ISO/FDIS 11855-2

Building environment design -- Embedded radiant heating and cooling systems

Conception de l'environnement des bâtiments -- Systèmes intégrés de chauffage et de refroidissement par rayonnement

General Information

• Status: Published
• ICS: 91.040.01 - Buildings in general
• Technical Committee: ISO/TC 205 - Building environment design
• Drafting Committee: ISO/TC 205/WG 8 - Radiant heating and cooling systems
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 14-Jun-2021
• Completion Date: 14-Jun-2021

RELATIONS

• Revised: ISO 11855-2:2012 - Building environment design -- Design, dimensioning, installation and control of embedded radiant heating and cooling systems
• Effective Date: 14-Oct-2017

Standards Content (sample)

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 11855-2
ISO/TC 205
Building environment design —
Secretariat: ANSI
Embedded radiant heating and cooling
Voting begins on:
2021-06-14 systems —
Voting terminates on:
Part 2:
2021-08-09
Determination of the design heating
and cooling capacity
Conception de l'environnement des bâtiments — Systèmes intégrés de
chauffage et de refroidissement par rayonnement —
Partie 2: Détermination de la puissance calorifique et frigorifique à la
conception
ISO/CEN PARALLEL PROCESSING
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 11855-2:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2021
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ISO/FDIS 11855-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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Published in Switzerland
ii © ISO 2021 – All rights reserved
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ISO/FDIS 11855-2:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols ......................................................................................................................................................................................................................... 1

5 Concept of the method to determine the heating and cooling capacity ......................................................3

6 Heat exchange coefficient between surface and space ................................................................................................. 4

7 Simplified calculation methods for determining heating and cooling capacity or

surface temperature ......................................................................................................................................................................................... 6

7.1 Universal single power function .............................................................................................................................................. 7

7.2 Thermal resistance methods ...................................................................................................................................................... 9

8 Use of basic calculation programmes ..........................................................................................................................................12

8.1 Basic calculation programmes ...............................................................................................................................................12

8.2 Items to be included in a complete computation documentation ...........................................................13

9 Calculation of the heating and cooling capacity ...............................................................................................................13

Annex A (normative) Calculation of the heat flux ................................................................................................................................14

Annex B (informative) General resistance method ............................................................................................................................36

Annex C (informative) Pipes embedded in wooden construction .......................................................................................42

Annex D (normative) Method for verification of FEM and FDM calculation programmes ........................50

Annex E (normative) Values for heat conductivity of materials and air layers .....................................................53

Annex F (informative) Maximal surface temperatures for floor heating systems .............................................55

Bibliography .............................................................................................................................................................................................................................56

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ISO/FDIS 11855-2:2021(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 11855-2:2012), which has been

technically revised.
The main changes compared to the previous edition are as follows:
— update of the figures for type A and C,
— update of the thermal, relevant material characteristics,
— editorial corrections.
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
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ISO/FDIS 11855-2:2021(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, this document, 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.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 11855-2:2021(E)
Building environment design — Embedded radiant heating
and cooling systems —
Part 2:
Determination of the design heating and cooling capacity
1 Scope

This document specifies procedures and conditions to enable the heat flux in water-based surface

heating and cooling systems to be determined relative to the medium differential temperature for

systems. The determination of thermal performance of water-based surface heating and cooling

systems and their conformity to this document is carried out by calculation in accordance with design

documents and a model. This enables a uniform assessment and calculation of water-based surface

heating and cooling systems.

The surface temperature and the temperature uniformity of the heated/cooled surface, nominal heat

flux between water and space, the associated nominal medium differential temperature, and the field

of characteristic curves for the relationship between heat flux and the determining variables are given

as the result.

This document includes a general method based on finite difference or finite element Methods and

simplified calculation methods depending on position of pipes and type of building structure.

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
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 Surface of the occupied area
A m Surface of the heating or cooling surface area
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ISO/FDIS 11855-2:2021(E)
Table 1 (continued)
Symbol Unit Quantity
A m Surface of the peripheral area
b — Calculation factor depending on the pipe spacing
B, B , B W/( m ⋅K) Coefficients depending on the system
G 0
D m External diameter of the pipe, including sheathing where used
d m External diameter of the pipe
d m Internal diameter of the pipe
d m External diameter of sheathing
c kJ/(kg⋅K) Specific heat capacity of water

h W/(m ⋅K) Total heat transfer coefficient (convection + radiation) between surface and space

h W/(m ⋅K) Total heat transfer coefficient (convection + radiation) between surface and space

A-F
(floor)

h W/(m ⋅K) Total heat transfer coefficient (convection + radiation) between surface and space

A-W
(wall)

h W/(m ⋅K) Total heat transfer coefficient (convection + radiation) between surface and space

A-C
(ceiling)
K W/(m ⋅K) Equivalent heat transmission coefficient
K — Parameter for heat conducting devices
k — Parameter for heat conducting layer
L m Width of heat conducting devices

L m Width of fin (horizontal part of heat conducting device seen as a heating fin)

fin
L m Length of installed pipes
m — Exponents for determination of characteristic curves
m — Exponents for determination of characteristic curves
m — Exponents for determination of characteristic curves
m — Exponents for determination of characteristic curves
m kg/s Design heating or cooling medium flow rate
n, n — Exponents
q W/m Heat flux at the surface
q W/m Heat flux in the occupied area
q W/m Design heat flux
des
q W/m Limit heat flux
q W/m Nominal heat flux
q W/m Heat flux in the peripheral area
q W/m Outward heat flux
R m ⋅K/W Partial inwards heat transmission resistance of surface structure
R m ⋅K/W Partial outwards heat transmission resistance of surface structure
R m ⋅K/W Thermal resistance of surface covering
λ,B
R m ⋅K/W Thermal resistance of thermal insulation
λ,ins

s m In type B systems, thickness of thermal insulation from the outward edge of the

insulation to the inward edge of the pipes (see Figure 2)

s m In type B systems, thickness of thermal insulation from the outward edge of the

insulation to the outward edge of the pipes (see Figure 2)
s m Thickness of thermal insulation
ins
s m Pipe wall thickness
s m Thickness of the layer above the pipe
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ISO/FDIS 11855-2:2021(E)
Table 1 (continued)
Symbol Unit Quantity
s m Thickness of heat conducting device
S m Thickness of the screed (excluding the pipes in type A systems)
W m Pipe spacing
h W/(m ⋅K) Heat exchange coefficient
α — Parameter factors for calculation of characteristic curves
λ W/(m⋅K) Heat conductivity of the heat diffusion device material
θ °C Maximum surface temperature
s,max
θ °C Minimum surface temperature
s,min
θ °C Design indoor temperature
θ °C Temperature of the heating or cooling medium
θ °C Average surface temperature
s,m
θ °C Return temperature of heating or cooling medium
θ °C Supply temperature of heating or cooling medium
θ °C Indoor temperature in an adjacent space
Δθ K Heating or cooling medium differential temperature
Δθ K Design heating or cooling medium differential temperature
H,des
Δθ K Limit of heating or cooling medium differential temperature
H,G
Δθ K Nominal heating or cooling medium differential temperature
Δθ K Heating or cooling medium differential supply temperature
Δθ K Design heating or cooling medium differential supply temperature
V,des
λ W/(m⋅K) Thermal conductivity
σ K Temperature drop θ −θ
V R
φ — Conversion factor for temperatures
ψ — Volume ratio of the attachment studs in the screed
5 Concept of the method to determine the heating and cooling capacity

A given type of surface (floor, wall, ceiling) delivers, at a given average surface temperature and indoor

temperature (operative temperature θ ), the same heat flux in any space independent of the type of

embedded system. It is, therefore, possible to establish a basic formula or characteristic curve for

cooling and a basic formula or characteristic curve for heating, for each of the type of surfaces (floor,

wall, ceiling), independent of the type of embedded system, which is applicable to all heating and

cooling surfaces (see Clause 6).
Two methods are included in this document:
— simplified calculation methods depending on the type of system (see Clause 7);
— finite element method and finite difference method (see Clause 8).

Different simplified calculation methods are included in Clause 7 for calculation of the surface

temperature (average, maximum and minimum temperature) depending on the system construction

(type of pipe, pipe diameter, pipe distance, mounting of pipe, heat conducting devices, distribution

layer) and construction of the floor/wall/ceiling [covering, insulation layer, trapped air layer (Annex E),

etc.]. The simplified calculation methods are specific for the given type of system, and the boundary

conditions listed in Clause 7 shall be met. In the calculation report, it shall be clearly stated which

calculation method has been applied.
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ISO/FDIS 11855-2:2021(E)

In case a simplified calculation method is not available for a given type of system, either a basic

calculation using two or three dimensional finite element or finite difference method can be applied

(see Clause 8 and Annex D).
NOTE In addition, laboratory testing (for example, EN 1264) can be applied.

Based on the calculated average surface temperature at given combinations of medium (water)

temperature and space temperature, it is possible to determine the steady state heating and cooling

capacity (see Clause 9).
6 Heat exchange coefficient between surface and space

The relationship between the heat flux and mean differential surface temperature [see Figure 1 and

Formulae (1) to (4)] depends on the type of surface (floor, wall, ceiling) and whether the temperature of

the surface is lower (cooling) or higher (heating) than the space temperature.
Key
X mean differential surface temperature (θ − θ ) in K
s,m i
Y heat flux q (W/m )
Figure 1 — Basic characteristic curve for floor heating and ceiling cooling
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ISO/FDIS 11855-2:2021(E)
For floor heating and ceiling cooling in Figure 1, the heat flux q is given by:
1,1
q = 8,92 (θ − θ ) (1)
S,m i
where
θ is the average surface temperature, in °C;
S,m
θ is the nominal indoor operative temperature, in °C.

For other types of surface heating and cooling systems, the heat flux q is given by:

Wall heating and wall cooling:
q = 8 (|θ − θ |) (2)
s,m i
Ceiling heating:
q = 6 (|θ − θ |) (3)
s,m i
Floor cooling:
q = 7 (|θ − θ |) (4)
s,m i
NOTE 1 Heat flux, q, is expressed in in W/m .
The heat transfer coefficient is combined convection and radiation.

NOTE 2 In many building system simulations using dynamic computer models, the heat transfer is often split

up in a convective part (between heated/cooled surface and space air) and a radiant part (between heated/

cooled surface and the surrounding surfaces or sources). The radiant heat transfer coefficient in the normal

temperature range (15-30) °C can be fixed to 5,5 W/m ·K. The convective heat transfer coefficient depends on

type of surface, heating or cooling, air velocity (forced convection) or temperature difference between surface

and air (natural convection).

By using the simplified calculation method in Annex A, the characteristic curves present the heat flux

as a function of the difference between the heating or cooling medium temperature and the indoor

temperature. For the user of Annex A, this means not to do any calculations by directly using values

of heat transfer coefficients. Consequently, Annex A does not include values for such an application or

special details or formulae concerning heat transfer coefficients on heating or cooling surfaces.

Thus, the values α of Table A.20 are not intended to calculate the heat flux directly. In fact, they are

provided exclusively for the conversion of characteristic curves in accordance with Formula (A.33). For

simplifications these calculations are based on the same heat transfer coefficient for floor cooling and

ceiling heating, 6,5 W/(m ·K).

For every surface heating and cooling system, there is a maximum allowable heat flux, the limit heat flux

q . This is determined for a selected design indoor room temperature of θ (for heating, often 20 °C and

G i

for cooling, often 26 °C) at the maximum or minimum surface temperature θ and a temperature

F,max
drop σ = 0 K.

For the calculations, the centre of the heating or cooling surface area, regardless of the type of system,

is used as a reference point for θ .
S,max

The average surface temperature, θ , which determines the heat flux (refer to the basic characteristic

S,m

curve) is linked with the maximum or minimum surface temperature: θ < θ and θ > θ

S,m S,max S,m S,min

always applies. (See Annex F for the maximal surface temperature for floor heating systems.)

The attainable value, θ , depends not only on the type of system, but also on the operating conditions

S,m

(temperature drop σ = θ −θ , outward heat flux q and heat resistance of the covering R ).

V R u λ,B
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ISO/FDIS 11855-2:2021(E)
The following assumptions form the basis for the calculation of the heat flux:

— the heat transfer between the heated or cooled surface and the space occurs in accordance with the

basic characteristic curve;

— the temperature drop is σ = 0 K. The dependence of the characteristic curve on the temperature

drop is determined by using the logarithmically determined mean differential heating medium

temperature Δθ [see Formula (1)];
m kg
— the turbulent pipe flow is: > 4 000 ;
d h×m
— there is no lateral heat flux;

— the heat-conducting layer of the floor heating system is thermally decoupled by thermal insulation

from the structural base of the building. The thermal insulation does not need to be directly below

the system.
7 Simplified calculation methods for determining heating and cooling capacity
or surface temperature

Two types of simplified calculation methods can be applied according to this document:

— one method is based on a single power function product of all relevant parameters developed from

the finite element method (FEM);

— another method is based on calculation of equivalent thermal resistance between the temperature

of the heating or cooling medium and the surface temperature (or room temperature).

A given system construction can only be calculated with one of the simplified methods. The correct

method to apply depends on the type of system, A to G (position of pipes, concrete or wooden

construction) and the boundary conditions listed in Table 2.

NOTE Type A is a system with pipes embedded in the thermal diffusion layer . Type C is a system with pipes

embedded in the adjustment layer.
Table 2 — Criteria for selection of simplified calculation method
Type of Reference to
Pipe position Figure Boundary conditions
system method
In screed A, C, H, I, J 2 a) W ≥ 0,050 m s ≥ 0,01 m 7.1
Thermally decoupled from the structural 0,008 m ≤ d ≤ 0,03 m A.2.2
base of the building by thermal insulation
s /λ ≥ 0,01
u e
In insulation, conductive devices B 2 b) 0,05 m ≤ W ≤ 0,45 m 7.1
Not wooden constructions except for 0,014 m ≤ d ≤ 0,022 m A.2.3
weight bearing and thermal diffusion layer
0,01 m ≤ s /λ ≤ 0,18 m
u e
Plane section system D 2 c) 7.1,
A.2.4
In concrete slab E 4 S /W ≥ 0,3 7.2,
B.1
Capillary tubes in concrete surface F 5 d /W ≤ 0,2 7.2, B.2
Wooden constructions, pipes in sub floor G 6 λ ≥ 10 λ 7.2, Annex C
or under sub floor, conductive devices
S ≥ 0,01
WL λ
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ISO/FDIS 11855-2:2021(E)
7.1 Universal single power function

The heat flux between embedded pipes (temperature of heating or cooling medium) and the space is

calculated by Formula (5):
qB=⋅ ()a ⋅Δθ (5)
∏ i
where

B is a system-dependent coefficient in W/(m ⋅K), this depends on the type of system;

i is the power product, which links the parameters of the structure (surface covering, pipe

()a
∏ i
spacing, pipe diameter and pipe covering).
NOTE Heat flux, q, is expressed in W/m .

This calculation method is given in Annex A for the following four types of systems:

— type A with pipes embedded in the screed or concrete (see Figure 2 and A.2.2);
— type B with pipes embedded outside the screed (see Figure 2 and A.2.3);
— type C with pipes embedded in the screed (see Figure 2 and A.2.2);
— type D plane section systems (see A.2.4).

Figure 2 shows the types as embedded in the floor, but the methods can also be applied for wall and

ceiling systems with a corresponding position of the pipes.

This method shall only be used for system configurations meeting the boundary conditions listed for

the different types of systems in Annex A.
a) Type A and C
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ISO/FDIS 11855-2:2021(E)
b) Type B
c) Type D
d) Type H
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ISO/FDIS 11855-2:2021(E)
d) Type I
f) Type J
Key
1 floor covering

2a weight bearing and thermal diffusion layer (cement screed, anhydrite screed, asphalt screed)

2b weight bearing and thermal diffusion layer (cement screed, anhydrite screed, asphalt screed, wood)

2c weight bearing and thermal diffusion layer (cement screed, anhydrite screed, asphalt screed, timber)

2d weight bearing and thermal diffusion layer
3 adjustment layer (cement screed, anhydrite screed, asphalt screed)
4 profile
5 heating and cooling pipe
6a protection layer (plastic foil)
6b protection layer
7 pipe anchorage
8 heat diffusion devices
9a insulation layer
9b thermal insulation
10 adjustment layer
11a structural bearing
11b structural bearing / existing floor
Figure 2 — System types A, B, C, D, H, I and J covered by the method in Annex A
7.2 Thermal resistance methods

The heat flux between embedded pipes (temperature of heating or cooling medium) and the space or

surface is calculated using thermal resistances.
The concept is shown in Figure 3.
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ISO/FDIS 11855-2:2021(E)

An equivalent resistance, R , between the heating or cooling medium to a fictive core (or heat

conduction layer) at the position of the pipes is determined. This resistance includes the influence of

the pipe type, pipe distance and method of pipe installation (in concrete, wooden construction, etc.).

This is how a fictive core temperature is calculated. The heat transfer between this fictive layer and

the surfaces, R and R (or space and neighbour space) is calculated using linear resistances (adding of

i e
resistance of the layers above and below the heat conductive layer).

The equivalent resistance of the heat conductive layer is calculated in different ways depending on the

type of system.
This calculation method, using the general res
...