EN ISO 11855-4:2021
(Main)Building environment design - Embedded radiant heating and cooling systems - Part 4: Dimensioning and calculation of the dynamic heating and cooling capacity of Thermo Active Building Systems (TABS) (ISO 11855-4:2021)
Building environment design - Embedded radiant heating and cooling systems - Part 4: Dimensioning and calculation of the dynamic heating and cooling capacity of Thermo Active Building Systems (TABS) (ISO 11855-4:2021)
This document allows the calculation of peak cooling capacity of Thermo Active Building Systems (TABS) – Type V according to ISO 11855-1, based on heat gains, such as solar gains, internal heat gains, and ventilation, and the calculation of the cooling power demand on the water side, to be used to size the cooling system, as regards, e.g. the chiller size, fluid flow rate.
This document defines a detailed method aimed at the calculation of heating and cooling capacity in non-steady state conditions.
Umweltgerechte Gebäudeplanung - Flächenintegrierte Strahlungsheiz und -kühlsysteme - Teil 4: Auslegung und Berechnung der dynamischen Heiz und Kühlleistung für thermoaktive Bauteilsysteme (TABS) (ISO 11855 4:2021))
Dieses Dokument ermöglicht die Berechnung der Spitzenkühlleistung thermoaktiver Bauteilsysteme (TABS) auf der Grundlage von Wärmeeinträgen, wie solaren Wärmeinträgen, internen Wärmeeinträgen und Lüftung, sowie in Hinblick auf Kühlergröße, Flüssigkeitsstrom usw. die Berechnung des wasserseitigen Bedarfs an Kühlleistung, die für das System vorgesehen ist.
In diesem Dokument wird ein detailliertes Verfahren für die Berechnung der Heiz und Kühlleistung bei nichtstationären Bedingungen festgelegt.
Conception de l'environnement des bâtiments - Systèmes intégrés de chauffage et de refroidissement par rayonnement - Partie 4: Dimensionnement et calculs relatifs au chauffage adiabatique et à la puissance frigorifique pour systèmes d'éléments de construction thermoactifs (TABS) (ISO 11855-4:2021)
Le présent document permet de calculer la puissance frigorifique de pointe de systèmes thermoactifs (TABS) en se fondant sur les apports de chaleur, tels que les apports solaires, les apports de chaleur internes et la ventilation, ainsi que de calculer la demande en puissance frigorifique côté eau, afin de les utiliser pour dimensionner le système de refroidissement en ce qui concerne les dimensions du refroidisseur, le débit de fluide, etc.
Le présent document présente une méthode détaillée visant à calculer la puissance calorifique et frigorifique dans des conditions non stabilisées.
Načrtovanje notranjega okolja v stavbah - Vgrajeni sevalni ogrevalni in hladilni sistemi - 4. del: Dimenzioniranje in izračun zmogljivosti dinamičnega ogrevanja in hlajenja toplotno-aktivnih delov stavbe (TABS) (ISO 11855-4:2021)
Ta dokument omogoča izračun konične zmogljivosti hlajenja termoaktivnih gradbenih sistemov
(TABS) na podlagi toplotnih dobitkov, npr. dobitkov sončnega sevanja, dobitkov notranjih virov toplote, in prezračevanja ter izračun
zahteve glede moči hlajenja v zvezi z vodo, ki se uporablja za ocenjevanje hladilnega sistema v smislu velikosti
naprave za hlajenje, hitrosti pretoka tekočine itd.
Ta dokument določa podrobno metodo za izračun zmogljivosti ogrevanja in hlajenja v
neustaljenih pogojih.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-november-2021
Nadomešča:
SIST EN ISO 11855-4:2015
Načrtovanje notranjega okolja v stavbah - Vgrajeni sevalni ogrevalni in hladilni
sistemi - 4. del: Dimenzioniranje in izračun zmogljivosti dinamičnega ogrevanja in
hlajenja toplotno-aktivnih delov stavbe (TABS) (ISO 11855-4:2021)
Building environment design - Embedded radiant heating and cooling systems - Part 4:
Dimensioning and calculation of the dynamic heating and cooling capacity of Thermo
Active Building Systems (TABS) (ISO 11855-4:2021)
Umweltgerechte Gebäudeplanung - Flächenintegrierte Strahlheizungs- und -
kühlsysteme - Teil 4: Auslegung und Berechnung der dynamischen Wärme- und
Kühlleistung für thermoaktive Bauteilsysteme (TABS) (ISO 11855-4:2021)
Conception de l'environnement des bâtiments - Systèmes intégrés de chauffage et de
refroidissement par rayonnement - Partie 4: Dimensionnement et calculs relatifs au
chauffage adiabatique et à la puissance frigorifique pour systèmes d'éléments de
construction thermoactifs (TABS) (ISO 11855-4:2021)
Ta slovenski standard je istoveten z: EN ISO 11855-4:2021
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 11855-4
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2021
EUROPÄISCHE NORM
ICS 91.040.01 Supersedes EN ISO 11855-4:2015
English Version
Building environment design - Embedded radiant heating
and cooling systems - Part 4: Dimensioning and calculation
of the dynamic heating and cooling capacity of Thermo
Active Building Systems (TABS) (ISO 11855-4:2021)
Conception de l'environnement des bâtiments - Umweltgerechte Gebäudeplanung - Flächenintegrierte
Systèmes intégrés de chauffage et de refroidissement Strahlheizungs- und -kühlsysteme - Teil 4: Auslegung
par rayonnement - Partie 4: Dimensionnement et und Berechnung der dynamischen Wärme- und
calculs relatifs au chauffage adiabatique et à la Kühlleistung für thermoaktive Bauteilsysteme (TABS)
puissance frigorifique pour systèmes d'éléments de (ISO 11855-4:2021)
construction thermoactifs (TABS) (ISO 11855-4:2021)
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 11855-4:2021 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
European foreword
This document (EN ISO 11855-4:2021) 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 March 2022, and conflicting national standards shall
be withdrawn at the latest by March 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 11855-4:2015.
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 11855-4:2021 has been approved by CEN as EN ISO 11855-4:2021 without any
modification.
INTERNATIONAL ISO
STANDARD 11855-4
Second edition
2021-08
Building environment design —
Embedded radiant heating and cooling
systems —
Part 4:
Dimensioning and calculation of the
dynamic heating and cooling capacity
of Thermo Active Building Systems
(TABS)
Conception de l'environnement des bâtiments — Systèmes intégrés de
chauffage et de refroidissement par rayonnement —
Partie 4: Dimensionnement et calculs relatifs au chauffage
adiabatique et à la puissance frigorifique pour systèmes d'éléments de
construction thermoactifs (TABS)
Reference number
ISO 11855-4:2021(E)
©
ISO 2021
ISO 11855-4:2021(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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 1
5 The concept of thermally active building surfaces (TABS) . 4
6 Calculation methods .10
6.1 General .10
6.2 Rough sizing method .13
6.3 Simplified sizing by diagrams .13
6.4 Simplified model based on FDM .19
6.4.1 Cooling system .20
6.4.2 Hydraulic circuit and slab .20
6.4.3 Room .22
6.4.4 Limits of the method .24
6.5 Dynamic building simulation programs.25
7 Effects of acoustic ceiling units on the cooling performance of TABS .25
8 Input for computer simulations of energy performance.25
Annex A (informative) Simplified diagrams .27
Annex B (normative) Calculation method .33
Annex C (informative) Tutorial guide for assessing the model.44
Annex D (informative) Computer program .47
Bibliography .58
ISO 11855-4:2021(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 11855-4:2012), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— editorial corrections;
— picture redraws;
— updated Bibliography;
— improved wording.
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 11855-4:2021(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 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, this document, 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 11855-4:2021(E)
Building environment design — Embedded radiant heating
and cooling systems —
Part 4:
Dimensioning and calculation of the dynamic heating and
cooling capacity of Thermo Active Building Systems (TABS)
1 Scope
This document allows the calculation of peak cooling capacity of Thermo Active Building Systems
(TABS), based on heat gains, such as solar gains, internal heat gains, and ventilation, and the calculation
of the cooling power demand on the water side, to be used to size the cooling system, as regards the
chiller size, fluid flow rate, etc.
This document defines a detailed method aimed at the calculation of heating and cooling capacity in
non-steady state conditions.
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 11855-1, Building environment design — Embedded radiant heating and cooling systems — Part 1:
Definitions, symbols, and comfort criteria
ISO 11855-2, Building environment design — Embedded radiant heating and cooling systems — Part 2:
Determination of the design heating and cooling capacity
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11855-1 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/
4 Symbols
For the purposes of this document, the symbols in Table 1 apply.
Table 1 — Symbols
Symbol Unit Quantity
A m Area of the heating/cooling surface area
F
A
m Total area of internal vertical walls (i.e. vertical walls, external façades excluded)
W
C J/(m ·K) Specific thermal capacity of the thermal node under consideration
ISO 11855-4:2021(E)
Table 1 (continued)
Symbol Unit Quantity
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
a
E kWh/m Specific daily energy gains
Day
Running mode (1 when the system is running; 0 when the system is switched off) in
h
-
f
rm
the h-th hour
f - Design safety factor
s
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
h W/(m ·K) Convective heat transfer coefficient between the air and the ceiling
A-C
h W/(m ·K) Convective heat transfer coefficient between the air and the floor
A-F
h W/(m ·K) Convective heat transfer coefficient between the air and the internal walls
A-W
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
Heat transfer coefficient between the thermal node under consideration and the air
H W/K
A
thermal node (“A”)
Heat transfer coefficient between the thermal node under consideration and the ceiling
H W/K
C
surface thermal node (“C”)
H W/K Heat transfer coefficient between the thermal node under consideration and the circuit
Cct
H W/K Heat transfer coefficient between the thermal node under consideration and the next one
CondDn
Heat transfer coefficient between the thermal node under consideration and the
H W/K
CondUp
previous one
H - Fraction of internal convective heat gains acting on the thermal node under consideration
Conv
Heat transfer coefficient between the thermal node under consideration and the floor
H W/K
F
surface thermal node (“F”)
H W/K Coefficient connected to the inertia contribution at the thermal node under consideration
I
Heat transfer coefficient between the thermal node under consideration and the
H W/K
IWS
internal wall surface thermal node (“IWS”)
H - Fraction of total radiant heat gains impinging on the thermal node under consideration
Rad
h W/(m ·K) Total heat transfer coefficient (convection + radiation) between surface and space
t
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
L m Length of installed pipes
R
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 iteration in iterative calculations
n h Number of operation hours of the circuit
h
n - Maximum number of iterations allowed in iterative calculations
Max
2 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Table 1 (continued)
Symbol Unit Quantity
Max,h
W Maximum cooling power reserved to the circuit under consideration in the h-th hour
P
Cct
Max
W/m Maximum specific cooling power (per floor square metre)
P
Cct,Spec
q W/m Inward specific heat flux
i
q W/m Outward specific heat flux
u
h
W Heat flux impinging on the ceiling surface (“C”) in the h-th hour
Q
C
h
W Heat flux extracted by the circuit in the h-th hour
Q
Cct
h
W Total convective heat gains in the h-th hour
Q
Conv
h
W Heat flux impinging on the floor surface (“F”) in the h-th hour
Q
F
h
W Internal convective heat gains in the h-th hour
Q
IntConv
h
W Internal radiant heat gains in the h-th hour
Q
IntRad
h
W Heat flux impinging on the internal wall surface (“IWS”) in the h-th hour
Q
IWS
h
W Primary air convective heat gains in the h-th hour
Q
PrimAir
h
W Total radiant heat gains in the h-th hour
Q
Rad
h
W Solar heat gains in the room in the h-th hour
Q
Sun
h
W Transmission heat gains in the h-th hour
Q
Transm
Q W/m Average specific cooling power
W
R (m ·K)/W Generic thermal resistance
R (m ·K)/W Additional thermal resistance covering the lower side of the slab
Add C
R (m ·K)/W Additional thermal resistance covering the upper side of the slab
Add F
R (m ·K)/W Internal thermal resistance of the slab conductive region
int
Conduction thermal resistance connecting the p-th thermal node with the boundary
R (m ·K)/W
L,p
of the (p+1)-th thermal node
R (m ·K)/W Pipe thickness thermal resistance
r
R (m ·K)/W Circuit total thermal resistance
t
Conduction thermal resistance connecting the p-th thermal node with the boundary
R (m ·K)/W
U,p
of the (p-1)-th thermal node
R (m ·K)/W Wall surface thermal resistance
W
R (m ·K)/W Water flow thermal resistance
Wa
R (m ·K)/W Pipe level thermal resistance
x
R (m ·K)/W Convection thermal resistance at the pipe inner side
z
s m Pipe wall thickness
r
s M Thickness of the upper part of the slab
s m Thickness of the lower part of the slab
m Pipe spacing
W
δ
m Thickness of the j-th layer of the slab
j
Δθ K Generic temperature difference
ISO 11855-4:2021(E)
Table 1 (continued)
Symbol Unit Quantity
Max
K Maximum operative temperature drift allowed for comfort conditions
Δθ
Comfort
s Calculation time step
Δt
h
°C Temperature of the air thermal node (“A”) in the h-th hour
θ
A
h
°C Temperature of the ceiling surface thermal node (“C”) in the h-th hour
θ
C
Max
°C Maximum operative temperature allowed for comfort conditions
Δθ
Comf
θ °C Maximum operative temperature allowed for comfort conditions in the reference case
Comf,Ref
h
°C Temperature of the floor surface thermal node (“F”) in the h-th hour
θ
F
h
°C Temperature of the core of the internal walls thermal node (“IW”) in the h-th hour
θ
IW
h
°C Temperature of the internal wall surface thermal node (“IWS”) in the h-th hour
θ
IWS
h
°C Room mean radiant temperature in the h-th hour
θ
MR
h
°C Room operative temperature in the h-th hour
θ
Op
h
°C Temperature of the p-th thermal node in the h-th hour
θ
p
h
°C Temperature of the pipe level thermal node (“PL”) in the h-th hour
θ
PL
Av
°C Daily average temperature of the conductive region of the slab
θ
Slab
h
°C Water inlet actual temperature in the h-th hour
θ
Wa,In
Setp,h
°C Water inlet set-point temperature in the h-th hour
θ
Wa,In
Setp
°C Water inlet set-point temperature in the reference case
θ
Wa,In,Ref
h
°C Water outlet temperature in the h-th hour
θ
Wa,Out
λ W/(m·K) Thermal conductivity of the material of the pipe embedded layer
b
λ W/(m·K) Thermal conductivity of the material constituting the j-th layer of the slab
j
λ W/(m·K) Thermal conductivity of the material constituting the pipe
r
ξ K Actual tolerance in iterative calculations
ξ K Maximum tolerance allowed in iterative calculations
Max
ρ kg/m Density of the material constituting the j-th layer of the slab
j
ω
various Slope of correlation curves
5 The concept of thermally active building surfaces (TABS)
A thermally active building surface (TABS) is an embedded water-based surface heating and cooling
system, where the pipe is embedded in the central concrete core of a building construction (see
Figure 1).
4 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Key
1 concrete
2 floor
3 pipes
4 room
5 reinforcement
6 window
Figure 1 — Example of position of pipes in TABS
The building constructions embedding the pipe are usually the horizontal ones. As a consequence,
in the following sections, floors and ceilings are usually referred to as active surfaces. Looking at a
typical structure of a thermally active building surfaces (TABS), heat is removed by a cooling system
(for instance, a chiller), connected to pipes embedded in the slab. The system can be divided into the
elements shown in Figure 2.
ISO 11855-4:2021(E)
Key
1 heating and cooling equipment
2 hydraulic circuit
3 slab including core layer with pipes
4 possible additional resistances (floor covering or suspended ceiling)
5 room below and room above
6 pipe level
Figure 2 — Simple scheme of a TABS
Thermally active surfaces exploit the high thermal inertia of the slab in order to perform the peak-
shaving. The peak-shaving consists in reducing the peak in the required cooling power (see Figure 3),
so that it is possible to cool the structures of the building during a period in which the occupants are
absent (during night time, in office premises). This way the energy consumption can be reduced and
a lower night time electricity rate can be used. At the same time a reduction in the size of heating and
cooling system components (including the chiller) is possible.
6 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Key
X time, h
Y cooling power, W
1 heat gain
2 cooling power needed for conditioning the ventilation air
3 cooling power needed on the water side
4 reduction of the required peak power
Figure 3 — Example of peak-shaving effect
TABS can be used both with natural and mechanical ventilation (depending on weather conditions).
Mechanical ventilation with dehumidifying can be required depending on external climate and
indoor humidity production. In the example in Figure 3, the required peak cooling power needed for
dehumidifying the air during day time is sufficient to cool the slab during night time.
As regards the design of TABS, the planner needs to know if the capacity at a given water temperature
is sufficient to keep the room temperature within a given comfort range. Moreover, the planner needs
also to know the heat flux on the water side to be able to dimension the heat distribution system and
the chiller and boiler. This document provides methods for both purposes.
When using TABS, the indoor temperature changes moderately during the day and the aim of a good
TABS design is to maintain internal conditions within the range of comfort, i.e. –0,5 < PMV < 0,5, during
the day, according to ISO 7730 (see Figure 4).
ISO 11855-4:2021(E)
Key
X time, h
Y temperature, °C
PMV predicted mean vote
θ air temperature
air
θ ceiling temperature
c
θ mean radiant temperature
mr
θ floor temperature
f
θ water return temperature
w exit
Figure 4 — Example of temperature profiles and PMV values vs. time
Some detailed building system calculation models have been developed to determine the heat exchanges
under unsteady state conditions in a single room, the thermal and hygrometric balance of the room air,
prediction of comfort conditions, check of condensation on surfaces, availability of control strategies
and calculation of the incoming solar radiation. The use of such detailed calculation models is, however,
limited due to the high amount of time needed for the simulations. The development of a more user-
friendly tool is required. Such a tool is provided in this document and allows the simulation of TABS.
The diagrams in Figure 5 show an example of the relation between internal heat gains, water supply
temperature, heat transfer on the room side, hours of operation and heat transfer on the water side.
The diagrams refer to a concrete slab with raised floor (R = 0,45 (m ·K)/W) and an allowed room
temperature range of 21 °C to 26 °C.
The upper diagram shows on the Y-axis the maximum permissible total heat gain in space (internal
heat gains plus solar gains) [W/m ], and on the X-axis the required water supply temperature. The
lines in the diagram correspond to different operation periods (8 h, 12 h, 16 h, and 24 h) and different
maximum amounts of energy supplied per day [Wh/(m ·d)].
The lower diagram shows the cooling power [W/m ] required on the water side (to dimension the
chiller) for TABS as a function of supply water temperature and operation time. Further, the amount of
energy rejected per day is indicated [Wh/(m ·d)].
The example shows that, for a maximum internal heat gain of 38 W/m and 8 h operation, a supply
water temperature of 18,2 °C is required. If, instead, the system is in operation for 12 h, a supply
8 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
water temperature of 19,3 °C is required. In total, the amount of energy rejected from the room is
approximately 335 Wh/m per day. In the same conditions, the required cooling power on the water side
2 2
is 37 W/m (for 8 h operation) and 25 W/m (for 12 h operation) respectively. Thus, by 12 h operation,
the chiller can be much smaller.
a)
ISO 11855-4:2021(E)
b)
Key
X (upper diagram) supply temperature tabs, °C
Y (upper diagram) maximum total heat gain in space (W/m , floor area)
Y (lower diagram) mean cooling power tabs (W/m , floor area)
1 maximurn temperature increase (21 °C - 26 °C)
2 self-regulating effect of slab
Figure 5 — Working principle of TABS
6 Calculation methods
6.1 General
TABS are systems with high thermal inertia. Therefore, for sizing chillers coupled with them, dynamic
simulations shall be carried out. In principle, the solution of heat transfer inside structures with
embedded pipes shall deal with 2-D calculations (see Figure 6). The calculation time required to
consider the 2-D thermal field and the overall balance with the rest of the room is usually too high.
Therefore, mathematical models in literature are usually based on a link between the pipe surface and
the upper and lower surfaces (i.e. floor and ceiling).
One possibility to model radiant systems is to apply response factors to the pipe surface, upper surface
and lower surface of the slab (see Figure 7). This way, the conduction heat transfer is defined via nine
response factor series, that can be reduced to six response factor series, because of reciprocity rules.
10 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Key
1 upper surface
2 pipe surface
3 lower surface
Figure 6 — Heat transfer through structures containing pipes
Key
1 impulse
Figure 7 — Transfer functions for building elements containing pipes
Another possibility is to consider a resistance between the external pipe surface and an equivalent core
temperature at pipe level, which represents the average temperature along the axial plane of the pipes
(see Figure 8). From the core level to upward and downward levels, a 1-D resistance-capacity network
or 1-D response factor series (or transfer function) can be applied.
ISO 11855-4:2021(E)
Key
1 lower part of the slab
2 lower surface temperature (ceiling)
3 circuit total thermal resistance
4 upper part of the slab
5 upper surface temperature (floor)
6 mean temperature at the pipe level
7 water supply temperature
Figure 8 — Simplified model for the conductive heat transfer in a structure containing pipes
In this document, the following calculation methods are presented.
— Rough-sizing method, based on a standard calculation of the cooling load (error: 20 % to 30 %). To
be used starting from the knowledge of the daily heat gains in the room (see 6.2).
— Simplified method using diagrams for sizing, based on the knowledge of the total energy to be
extracted daily to ensure comfort conditions (error: 15 % to 20 %). For details, see 6.3.
— Simplified model based on finite difference method (FDM) (error: 10 % to 15 %). It consists in
detailed dynamic simulations predicting the heat transfers in the slab and even in the room via
FDM. Based on the knowledge of the values of the variable cooling loads of the room during each
hour of the day. For further details, see 6.4. Annex A describes simplified diagrams based on the
simplified calculation method reported in 6.4,
— Detailed simulation models (error: 6 % to 10 %). It implies the overall dynamic simulation model for
the radiant system and the room via detailed building-system simulation software (see 6.5).
12 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
6.2 Rough sizing method
The cooling system shall be sized via the following Formula (1):
E
Day
Max
P = ⋅⋅1 000 f (1)
Cct,Spec s
n
h
where
Max
is the maximum specific cooling power (per floor square metre) in W/m ;
P
Cct,Spec
E is the specific daily energy gains in kWh/m ;
Day
n is the number of operation hours of the circuit in h;
h
f is the safe design factor (greater than one, usually 1,15) in -.
s
For this purpose, E shall be calculated in the following way:
Day
— the hourly values of heat gains are calculated for the room under the design conditions and occupancy
schedules, via an energy simulation tool or a proper method for the calculation of heat gains;
— E is the sum of the 24 values of heat gains.
Day
The heat gains calculation shall be carried out using an operative temperature 0,5 °C lower than the
average operative temperature during occupancy hours, for the sake of safe design. As a consequence,
if the room operative temperature drift during occupancy hours is 21,0 °C to 26,0 °C, then the room
average operative temperature during occupancy hours is 23,5 °C, and the reference room operative
temperature for the calculation of heat gains is 23,0 °C.
6.3 Simplified sizing by diagrams
In this case, the calculation of the heat gains shall be carried out by means of the value of the total
cooling energy to be provided during the day in order to ensure comfort conditions at the average
operative temperature (for instance, 23,0 °C). This method is based on the assumption that the entire
thermally conductive part of the slab is maintained at an almost constant temperature during the whole
day, due to its own thermal inertia and the thermal resistance dividing it from the rooms over and
below. This average temperature of the slab is calculated by the method itself and is used to calculate
the water supply temperature depending on the running time of the circuit.
The following magnitudes are involved in this method.
— E : specific daily energy gains in the room during the design day. It consists of the sum of heat
Day
gains values acting during the whole design day, divided by the floor area, in kWh/m .
Max
— θ : maximum operative room temperature allowed for comfort conditions, in °C.
Comf
— Orientation of the room: used to determine when the peak load in heat gains happens [east (morning),
south (noon) or west (afternoon)].
— Number of active surfaces: distinguishes whether the slab works transferring heat both through the
floor side and through the ceiling side or just through the ceiling side (see Figures 9, 10 and 11).
— n : number of operation hours of the circuit in h.
h
— R : internal thermal resistance of the slab conductive region in (m ·K)/W. It is the average thermal
Int
resistance that connects the conductive parts of the slab placed near the pipe level to the pipe level
itself [see Formula (4)].
ISO 11855-4:2021(E)
Av
— θ : daily average temperature of the conductive region of the slab in °C. It is a result of the present
Slab
method and depends on the number of active surfaces (ceiling only, or ceiling and floor), the running
mode (24 h or 8 h) and the shape of the internal load profile (lunch break or not) and room orientation.
The average temperature of the slab is achieved through coefficients included in the method by
Formula (2).
Av Max
θθ=+ω⋅E (2)
Slab Comf Day
where ω is a coefficient, whose values are given in Tables 2 and 3.
— R : circuit total thermal resistance, obtained via the resistance method (for further details, see
t
ISO 11855-2) in (m ·K)/W. This thermal resistance depends on the characteristics of the circuit,
pipe, and conductive slab (see Figure 14).
Setp,h
— θ : water supply temperature required for ensuring comfort conditions in °C.
Wa,In
It is obtained through Formula (3):
E ⋅1000
Day
Setp,h Av
θθ=− ⋅+RR (3)
()
Slab int t
Wa,In
h
Dimensions in metres
Key
1 concrete
2 reinforced concrete
3 active surfaces
Conductive region: material 1 and material 2.
Number of active surfaces: 2.
Figure 9 — Example 1 — Conductive regions and numbers of active surfaces
14 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Dimensions in metres
Key
1 wood
2 air
3 reinforced concrete
4 Active surface
Conductive region: material 3.
Number of active surfaces: 1.
Figure 10 — Example 2 — Conductive regions and numbers of active surfaces
ISO 11855-4:2021(E)
Dimensions in metres
Key
1 wood
2 concrete
3 fibreglass
4 reinforced concrete
5 active surface
Conductive region: material 4.
Number of active surfaces: 1.
Figure 11 — Example 3 — Conductive regions and numbers of active surfaces
The internal thermal resistance of the slab conductive region (R ) expressed in (m ·K)/W is the
int
average thermal resistance that connects the conductive parts of the slab placed near the pipe level to
the pipe level itself. Formula (4) describes how to calculate it.
R
R
Up
Down
⋅
R = (4)
int
R
R
Up
Down
+
where
R is the total thermal resistance of the lower part of the slab conductive region;
Down
R is the internal thermal resistance of the slab conductive region;
int
R is the total thermal resistance of the upper part of the slab conductive region.
Up
16 © ISO 2021 – All rights reserved
ISO 11855-4:2021(E)
Table 2 — Constant internal heat gains from 8:00 to 18:00
Orientation of the room
Number of active sur-
Circuit running mode East (E) South (S) West (W)
faces
ω
Floor and ceiling (C2) -4,6 816 -5,3 696 -5,935
Continuous (24 h)
Only ceiling (C1) -6,3 022 -7,2 237 -7,7 982
Floor and ceiling (I2) -5,5 273 -6,1 701 -6,7 323
Intermittent (8 h)
Only ceiling (I1) -7,2 853 -7,8 562 -8,5 791
Table 3 — Constant internal heat gains from 8:00 to 12:00 and from 14:00 to 18:00
Orientation of the room
Number of active sur-
Circuit running mode East (E) South (S) West (W)
faces
ω
Floor and ceiling (C2) -6,279 -7,1 094 -7,3 681
Continuous (24 h)
Only ceiling (C1) -7,9 663 -8,7 989 -8,7 455
Floor and ceiling (I2) -8,1 474 -8,758 -9,3 264
Intermittent (8 h)
Only ceiling (I1) -10,029 -10,685 -10,967
Max
By the choice ofθ , it is possible to adapt the method to different maximum room operative
Comf
temperatures, if the same maximum operative temperature drift allowed for comfort conditions is kept.
Max Max
Once θ is defined, the tables can be summarized by diagrams. For example, if θ = 26 °C, the
Comf Comf
diagram for constant internal heat gains from 8:00 to 18:00 is as given in Figure 12.
ISO 11855-4:2021(E)
Key
X E , kWh/m
Day
Y θ , °C
slab
Figure 12 — Diagram for determining θ as a function of the specific daily energy, exposure
slab
of the room (E = east, S = south, W = west), running mode of the circuit (C = continuous - 24 h, I
= intermittent - 8 h), and number of active surfaces (1 or 2), in the case of constant internal heat
gains during the day
Examples of calculation with input data and steps are presented in Table 4.
18 © ISO 2021 – All rights reserved
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