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The heating system is one of the biggest energy and cost factors in a building. For this reason, in this article, I will show you the structure of heating systems: from heat generation to heat distribution and heat transfer into the rooms. This article explains the structure of a heating system step by step, broken down into the most important components, as an easy introduction to heating technology.
Table of Contents
Components in your building
If you look around your building, you will probably immediately see typical components of a heating system. These include, for example, radiators on the wall, a thermostatic head on the radiator or an underfloor heating manifold in the hallway or utility room (see illustrations below). It is precisely these visible components that you come into direct contact with on a daily basis.
However, there are also areas of a heating system that are not immediately visible, but are nevertheless part of the heating system. These are, for example, heating pipes that are laid in walls, shafts or in the screed, as well as the heat source, which is often located in the boiler room, in the attic or in a technical centre. In many apartment blocks with apartment gas boilers (gas combi boilers), the heat source is even located directly in the flat, for example in the kitchen, bathroom or hallway.
The special thing about all these components is that they are interconnected. In a technical illustration, this results in a so-called heating system schematic or hydronic schematic. At first glance, such diagrams can seem very complex and overwhelming. Figure 1 therefore shows an example of such a diagram. Based on this, however, it is easy to explain how every heating system is structured in principle. The basic structure is always similar, regardless of whether it is decentralised gas heating, a central heat pump or district heating.
Structure of heating systems: three areas
How is a heating system structured? First of all, it can be said that every heating system is divided into three areas. These include heat generation, heat distribution and heat transfer. This categorisation helps enormously to be able to classify components in the building and better understand what function they have in the overall system. If you have these three areas in mind, you can “read” almost any system, even if the diagram looks complicated at first glance. The following simplified hydraulic diagram is intended to illustrate this.
Heat generation
In a simplified diagram, heat generation is usually shown as a central block (see Figure 2). This is where the heat is provided that is later used in the building. Typical heat sources are, for example, gas or oil condensing boilers, biomass boilers and heat pumps.
In a classic hot-water heating system, also known as a hydronic heating system, water is heated as the heat transfer medium in the heat source. This warm heating water then flows into the heat distribution system. At the “heat generation” level, it does not matter whether the energy source is gas, oil, wood, environmental heat or district heating – the decisive factor is that hot heating water is available at the end, which can be distributed throughout the building.
Heat distribution
Heat distribution ensures that the heated heating water reaches the individual areas of the building (Figure 3). For this purpose, each system has a supply (flow) pipe in which the hot water flows away from the heat source and a return pipe in which the cooled water flows back to the heat source. In a closed heating circuit, the water therefore always flows in one direction: from the heat source to the heat transfer and from there back again.
In order for the water to be transported, we have a component that is responsible for this. This is the heating circulation pump (Figure 4), which circulates the water to the individual areas of the building.
Without a pump, the heating water would virtually stand still and the heating surfaces would not become sufficiently warm. In real heating systems, other components such as mixing valves, distributors and control valves are also used, for example to supply different heating circuits with different temperatures or to hydraulically regulate individual areas. Different hydraulic circuits help with this.
Heat transfer
At the end of the heat distribution is the heat transfer. In the diagram, this is often shown as a “heating circuit”, often symbolised as a circle within a circle. The heating circuit represents the sum of all the heat transfer points behind it. Heat transfer includes all components that can transfer heat to a building area or room. These can be classic radiators, for example, but also panel heating systems such as underfloor or wall heating. In larger buildings or special applications, there are also air heaters or fan convectors, which are also part of the heat transfer.
When the warm heating water arrives at the heat transfer point, it releases its heat into the room. The heating water cools down in the process, then flows back to the heat source via the return flow and can be reheated there. This creates a closed water circuit.
Design temperatures
On many heating system schematics, you will often find information such as 75/55/20. These are the so-called design temperatures of the heating system. The first number stands for the flow temperature, the second number for the return temperature, in each case in the design case, and the third number for the desired room temperature (Figure 6).
The figures 75/55/20 mean, for example, 75 °C flow, 55 °C return and 20 °C room temperature. The temperature difference between the flow and return is also referred to as the temperature difference (ΔT), often called the “temperature spread”, and is necessary for calculating the volume flow of radiators, i.e. the amount of water required for a radiator to provide sufficient heat.
The design case or full load case describes the case with the lowest outside temperature for which the heating is designed. This refers to the heating load of the building, i.e. the amount of heat that must be provided on a very cold day so that the building still has a comfortable temperature. It is precisely this heating load that forms the basis for the dimensioning of heating surfaces and heat sources and is therefore a key planning tool.
It is important to note that modern low-temperature or heat pump systems also follow precisely this principle. However, they work with significantly lower design temperatures, for example 45/35/20 or 35/28/20. The lower these temperatures are, the more efficiently a heat pump or condensing boiler can generally work. Keeping the design temperature as low as possible is therefore an important building block for efficient heating systems.
Below you will find an overview of common design temperatures for heating systems with various heat sources.
| Heat Source | Flow | Return flow | Temperature spread | Design temperatures |
| Low temperature boiler | 75 °C | 55 °C | 20 K | 75/55 |
| Condensing boiler | 50 – 75 °C | 35 – 55 °C | 15 – 20 K | 50/35, 60/40, 75/55 |
| Heat pump | 35 – 55 °C | 30 – 45 °C | 5 – 10 K | 35/30, 45/35, 55/45 |
In relation to our original heating system schematic, the three areas can be divided as follows (see Figure X): Heat generation (green), Heat distribution (blue) and Heat transfer (red).
Types of heating systems
This basic structure of heat generation, heat distribution and heat transfer can be found in every heating system schematic. A distinction is now made between the type of heating system and the arrangement of heat sources. In practice, there are three main types of heating systems: centralised heating, decentralised heating and district heating. Regardless of which system is in use, the three areas of heat generation, heat distribution and heat transfer occur again and again – sometimes at building level, sometimes at flat level. In the following, I would like to introduce you to the different types of heating systems.
Central heating
With centralised heating, there is a common heat source for the entire building (Figure 8). This can be located in the boiler room, in the attic or in a central technical centre, for example. The heat is generated there and then transported to the individual heating surfaces in the building via the heat distribution system, i.e. via the supply (flow) pipe.
The radiators or panel heating systems transfer the heat to the respective rooms. The heating water cools down in the process and flows back to the heat source via the return pipe, where it is reheated. Central heating systems can be found both in detached houses and in apartment blocks with a common heat source for all flats. At building level, you therefore have a classic combination of centralised heat generation, a common distribution network and many heat transfer points.
Decentralised heating
The situation is different with decentralised heating. Here, the entire building does not have a common heat source, but several units each have their own heat source (Figure 9). Typical examples are apartment blocks in urban areas where each flat has its own gas connection. In these flats, there are usually individual gas boiler per apartment (so-called gas floor heating), often in the hallway, kitchen or bathroom.
Each of these gas boilers heats the heating water (and usually also the domestic hot water) for the respective flat only. A small integrated pump ensures that the water is transported to the radiators or underfloor heating circuits in the flat. There, the heat is transferred to the rooms, the water cools down and flows back to the respective boiler, where it can be reheated. From the perspective of the individual flat, we also have the individual areas of a heating system here: heat generation, heat distribution and heat transfer, but only at flat level instead of for the entire building.
District heating
In a district heating system, the actual heat source is not located in the building, but at a location outside the building, for example in a combined heat and power plant or in a large heating centre (see Figure 10).
In a combined heat and power (CHP) plant, heat is generated as a by-product of electricity production. Such plants are therefore referred to as combined heat and power (CHP) plants. The waste heat generated during electricity production is not released into the environment unused, but is transported via a district heating network to neighbouring buildings for heating. However, district heating can also be generated in a pure heating plant, in which only heat is provided, for example by burning a fuel.
The building itself does not have its own heat source, but a district heating substation. This consists of a heat exchanger, a pump and the necessary control technology. The heat exchanger ensures that the water from the district heating network and the heating water in the building remain hydraulically separated. The heat is transferred, but the two water circuits do not come into contact with each other.
The heating water in the building absorbs the heat at the heat exchanger and is distributed in the building via the pump. It releases the heat to the heating surfaces and then flows back to the transfer station, where it can be reheated. Once the heat has been transferred, the district heating water itself flows back to the combined heat and power plant or heating plant where it is reheated.
Note: There are still older district heating systems without heat exchangers in which the district heating water flows directly through the radiators in the building. However, this is rather uncommon today, as modern systems endeavour to achieve system separation by means of heat exchangers, for example for reasons of water quality and system hygiene.
Frequently asked questions about the structure of heating systems
What is the difference between central and decentralised heating?
In a central heating system there is one common heat source for the entire building, for example a boiler or a heat pump in the plant room or basement. From there, the heating water is distributed through the pipework to all radiators or surface heating circuits in the building. In a decentralised heating system, each flat or unit has its own heat source, for example a small gas boiler (gas combi boiler) inside the flat. Each unit therefore provides its own heat, with its own distribution pipework and its own heat emitters.
How can I tell whether I have district heating?
A typical sign is that there is no boiler or heat pump in the building, but instead a district heating substation with a heat exchanger and usually a district heating meter. On bills or documents from your energy supplier you will often find terms like “district heating”, “local heating” or the name of the local district heating provider. If you are unsure, a quick look into the plant room or a short call to the building management will usually clarify it.
What is the role of the circulation pump?
The circulation pump is one of the most important components in a heating system, as it is responsible for moving the water around the system. It ensures that the heated water reaches the heat emitters and that the cooled water flows back to the heat source. Without a pump, the water would more or less stand still and the radiators or surface heating systems would not get sufficiently warm. Modern electronically controlled circulation pumps are very efficient and automatically adjust their output to the actual demand, which can save a significant amount of electricity.
What do values such as 75/55/20 or 45/35/20 mean?
These values describe the design temperatures of a heating system. The first number is the design flow temperature, the second number is the design return temperature and the third number is the desired room temperature. For example, 45/35/20 means 45 °C flow, 35 °C return and 20 °C room temperature. Depending on the heat source, lower temperatures are usually targeted for efficient operation, especially with heat pumps and condensing boilers. However, there are still very old heat sources in operation (for example so-called low-temperature boilers – the name made sense decades ago), which need relatively high temperatures to work properly.
Can I work out the structure of my heating system myself?
To a certain extent, yes. Using the tips in this article, you can check whether your building has a single central heat source, several small decentralised heat sources or a district heating substation. You can also observe where the heating pipes run and where radiators or other heat emitters are located. With a bit of practice, you may even be able to assign an existing heating schematic to the three areas of heat generation, heat distribution and heat transfer. For detailed questions about design, hydraulics or planned modernisation measures, it is always a good idea to involve a qualified heating contractor or an independent energy consultant.
Conclusion
To summarise, it can be said that every heating system is divided into three areas: heat generation, heat distribution and heat transfer. There are also three types of heating systems: central heating, decentralised heating and district heating.
If you look around your building and stand in front of any part of a heating system, you should be able to recognise the area and type. That’s it for today, take care and see you next time. If you have any questions, suggestions or criticism, I look forward to your comments.
Best regards, Martin
