Building environment design - Embedded radiant heating and cooling systems - Part 1: Definitions, symbols, and comfort criteria - Amendment 1 (ISO 11855-1:2021/Amd 1:2023)
• Effective Date: 06-Oct-2021

Overview

EN ISO 11855-1:2021 / ISO 11855-1:2021 establishes the basic definitions, symbols and comfort criteria for embedded radiant heating and cooling systems used in building environment design. Adopted by CEN as EN ISO 11855-1:2021 (superseding the 2015 edition), this part of the ISO 11855 series frames the terminology and thermal comfort parameters that designers, specifiers and regulators rely on when working with embedded radiant systems, including Thermo Active Building Systems (TABS).

Key Topics

• Terms and definitions for embedded radiant heating and cooling systems (e.g., additional thermal resistance, average surface temperature, circuit).
• Symbols and abbreviated terms used consistently across the ISO 11855 series to support clear technical communication.
• General thermal comfort metrics: operative temperature, PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied).
• Local thermal discomfort considerations: surface temperature limits, radiant temperature asymmetry, vertical air temperature difference.
• Acoustical comfort criteria relevant to water-based embedded systems including noise related to water velocity and TABS operation.
• Informative annexes covering floor surface temperature for thermal comfort and draught guidance.
• Normative relationships to other parts of the ISO 11855 series (design, dimensioning, installation, control, dynamic analysis, and energy input parameters).

Applications

EN ISO 11855-1:2021 is practical for professionals involved in the design, specification and operation of building HVAC and radiant systems:

• HVAC engineers & mechanical designers: use the defined comfort criteria to size embedded radiant floors, walls or ceilings and to align system outputs with occupant comfort targets.
• Architects & building designers: integrate surface temperature and radiant asymmetry limits into architectural detailing and material selection.
• Energy modellers & simulation specialists: apply the standard’s operative temperature and comfort definitions when running steady-state and dynamic thermal analyses (linked to other ISO 11855 parts).
• Installers & manufacturers: refer to standardized symbols and terms for clear installation instructions and product information.
• Facility managers & spec writers: specify comfort thresholds and acoustical limits for procurement and commissioning.

Related Standards

• ISO 11855 series (Parts 2–7): determination of capacity, design & dimensioning, TABS dynamic calculation, installation, control, and input parameters for energy calculation.
• ISO 11855-5:2021 (installation) and ISO 52031 (energy calculation inputs) are specifically referenced within the series.

This standard is essential for consistent, interoperable practice in embedded radiant heating and cooling design, ensuring thermal and acoustical comfort criteria are clearly defined and consistently applied across projects.

Standards Content (Sample)

SLOVENSKI STANDARD
01-november-2021
Nadomešča:
SIST EN ISO 11855-1:2015
Načrtovanje notranjega okolja v stavbah - Vgrajeni sevalni ogrevalni in hladilni
sistemi - 1. del: Definicije, simboli in merila za ugodje (ISO 11855-1:2021)
Building environment design - Embedded radiant heating and cooling systems - Part 1:
Definitions, symbols, and comfort criteria (ISO 11855-1:2021)
Umweltgerechte Gebäudeplanung - Flächenintegrierte Strahlheizungs- und -
kühlsysteme - Teil 1: Begriffe, Symbole und Komfortkriterien (ISO 11855-1:2021)
Conception de l'environnement des bâtiments - Systèmes intégrés de chauffage et de
refroidissement par rayonnement - Partie 1: Définitions, symboles et critères de confort
(ISO 11855-1:2021)
Ta slovenski standard je istoveten z: EN ISO 11855-1:2021
ICS:
91.140.10 Sistemi centralnega Central heating systems
ogrevanja
91.140.30 Prezračevalni in klimatski Ventilation and air-
sistemi conditioning systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 11855-1
EUROPEAN STANDARD
NORME EUROPÉENNE
August 2021
EUROPÄISCHE NORM
ICS 91.040.01 Supersedes EN ISO 11855-1:2015
English Version
Building environment design - Embedded radiant heating
and cooling systems - Part 1: Definitions, symbols, and
comfort criteria (ISO 11855-1:2021)
Conception de l'environnement des bâtiments - Umweltgerechte Gebäudeplanung - Flächenintegrierte
Systèmes intégrés de chauffage et de refroidissement Strahlheizungs- und -kühlsysteme - Teil 1: Begriffe,
par rayonnement - Partie 1: Définitions, symboles et Symbole und Komfortkriterien (ISO 11855-1:2021)
critères de confort (ISO 11855-1:2021)
This European Standard was approved by CEN on 29 July 2021.

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, Turkey 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
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 11855-1:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 11855-1:2021) has been prepared by Technical Committee ISO/TC 205
"Building environment design" in collaboration with Technical Committee CEN/TC 228 “Heating
systems and water based cooling systems in buildings” the secretariat of which is held by DIN.
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 February 2022, and conflicting national standards
shall be withdrawn at the latest by February 2022.
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 ISO 11855-1:2015.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN websites.
According to the CEN-CENELEC Internal Regulations, the national standards organizations 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, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 11855-1:2021 has been approved by CEN as EN ISO 11855-1:2021 without any
modification.
INTERNATIONAL ISO
STANDARD 11855-1
Second edition
2021-08
Building environment design —
Embedded radiant heating and cooling
systems —
Part 1:
Definitions, symbols, and comfort
criteria
Conception de l'environnement des bâtiments — Systèmes intégrés de
chauffage et de refroidissement par rayonnement —
Partie 1: Définitions, symboles et critères de confort
Reference number
ISO 11855-1:2021(E)
©
ISO 2021
ISO 11855-1:2021(E)
© 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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms .11
5 Comfort criteria .16
5.1 General .16
5.2 General thermal comfort .16
5.2.1 Operative temperature .17
5.2.2 PMV (predicted mean vote) and PPD (predicted percentage of dissatisfied) .17
5.3 Local thermal discomfort .18
5.3.1 Surface temperature limit .18
5.3.2 Radiant temperature asymmetry .19
5.3.3 Vertical air temperature difference .19
5.4 Acoustical comfort .20
5.4.1 Water velocity and noise .20
5.4.2 Acoustical comfort in water-based heating and cooling systems .20
5.4.3 Acoustical comfort in thermally active building systems (TABS) .21
Annex A (informative) Floor surface temperature for thermal comfort .22
Annex B (informative) Draught .25
Bibliography .26
ISO 11855-1:2021(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 205, Building environment design, in
collaboration with the European Committee for Standardization (CEN) Technical Committee CEN/
TC 228, Heating systems and water based cooling systems in buildings, in accordance with the Agreement
on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 11855-1:2012), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— only references cited normatively were kept in Clause 2, the others were moved to Bibliography;
— in Clause 3, self-explanatory terms were removed, two similar terms representing the same concept
were unified into one term, and one term explaining two concepts were divided into two terms each
having one concept;
— editorial changes were performed.
A list of all parts in the ISO 11855 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.
iv © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
Introduction
The radiant heating and cooling system consists of heat emitting/absorbing, heat supply, distribution,
and control systems. The ISO 11855 series deals with the embedded surface heating and cooling system
that directly controls heat exchange within the space. It does not include the system equipment itself,
such as heat source, distribution system and controller.
The ISO 11855 series addresses an embedded system that is integrated with the building structure.
Therefore, the panel system with open air gap, which is not integrated with the building structure, is
not covered by this series.
The ISO 11855 series is applicable to water-based embedded surface heating and cooling systems
in buildings. The ISO 11855 series is applied to systems using not only water but also other fluids or
electricity as a heating or cooling medium. The ISO 11855 series is not applicable for testing of systems.
The methods do not apply to heated or chilled ceiling panels or beams.
The object of the ISO 11855 series is to provide criteria to effectively design embedded systems. To do
this, it presents comfort criteria for the space served by embedded systems, heat output calculation,
dimensioning, dynamic analysis, installation, control method of embedded systems, and input
parameters for the energy calculations.
The ISO 11855 series consists of the following parts, under the general title Building environment
design — Embedded radiant heating and cooling systems:
— Part 1: Definitions, symbols, and comfort criteria
— Part 2: Determination of the design heating and cooling capacity
— Part 3: Design and dimensioning
— Part 4: Dimensioning and calculation of the dynamic heating and cooling capacity of Thermo Active
Building Systems (TABS)
— Part 5: Installation
— Part 6: Control
— Part 7: Input parameters for the energy calculation
ISO 11855-1, this document, specifies the comfort criteria which should be considered in designing
embedded radiant heating and cooling systems, since the main objective of the radiant heating and
cooling system is to satisfy thermal comfort of the occupants. ISO 11855-2 provides steady-state
calculation methods for determination of the heating and cooling capacity. ISO 11855-3 specifies design
and dimensioning methods of radiant heating and cooling systems to ensure the heating and cooling
capacity. ISO 11855-4 provides a dimensioning and calculation method to design Thermo Active
Building Systems (TABS) for energy saving purposes, since radiant heating and cooling systems can
reduce energy consumption and heat source size by using renewable energy. ISO 11855-5 addresses the
installation process for the system to operate as intended. ISO 11855-6 shows a proper control method
of the radiant heating and cooling systems to ensure the maximum performance which was intended
in the design stage when the system is actually being operated in a building. ISO 11855-7 presents a
calculation method for input parameters to ISO 52031.
INTERNATIONAL STANDARD ISO 11855-1:2021(E)
Building environment design — Embedded radiant heating
and cooling systems —
Part 1:
Definitions, symbols, and comfort criteria
1 Scope
This document specifies the basic definitions, symbols, and comfort criteria for embedded radiant
heating and cooling systems.
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 11855-5:2021, Building environment design —Embedded radiant heating and cooling systems — Part 5:
Installation
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological 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
additional thermal resistance
thermal resistance representing layers added to the building structure and acting mostly as thermal
resistances because of their own low thermal inertia
EXAMPLE Carpets, moquette, and suspended ceilings.
3.2
average specific thermal capacity of the internal walls
thermal capacity related to one square metre of the internal walls
Note 1 to entry: Since internal walls are shared with other rooms, then just half of the total specific thermal
capacity of the wall shall be taken into account, since the second half is influenced by the opposite rooms that are
considered to be at the same thermal conditions as the one under consideration.
3.3
average surface temperature
θ
s,m
average value of all surface temperatures in the occupied or peripheral area (3.62)
ISO 11855-1:2021(E)
3.4
basic characteristic curve
curve reflecting the relationship between the heat flux (3.31) and the mean surface temperature
difference (3.47)
Note 1 to entry: This depends on the heating or cooling and the surface (floor, wall or ceiling) but not on the type
of embedded system.
3.5
calculation time step
length of time considered for the calculation
Note 1 to entry: This is typically assumed to equal 3 600 s.
3.6
circuit
section of system connected to a distributor (3.25) which can be independently switched and controlled
3.7
circuit total thermal resistance
thermal resistance representing the circuit (3.6) as a whole, determining a straight connection between
the water inlet temperature and the mean temperature at the pipe level (3.63)
Note 1 to entry: It includes the water flow thermal resistance (3.92), the convection thermal resistance at the pipe
inner side (3.10), the pipe thickness thermal resistance (3.66), and the pipe level thermal resistance (3.64).
3.8
clothing insulation
resistance of a uniform layer of insulation covering the entire body that has the same effect on sensible
heat flow as the actual clothing under standardized (static, wind-still) conditions
Note 1 to entry: The definition of clothing insulation also includes the uncovered parts of the body, e.g. the head. It
is specified as the intrinsic insulation from the skin to the clothing surface, not including the resistance provided
2 2
by the air layer around the clothed body, and is expressed in the clo unit or in m K/W; 1 clo = 0,155 m K/W.
3.9
conductive region of the slab
region of the slab (3.75) that includes the pipes with thermal conductivities of the layers higher than
0,8 W/(m·K)
Note 1 to entry: Due to the subdivision of the slab into an upper slab and a lower slab, the conductive region is
also subdivided into an upper conductive region and a lower conductive region.
3.10
convection thermal resistance at the pipe inner side
thermal resistance associated to the convection heat transfer taking place between the water flowing
in the pipe and the pipe inner side, thus connecting the mean water temperature along the circuit (3.6)
with the mean temperature of the pipe inner side
3.11
convective heating and cooling system
system that directly conditions the air in the room for the purpose of heating and cooling
3.12
convective peak load
maximum cooling load to be extracted by a virtual convective system used to keep comfort conditions
in the room
3.13
design cooling capacity
Q
H,c
thermal output by a cooling surface at design conditions
2 © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
3.14
design cooling load
Q
N,c
required thermal output necessary to achieve the specified design conditions in outside summer design
conditions
3.15
design sensible cooling load
required sensible thermal output necessary to achieve the specified design conditions in outside
summer design conditions
3.16
design supply temperature of heating medium
θ
V,des
value of flow water temperature with the thermal resistance of the chosen floor covering, at maximum
value of heat flux q
max
Note 1 to entry: The flow and the supply temperature are the same throughout the EN 1264 series.
Note 2 to entry: For the radiant cooling system, the design supply temperature of cooling medium applies instead
of design supply temperature of heating medium.
3.17
design heat flux
q
des
heat flow divided by the heating or cooling surface, taking into account the surface temperature
required to reach the design thermal capacity of a surface heated or cooled space, Q , reduced by the
H
thermal capacity of any supplementary heating or cooling equipment, if applicable
3.18
design heating capacity
Q
H,h
thermal output from a heating surface (3.33) at design conditions
3.19
design heating load
Q
N,h
required thermal output necessary to achieve the specified design conditions in outside winter design
conditions
Note 1 to entry: When calculating the value of the design heat load, the heat flow from embedded heating systems
into neighbouring rooms is not taken into account.
3.20
design heating medium differential temperature
Δθ
H,des
temperature difference of heating medium at design heat flux (3.17)
3.21
design cooling medium differential temperature
Δθ
C,des
temperature difference of cooling medium at design heat flux (3.17)
3.22
design heating medium differential supply temperature
Δθ
V,des
temperature difference between the design supply medium temperature and indoor temperature at
design heat flux (3.17)
ISO 11855-1:2021(E)
3.23
design heating medium flow rate
m
H
mass flow rate in a circuit (3.6) which is needed to achieve the design heat flux (3.17)
Note 1 to entry: The design cooling medium flow rate is similar with the only difference being that it has an
embedded radiant cooling system.
3.24
design indoor temperature
θ
i
operative temperature (3.58) at the centre of the conditioned space used for calculation of the design
load and capacity
Note 1 to entry: The operative temperature is considered relevant for thermal comfort assessment and heat loss
calculations. This value of internal temperature is used for the calculation method.
3.25
distributor
common connection point for several circuits (3.6)
3.26
draught
unwanted local cooling of a body caused by movement of air and related to temperature
3.27
electric heating system
several panel systems that convert electrical energy to heat, raising the temperature of conditioned
indoor surfaces and the indoor air
Note 1 to entry: The electric heating system can be applied to floor, walls and ceiling.
3.28
embedded surface heating and cooling system
system consisting of circuits (3.6) of pipes embedded in floor, wall or ceiling construction, distributors
(3.25) and control equipment
3.29
equivalent heat transmission coefficient
K
H
coefficient describing the relationship between the heat flux (3.31) from the surface and the heating
medium differential temperature (3.36)
Note 1 to entry: For the radiant cooling system, the cooling medium differential temperature applies instead of
heating medium differential temperature.
3.30
family of characteristic curves
curves denoting the system-specific relationship between the heat flux (q) (3.31) and the required
heating medium differential temperature (ΔθH) (3.36) for conduction resistance of various floor
coverings
3.31
heat flux
q
heat flow between the space and surface divided by the heated or cooled surface
Note 1 to entry: For heating it is a positive value and for cooling it is a negative value.
4 © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
3.32
heat transfer coefficient
h
t
combined convective and radiative heat transfer coefficient between the heated or cooled surface and
the space operative temperature (3.58) [design indoor temperature (3.24)]
3.33
heating surface
surface (floor, wall, ceiling) covered by the embedded surface heating system between the pipes at the
outer edges of the system with the addition of a strip at each edge of width equal to half the pipe spacing
(3.65), but not exceeding 0,15 m
Note 1 to entry: The cooling surface is similar with the only difference being that it has an embedded surface
cooling system.
3.34
heating surface area
A
F
area of surface (floor, wall, ceiling) covered by the embedded surface heating system between the pipes
at the outer edges of the system with the addition of a strip at each edge of width equal to half the pipe
spacing (3.65), but not exceeding 0,15 m
Note 1 to entry: The same concept of cooling surface area applies to the embedded cooling system.
3.35
heating capacity for circuit
Q
HC
heat exchange between a pipe circuit (3.6) and the conditioned room
Note 1 to entry: The same concept of cooling capacity for circuit applies to the embedded cooling system.
3.36
heating medium differential temperature
Δθ
H
logarithmically determined average difference between the temperature of the heating medium (3.83)
and the design indoor temperature (3.24)
Note 1 to entry: The same concept of cooling medium differential temperature applies to the embedded cooling
system.
3.37
internal convective heat gain
convective contributions by internal heat gains acting in the room
Note 1 to entry: Mainly due to people or electrical equipment.
3.38
internal radiant heat gain
radiant contributions by internal heat gains acting in the room
Note 1 to entry: This is mainly due to people or electrical equipment.
3.39
internal thermal resistance of the slab conductive region
total thermal resistance connecting the pipe level (3.63) with the middle points of the upper conductive
region and lower conductive region of the slab (3.9)
3.40
limit curve
curve in the field of characteristic curves showing the pattern of the limit heat flux (3.41) depending on
the heating medium differential temperature (3.36) and the floor covering
ISO 11855-1:2021(E)
3.41
limit heat flux
q
G
heat flux (3.31) at which the maximum (3.45) or minimum permissible surface temperature (3.49) is
achieved
3.42
limit heating medium temperature difference
Δθ
H,G
intersection of the system characteristic curve with the limit curve (3.40)
3.43
maximum cooling power
maximum thermal power of the cooling equipment, referring only to the room under consideration
3.44
maximum design heat flux
q
max
required design heat flux (3.17) in the room in order to design supply medium temperature
3.45
maximum permissible surface temperature
θ
S,max
maximum temperature permissible for physiological reasons or for the physical building, for calculation
of the limit curves (3.40), which may occur at a point on the surface (floor, wall, ceiling) in the occupied
or peripheral area (3.62) depending on the particular usage at a temperature drop (σ) (3.82) of the
heating medium equal to 0
3.46
mean radiant temperature
uniform surface temperature of an imaginary black enclosure in which an occupant would exchange
the same amount of radiant heat as in the actual non-uniform enclosure
3.47
mean surface temperature difference
difference between the average surface temperature (3.3) and the design indoor temperature(θi) (3.24)
Note 1 to entry: It determines the heat flux (3.31).
3.48
metabolic rate
rate of transformation of chemical energy into heat and mechanical work by aerobic and anaerobic
metabolic activities within an organism, usually expressed in terms of unit area of the total body
surfaces
Note 1 to entry: The metabolic rate varies with each activity. It is expressed in the met unit or in W/m ;
1 met = 58,2 W/m . 1 met is the energy produced per unit surface area of a sedentary person at rest. The surface
area of an average person can be determined by Dubois equation, body surface area, in m = 0,20 247 × height
0,725 0,425
(m) × weight (kg) .
3.49
minimum permissible surface temperature
θ
S,min
minimum temperature permissible for physiological reasons or for the physical building, for calculation
of the limit curves (3.40), which may occur at a point on the surface (floor, wall, ceiling) in the occupied
or peripheral area (3.62) depending on the particular usage at a temperature drop (σ) (3.82) of the
heating medium equal to 0
6 © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
3.50
nominal heat flux
q
N
limit heat flux (3.41) achieved without surface covering
3.51
nominal heating or cooling medium differential temperature
Δθ
N
absolute temperature difference at nominal heat flux (qN) (3.50)
3.52
non-active area
area of the surface not covered by a radiant heating or cooling system
3.53
number of active surfaces
number of surfaces in straight thermal connection with the pipe level (3.63), so that it distinguishes
whether the slab (3.75) transfers heat both through the floor side and through the ceiling side or
whether the ceiling side is much more active than the floor side
Note 1 to entry: Two active surfaces when the conductive region extends from the floor to the ceiling, one active
surface otherwise.
3.54
number of operation hours of the circuit
length of time during which the system runs in the day
3.55
occupied area
A
A
surface area which is heated or cooled, excluding peripheral area (3.62)
3.56
occupied zone
part of the conditioned zone in which persons normally reside and where requirements as to the
internal environment are satisfied
Note 1 to entry: Normally, the zone between the floor and 1,8 m above the floor and 1,0 m from outside walls or
windows and heating or cooling appliances, 0,5 m from internal surfaces.
3.57
open air gap
air gap in the floor, wall, or ceiling construction, where air exchange with space or the outside may
occur
3.58
operative temperature
θ
o
uniform temperature of an imaginary black enclosure in which an occupant exchanges the same amount
of heat by radiation and convection as in the actual non-uniform environment
3.59
orientation of the room
orientation of the main windowed external wall: East, South, West or North
Note 1 to entry: It is used to determine when the peak load (3.61) from heat gains happens, since internal heat
gains are considered almost constant and the widest variation is expected to happen in solar heat gains (3.76).
ISO 11855-1:2021(E)
3.60
outward heat flux
q
U
heat flow which is exchanged through the construction with unconditioned spaces, another building
entity, the ground or outdoor air
3.61
peak load
maximum cooling load to be extracted by the system used to keep comfort conditions in the room
3.62
peripheral area
A
R
surface area which is heated or cooled to a higher or lower temperature
Note 1 to entry: It is generally an area of 1 m maximum in width along exterior walls. It is not an occupied area
(3.55).
3.63
pipe level
virtual plane where the pipe circuit (3.6) lies
3.64
pipe level thermal resistance
thermal resistance associated to the 2-D conduction heat transfer taking place between the pipes and
the embedding layer, virtually referred to the pipe level (3.63), thus connecting the mean temperature
of the pipe outer side with the mean temperature at the pipe level
3.65
pipe spacing
spacing or distance between pipes embedded in the surface
3.66
pipe thickness thermal resistance
thermal resistance associated to the conduction heat transfer taking place through the pipe wall, thus
connecting the mean temperature of the pipe inner side with the mean temperature of the pipe outer
side
3.67
predicted mean vote
PMV
index that predicts the mean value of the thermal sensation votes of a large group of persons on a
7-point thermal sensation scale
3.68
predicted percentage of dissatisfied
PPD
index that establishes a quantitative prediction of the percentage of thermally dissatisfied people who
are either too warm of too cool
3.69
primary air convective heat gains
heat gains acting in the room due to the infiltration or primary air inflow
3.70
radiant surface heating and cooling system
heating and cooling system that controls the temperature of indoor surfaces on the floor, walls or ceiling
3.71
radiant temperature asymmetry
difference between the plane radiant temperature of the two opposite sides of a small plane element
8 © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
3.72
relative air velocity
air velocity relative to the occupant, including body movements
3.73
regional dew point
θ
Dp,R
dew point specified depending on the climatic conditions of the region
3.74
running mode
mode of the circuit (3.6) that defines whether the system is currently switched on or off
3.75
slab
horizontal building structure separating two rooms placed one below the other, hence being the ceiling
for one and the floor for the other
3.76
solar heat gain
heat gain from solar energy acting in the room due to high-frequency radiation transmission through
windows
3.77
specific daily energy gain
total energy to be extracted during the day in order to avoid a net increase in internal energy in the
room and maintain comfort conditions
3.78
supplementary heating equipment
additional heating facility with the additional heat output Q
out
EXAMPLE Convector, radiators.
Note 1 to entry: It may have its own control equipment.
3.79
surface heating and cooling components
insulating layer (for thermal and/or impact noise insulation), protection layer (to protect the insulating
layer), the pipes or plane sections, the load and thermal distribution layer where pipes are embedded,
covering and other items
Note 1 to entry: Other items include conducting devices, peripheral strips, attachment items, etc.
Note 2 to entry: Components may differ depending on the system.
3.80
system insulation
insulation with the thermal resistance R to limit the heat loss of heating and cooling systems
λ,ins
Note 1 to entry: R shall be in accordance to ISO 11855-5:2021, 5.1.2.3.2.
λ,ins
Note 2 to entry: For floor heating and cooling systems, as a rule the thermal resistance R is provided by the
λ,ins
insulation layers which are integral parts of the system. National rules can be consulted for this subject. For wall
and ceiling heating and cooling systems, the thermal resistance R may be determined taking into account the
λ,ins
effective thermal resistance of the building structure.
3.81
thermally active building system
TABS
water-based heating and cooling system (3.91) where the pipes are embedded in the central concrete
core of a building construction
ISO 11855-1:2021(E)
3.82
temperature drop
σ
difference between the supply and return temperature of the heating or cooling medium in a circuit
(3.6)
3.83
temperature of the heating medium
θ
m
average temperature between the supply and the return temperature defined as θ = θ + Δθ
m i H
Note 1 to entry: The same concept of temperature of the cooling medium applies to the embedded cooling system.
3.84
thermal node
node summarizing the thermal behaviour of a material or air volume as regards heat transfer
calculations
3.85
thermal output of surface system
Q
S
sum of the products of the heating or cooled surfaces of a space with the associated design heat fluxes
(3.17)
Note 1 to entry: For heating it is a positive value. For cooling it is a negative value.
3.86
total convective heat gain
sum of all convective contributions from heat gains acting in the room, hence it is the sum of internal
convective heat gains (3.37), primary air convective heat gains (3.69) and a fraction of transmission heat
gains (3.88)
3.87
total radiant heat gain
sum of all radiant contributions from heat gains acting in the room
Note 1 to entry: The heat gains acting in the room comprise internal radiant heat gains (3.38), solar heat gains
(3.76) and a fraction of transmission heat gains (3.88).
3.88
transmission heat gain
heat gains acting in the room due to conductive heat transmission through the external walls and
windows
3.89
vertical air temperature difference
difference in air temperature measured at 1,1 m and 0,1 m above the floor
Note 1 to entry: The distances 1,1 m and 0,1 m are theoretical average values for head and ankle height of a
sedentary person.
3.90
wall surface thermal resistance
thermal resistance representing the connection between the core of the internal walls and their surface
on the room side
Note 1 to entry: It usually corresponds to the layer of plaster covering the internal side of the walls.
3.91
water-based heating and cooling system
floor, wall or ceiling system where pipes carrying water with or without additives as a medium are laid
in the floor, wall or ceiling
10 © ISO 2021 – All rights reserved

ISO 11855-1:2021(E)
3.92
water flow thermal resistance
thermal resistance that expresses the variation in temperature of the water flowing in the pipe along
the circuit (3.6), so it connects the water inlet temperature with the mean water temperature along the
circuit
4 Symbols and abbreviated terms
For the purposes of this document, the symbols and abbreviations in Table 1 apply.
Table 1 — Symbols and abbreviated terms
Symbol Unit Quantity
A m Area of the occupied surface
A
A m Area of the heating or cooling surface
F
A m Area of the peripheral surface
R
A m Total area of internal vertical walls (i.e. vertical walls, external façades excluded)
W
a — Parameter factors for calculation of characteristic curves
i
B, B , B W/(m ·K) Coefficients depending on the system
G 0
b — Calculation factor depending on the pipe spacing
u
C J/(m ·K) Specific thermal capacity of the thermal node under consideration
C J/(m ·K) Average specific thermal capacity of the internal walls
W
c J/(kg·K) Specific heat of the material constituting the j-th layer of the slab
j
c J/(kg·K) Specific heat of water
Wa
D m External diameter of the pipe, including sheathing where used
d m External diameter of the pipe
a
d m Internal diameter of the pipe
i
d m External diameter of sheathing
M
E kWh/m Specific daily energy gains
Day
F — View factor between the floor and the ceiling
v F-C
F — View factor between the floor and the external walls
v F-EW
F — View factor between the floor and the internal walls
v F-W
f — Design safety factor
s
h
Running mode (1 when the system is running; 0 when the system is switched off)
f
—
rm
in the h-th hour
H Heat transfer coefficient between the thermal node under consideration and the air
A
W/K
thermal node (“A”)
H Heat transfer coefficient between the thermal node under consideration and the
C
W/K
ceiling surface thermal node (“C”)
H Heat transfer coefficient between the thermal node under consideration and the
Cct
W/K
circuit
H Heat transfer coefficient between the thermal node under consideration and the
CondDn
W/K
next one
H Heat transfer coefficient between the thermal node under consideration and the
CondUp
W/K
previous one
H Fraction of internal convective heat gains acting on the thermal node under consid-
conv
—
eration
H Heat transfer coefficient between the thermal node under consideration and the
F
W/K
floor surface thermal node (“F”)
H Coefficient connected to the inertia contribution at the thermal node under consid-
I
W/K
eration
ISO 11855-1:2021(E)
Table 1 (continued)
Symbol Unit Quantity
H Heat transfer coefficient between the thermal node under consideration and the
IWS
W/K
internal wall surface thermal node (“IWS”)
H Fraction of total radiant heat gains impinging on the thermal node under consider-
Rad
—
ation
h Total heat transfer coefficient (convection + radiation) between surface and space
A-C
W/(m ·K)
(ceilling)
h Total heat transfer coefficient (convection + radiation) between surface and space
A-F 2
W/(m ·K)
(Floor)
h Total heat transfer coefficient (convection + radiation) between surface and space
A-W 2
W/(m ·K)
(Wall)
h W/(m ·K) Convective heat transfer coefficient
c
h W/(m K) Heat transfer coefficient at floor heating surface
F
h W/(m ·K) Radiant heat transfer coefficient between the floor and the ceiling
F-C
h W/(m ·K) Radiant heat transfer coefficient between the floor and the internal walls
F-W
h W/(m ·K) Radiant heat transfer coefficient
r
h W/(m ·K) Total heat transfer coefficient (convection + radiation) between surface and space
t
h W/(m K) Heat transfer coefficient at wall heating surface
W
J — Number of layers constituting the slab as a whole
J — Number of layers constituting the upper part of the slab
J — Number of layers constituting the lower part of the slab
K W/(m ·K) Equivalent heat transmission coefficient
H
K — Parameter for heat conducting devices
WL
k — Parameter for heat conducting layer
CL
L m Width of fin (horizontal part of heat conducting device seen as a heating fin)
fin
L m Length of installed pipes
R
L m Width of heat conducting devices
WL
m — Exponents for determination of characteristic curves
m kg/s Design cooling medium flow rate
C
m kg/s Design heating medium flow rate
H

m
kg/(m ·s) Specific water flow in the circuit, calculated on the area covered by the circuit
H,sp
m Number of partitions of the j-th layer of the slab
j
n — Actual number of iterations in iterative calculations
n, n — Exponents
G
n h Number of operation hours of the circuit
h
Max
n — Maximum number of iterations allowed in iterative calculations
Max,h
Maximum cooling power reserved to the circuit under consideration in the h-th
P
W
Cct
hour
Max
W Maximum specific cooling power (per floor square metre)
P
Cct,Spec
PB — Polybutylene
PE-MDX — Cross-linked polyethylene, medium density
PE-RT-Sys
...