ISO 21026-1:2026

International
Standard
ISO 21026-1
First edition
Heat supply network —
2026-10
Vocabulary —
Part 1:
System
Réseau de distribution de chaleur — Vocabulaire —
Partie 1: Système
Reference number
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 District heating and/or cooling system .1
3.2 Heat and cold source .4
3.3 Pipeline .4
3.4 Substation and station .6
3.5 Heat and cold user .7
Bibliography . 9
Index . 10

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 341, Heat supply network.
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iv
Introduction
This document provides terms and definitions for district heating and/or cooling systems and their
components to support consistent use of terminology in International Standards.
This document aims to facilitate effective communication among the following groups:
— experts engaged in the development of ISO standards for heat supply networks;
— professionals involved in the design, construction, operation, maintenance, and decommissioning of heat
supply networks;
— new entrants to the field and external stakeholders seeking to acquire basic knowledge of heat supply
networks.
v
International Standard ISO 21026-1:2026(en)
Heat supply network — Vocabulary —
Part 1:
System
1 Scope
This document defines general terms for district heating and/or cooling systems.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 District heating and/or cooling system
3.1.1
district heating and/or cooling system
system that transports and distributes heating and/or cooling medium through pipes from heat and/or cold
sources to a number of users across a district
Note 1 to entry: A district heating and/or cooling system includes sources, pipelines, substations, and users.
Note 2 to entry: The medium can be water or steam. When the medium is water, it is referred to as a water district
heating and/or cooling system. When the medium is steam, it is referred to as a steam district heating system.
Note 3 to entry: For the principle of the structure of a district heating and/or cooling system, see Figure 1.

Key
1 heat or cold source
2 thermal energy storage
3 area substation
4 user substation
5 transmission pipeline
6 distribution pipeline
7 user connection
8 primary system
9 secondary system
Figure 1 — Principle for the structure of a district heating and/or cooling system
3.1.2
condensate return system
system that collects condensed steam and returns it to the heat sources
Note 1 to entry: Depending on the temperature and pressure, condensed steam can be present as water or consist of
water and steam fractions (wet steam, flash steam).
Note 2 to entry: A condensate return system is normally part of a steam district heating system.

3.1.3
circulation system
system in which the medium is returned to heat or cold sources
3.1.4
closed system
sealed system
system in which the medium is closed to the atmosphere
3.1.5
open system
system in which the medium is open to the atmosphere
EXAMPLE A steam district heating system is an open system, in which steam is extracted and directly in contact
with the atmosphere or when open water tanks are used in the condensate return system. A water district heating
system is an open system, in which water is extracted and directly in contact with the atmosphere.
3.1.6
direct connection
configuration in which medium flows from heat or cold sources directly to users without hydraulic
separation
3.1.7
indirect connection
configuration that separates media of two hydraulically independent systems but can transfer heat between
them
Note 1 to entry: The two hydraulically independent systems can be primary and secondary systems, or secondary and
user systems.
3.1.8
mixing loop
configuration that adjusts the temperature of supply medium by blending water from a return pipeline or
other heat or cold sources to achieve a desired temperature
Note 1 to entry: Water flows from heat or cold sources to users in a mixing loop without hydraulic separation.
Note 2 to entry: For the example of a mixing loop, see Figure 2.
Key
1 supply pipeline
2 return pipeline
3 pump
Figure 2 — Principle for the structure of a mixing loop
3.1.9
primary system
system from a heat or cold source to a substation
Note 1 to entry: For the principle of the structure of the primary system, see Figure 1.
...