Today's article covers a number of aspects related to the use of heat exchangers for underfloor heating. Not every underfloor heating installation includes a heat exchanger, and for some people this may sound rather mysterious. We will look at the technical and practical aspects of the topic in order to explain exactly what it is about.
✅ The article refers to various situations in which heat exchangers are used with underfloor heating – not all of them may apply to your heating system. If a given paragraph does not relate to what you are looking for, you can move on – at the end of the article you will also find important information that will help you avoid installation mistakes.
A heat exchanger for underfloor heating is a device that enables the transfer of thermal energy between two circuits of a heating installation – i.e. from the heat source (for example a heating boiler, fireplace or heat pump) – without their direct mixing. Most often a plate heat exchanger is used, which separates the heat source circuit from the underfloor heating circuit – it separates the circuits but still allows heat to be transferred from the hot circuit to the underfloor heating. Thanks to this, it is possible to safely connect different heating systems such as a heat pump, gas boiler, solid fuel boiler or a fireplace with a water jacket.
Why a heat exchanger is used in an underfloor heating installation is basically the key issue.
✅ First of all, not every underfloor heating installation requires a heat exchanger. As mentioned above, the exchanger ensures separation of the hydraulic circuits of the heat source and the underfloor heating. In some situations it is necessary – even essential. In which ones? We will discuss that in a moment.
However, there are also underfloor heating systems that can operate in a single circuit with the heat source without requiring separation. What does it depend on? There are several reasons why separating underfloor heating with a heat exchanger may be necessary. We will review the most important ones, but it can already be noted that it usually depends on the type of heat source used.
Product available with different options
The use of a heat exchanger for underfloor heating is particularly useful when the heat source is of an older type and requires operation in an open system. An open system is a central heating system that is not tightly sealed and pressurized. Such a system usually has, at the highest point – for example in the attic – a so-called open expansion tank through which it has access to air. If the water in an open system overheats and the pressure increases dangerously, it releases automatically through the open expansion tank.
In such open systems all older-type devices operate, such as coal boilers, but also fireplaces or wood boilers. These devices lack temperature sensors and are relatively inert – they will not shut down due to system overheating (unlike, for example, a heat pump that switches off after reaching a set temperature). In these systems the safety function is performed by the open expansion tank, which releases excess pressure. The easiest way to explain this is with a fireplace example – it is impossible to simply reduce the flame to regulate the temperature. It is an inert heat source and therefore intended mainly for open systems.
Unfortunately, underfloor heating cannot operate in an open system. It is a modern system based on thin pipes designed for closed systems. Underfloor heating cannot operate in an open system.
This raises the question – how to connect the closed underfloor heating circuit with the open circuit of a fireplace, coal boiler or wood boiler? This is exactly why a heat exchanger for underfloor heating is used.
Nordic Tec heat exchanger for underfloor heating – on the left in a version without accessories, on the right in a full installation kit with valves and thermometers for monitoring efficient heat exchange.
Some people may wonder why underfloor heating cannot operate in an open system. Unfortunately, such systems are considered less suitable for modern installations. A heat exchanger used together with underfloor heating separates the circuits and protects the system from contamination, excessive pressure or unsuitable water quality in the heat source circuit.
Open systems also tend to create a better environment for scale buildup inside the installation. While this may still function in thick pipes of an open system (although scale always reduces heating efficiency), in underfloor heating systems based on thin pipes it cannot be allowed.
✅ Less commonly, the opposite situation occurs – when underfloor heating could potentially harm a modern heat source such as an expensive heat pump or a modern condensing boiler. How is this possible? After all, underfloor heating is considered a relatively clean and reliable part of heating systems...
In older underfloor heating installations, pipes without an oxygen barrier were often used. This means that over time they may allow oxygen from the air to penetrate into the installation water. In modern heating systems powered by devices such as heat pumps or condensing boilers, the presence of oxygen in the installation is undesirable – it can accelerate corrosion of metal components, cause magnetite formation and create problems with pumps and valves.
Using a heat exchanger allows the old underfloor heating circuit to be separated from the modern heat source. As a result, oxygen entering through old pipes remains only in the underfloor heating circuit and does not enter the main installation circuit, significantly increasing the durability of the entire heating system. This greatly reduces the risk of heat pump failure after several years as well as the risk of premature clogging of a condensing boiler heat exchanger.
However, in most cases a heat exchanger is used in the opposite direction – to protect the pipes of the underfloor heating system.
Another situation that should be considered is when the heating medium in the underfloor heating circuit differs from that in the heat source circuit. In such cases, the only way to separate the two systems is a heat exchanger. Although it may be difficult for non-specialists to imagine, such situations do occur:
A plate heat exchanger consists of a package of thin steel plates arranged so that flow channels are created between them for two separate heating media. One circuit releases heat through the plate surfaces, while the other absorbs it. The flows are usually arranged in counterflow, which increases heat exchange efficiency. Thanks to the large plate surface area, a plate heat exchanger can transfer a large amount of energy despite relatively small device dimensions.
Selecting a heat exchanger for underfloor heating should take into account several key parameters. The most important are the heating system power, supply and return temperatures, and water flow in both circuits. In underfloor heating systems it is often necessary to use a larger heat exchange surface because the system operates at relatively low temperatures. A properly selected exchanger ensures stable installation temperature and minimal energy loss.
✅ The manufacturer Nordic Tec heat exchangers often uses a conversion factor of at least 0.30–0.35 m² of plate surface per 100 m² of underfloor heating in modern buildings. However, this ratio cannot be applied universally because the exchanger selection must also take the heat source into account. If the boiler or fireplace is significantly larger than required by the floor heating area, the exchanger must also be oversized so it can absorb all the heat – including excess heat from the heat source circuit. Otherwise the water in the heat source circuit could boil, which is very dangerous.
☑️✅ If you are unsure about the correct selection – it is best to contact our service team.
If the heat source (like underfloor heating) operates in a closed system, a heat exchanger is usually unnecessary and situations where it is used are rare. Therefore, if you are building a new house and installing a modern heating device – such as a condensing boiler, biomass boiler, pellet boiler or a heat pump – a heat exchanger will typically not be needed. However, it may be useful if an additional heat source operating with an open flame, such as a fireplace, is installed in the house. Since fireplaces operate most safely in open systems, it is advisable to separate them from underfloor heating using a plate heat exchanger for fireplaces.
Sometimes in large heating systems or multi-storey buildings the system may be poorly balanced in terms of pressure. This means that certain parts of the system may lack sufficient pressure, resulting in underheated zones. In such cases – even if both the underfloor heating and the heat source are designed for the same type of system – hydraulic separation may still be required.
A plate heat exchanger is primarily designed to separate open and closed systems. In the situation described above, where separation is needed not to protect the systems but rather to properly balance system pressure, a hydraulic separator can be used instead. A hydraulic separator for underfloor heating divides the system into two parts, leaving underfloor heating on one side and the heat source on the other. It theoretically resembles a heat exchanger. However, there is a key difference – in a hydraulic separator water from both circuits still mixes, which is why it is used when we are not connecting open and closed systems within the same building.
Correct installation of a heat exchanger in an underfloor heating system is crucial for the efficiency of the entire system. In fact, proper installation is absolutely essential. The exchanger should be installed in a location that allows easy service access. It is also recommended to install mesh filters before the exchanger and appropriate circulation pumps on both sides of the installation. This ensures stable water flow and protects the device from contamination.
One of the most common mistakes is using a heat exchanger that is too small. In underfloor heating installations this can lead to reduced supply temperature and decreased system performance. Another mistake is insufficient filtration before the exchanger, which may cause contaminants to accumulate in the flow channels. It is also important to properly select circulation pumps to ensure adequate flow in both circuits.
There are many other possible installation mistakes. We recommend the article below, which will also help you check whether the installer of your exchanger made any errors (which can happen even to the best professionals).
A heat exchanger for underfloor heating is a solution that in many installations significantly improves the safety and flexibility of the heating system. It allows separation of hydraulic circuits, stabilizes system operation and enables integration of different heat sources. A properly selected and installed plate heat exchanger can significantly increase the efficiency of underfloor heating and ensure reliable operation for many years.
However, the question should not really be “is it worth it” – because a heat exchanger is not a magical component that automatically improves heating performance in every installation. A heat exchanger is a specific solution used to provide hydraulic separation – and it is applied only when there are clear technical reasons for it. In those cases it is essential. In systems that do not require hydraulic separation, it is generally unnecessary.
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