Connecting a gas boiler to an open system via a plate heat exchanger

✅ Connecting a gas boiler operating in an sealed system to an open installation via a plate heat exchanger

How is a gas or condensing boiler, which as we know should operate in a closed system, connected to an open installation via a plate heat exchanger? We discuss all the technical and practical aspects related to this, including how to select a heat exchanger for a condensing boiler, what to watch out for when connecting it, and we provide basic information about connecting an open system and a closed system within one central heating setup.

• BLOG ➡️: If you need to learn more about what a sealed and open central heating system is, check out our other posts.

▶️ We note that in the article, we use the terms "gas boiler" and "gas furnace," even though the correct technical term is only "gas boiler." However, in everyday language, the incorrect term "gas furnace" is often used as well.

Gas boilers are intended for closed-loop systems

Gas boilers, as modern devices equipped with temperature sensors, are relatively delicate compared to old cast iron boilers or even slightly newer coal boilers. Therefore, gas boilers are designed to work in a closed-loop system to ensure their safety and isolate them from contaminants that naturally occur in open systems. This also separates the gas boiler from air access, which happens in an open installation (open-type expansion vessel), thereby reducing corrosive properties and slowing down the formation of limescale – the main enemy of efficient and effective heating systems.

Warranty for gas boilers is generally conditional on operation in a closed-loop system

Practically every gas boiler manufacturer states in their warranty conditions that a gas boiler must be used in a closed-loop system. This means that installing it in an open system essentially voids the warranty. Therefore, it is not worth doing this, especially since there are ways to avoid it even if you have an open domestic installation. In this case, we do not mean converting the entire installation to a closed type (this often requires significant financial outlays and a kind of revolution in the house), but rather methods to connect both systems in a relatively inexpensive way using a plate heat exchanger for the gas boiler.

Below, we provide more information on this topic.

Plate heat exchanger for connecting a gas boiler to an open vented system – a handful of information

A heat exchanger is a device created to transfer heat from one medium to another. There are, of course, many types of heat exchangers – for example, the coil in a domestic hot water tank, shell-and-tube heat exchangers, JAD exchangers, etc. A plate heat exchanger is used to connect a gas boiler to an open vented system because it works best for this application, has small dimensions, and is reasonably priced.

The entire principle of a plate heat exchanger’s operation is that hot medium from the closed-loop system, where the gas boiler operates, flows through it – and, on the other side, the cold medium from the open vented system, where the heat receivers are working – more simply put: the radiators. Of course, these media do not mix inside the exchanger, but the heat from the closed-loop system transfers to the open system, which heats up by passing the temperature to the radiators. In this way, the plate heat exchanger allows the gas boiler to be connected to an open system, although it itself remains in the closed-loop system. This will be shown more clearly in the diagram of connecting the gas boiler to the open system using the exchanger, included later in the article.

• ☑️ BLOG: How a plate heat exchanger works – the principle of operation in practice

Plate heat exchanger – for connecting a gas boiler to an open system can also work in the opposite direction, i.e., transferring heat from the open vented system (if there is a heat source in it, such as a coal boiler) to the closed-loop system (if our radiators or underfloor heating operate in a closed installation). In the photo, the Ba-16-50 plate heat exchanger by Nordic Tec – with thermal insulation – capable of handling standard-sized gas boilers with a capacity of up to 24 kW.

How to select a heat exchanger to connect a gas boiler to a vented system?

This is a key issue – for everything to work correctly and for the gas boiler to operate economically, it is necessary to properly select the heat exchanger, which, as practice shows, many professionals unfortunately cannot do. The selection of a heat exchanger for a gas furnace should be based on the relationship between the surface area of the exchanger’s plates and the furnace’s power, expressed in kW (this means that ❌ the selection of a heat exchanger for a gas furnace should not be based on comparing the exchanger’s power and the furnace’s power).

Nordic Tec suggests that for heat exchangers of this brand, 0.30 m² of plate surface should correspond to 10 kW of furnace power, or slightly less (0.25 m²) in the case of central heating systems in relatively good condition (although this is a rarer situation, as open central heating systems in our country are generally quite worn out).

✅☑️ The above ratio of 0.30 m² of plates/10 kW of gas furnace power works for Nordic Tec heat exchangers. It may not necessarily be compatible with heat exchangers from other manufacturers (heat transfer plates may have different degrees of corrugation, different thicknesses, and their efficiency is not always the same).

For example, for a gas furnace with a power of approximately 15 kW, a heat exchanger with a plate surface of at least 0.45 m² should be selected.

✅ We have described this matter in more detail in the following posts:

• ☑️ BLOG: How to select a heat exchanger for a gas furnace?
• ☑️ BLOG: Which plate heat exchanger to choose for a 20 kW furnace?

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Other aspects of selecting heat exchangers for gas furnaces / boilers

In addition to the heat exchange surface, attention must certainly be paid to the connection size of the exchanger so that it is reasonably compatible with the gas furnace and the installation. Usually, the connection size on the receiving installation side to which the gas furnace is connected should be considered – in single-family houses with open systems, it is typically 1 inch or 1 1/4 inch (although "typically" does not necessarily mean "always"). On the gas furnace side, it will generally be necessary to use, for example, a reducer, but these are relatively simple matters that do not pose any particular difficulties.

Does a heat exchanger for a gas boiler need thermal insulation?

In addition, it is worth considering insulation for the heat exchanger if it is intended to be used with a gas boiler. Gas boilers are low-temperature heat sources – the fact that we can set the furnace temperature even 1–2 degrees lower will already make a significant difference in gas combustion (and thus in our heating bills). However, this requires an efficient heating system that does not have pointless heat losses. Meanwhile, an uninsulated plate heat exchanger installed somewhere in a basement or boiler room unnecessarily releases heat like a small radiator. To produce this heat, gas is burned, which you pay for. The cost of insulation is 28–45 €, and under today’s conditions, it pays for itself quickly.

• ☑️ BLOG: When is it worth using insulation for a heat exchanger?

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Diagram of connecting a gas boiler to an open system with a plate heat exchanger

The diagram showing the placement of the gas boiler in the system we are describing should clarify most doubts. Please note that it is simplified and serves only as a conceptual diagram for connecting an open system and a closed-loop system within a single central heating setup.

At the central point of the diagram is the plate heat exchanger, which connects both parts of the system. It can be said that the exchanger marks the boundary between them – everything to the left of the exchanger is the closed-loop section, where the gas boiler operates. Everything to the right is the open system, where the radiators are located.

Below is an explanation of the individual components of the diagram:

1. Gas boiler operating in the closed-loop system
2. Circulation pumps, in this case, one on each side
3. Plate heat exchanger ensuring heat transfer from the gas boiler
4. Receiving system, meaning the radiators

The connection diagram with a gas boiler and a plate heat exchanger is very simplified and can be subject to various modifications depending on the specifics of the building and the central heating system. For example, it can be added that the gas boiler does not have to be the only heat source – in many cases, the building owner wants to keep a coal boiler (commonly called a "junk boiler") as a backup, which, as we know, should operate in an open system. In this situation, the system would look as shown below.

Diagram of connecting a gas boiler and a coal boiler using a heat exchanger

The diagram of connecting a gas boiler and a coal boiler with the key role of the heat exchanger would look as shown below. Essentially, it should be noted that the conceptual diagram could be similar in many cases. Instead of the gas boiler (1), any heat source operating in a closed-loop system can be used – and the diagram would look the same. This could, for example, be a heat pump or an electric boiler. On the other hand, instead of the coal boiler (4), it could just as easily be a fireplace with a water jacket in an open system.

Below is a description of the individual points of the connection diagram:

1. Gas boiler in a pressurized, closed-loop system
2. Circulation pumps supporting water circulation
3. Plate heat exchanger facilitating heat transfer from the gas boiler
4. Additional heat source in an open system, in this case, a coal boiler
5. Heat receiving system / radiators

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Connection of a gas boiler – is there any heat loss on the heat exchanger?

This question sometimes arises in online discussions, where so-called “experts” claim that using a heat exchanger results in a 5 or even 10°C heat loss. For example, if the boiler operates at 50°C, the receiving system would supposedly receive only 40°C. This is, of course, nonsense and has no technical justification – unless the heat exchanger selected is too small and cannot handle the heat transfer. However, this is a case of incorrect device selection, which is technically flawed by nature. Similarly, a gas boiler that is too small will not handle a large building, but that says more about improper equipment selection than the boiler itself.

With the correct selection of a heat exchanger and proper device connection, the loss is negligible, and in a functioning system, it drops below 1°C. If we break this myth down, we quickly realize that if, for any reason, a heat exchanger caused a loss of 1/5 of the temperature, such a solution would not survive on the market – nobody would use it. Meanwhile, it has been commonly used for decades and has become even more popular over the last 20 years.

The mythical loss on a plate heat exchanger can only occur temporarily during the startup of a cooled system, for example, when the heating has not been in use for several days in the middle of winter (in this case, water returning from the heating system is quite cold and consumes slightly more heat). After a few hours, the system balances itself, and the loss disappears – this is the law of physics, and there is nothing surprising about it.

More observant readers should also note another absurdity contained in this myth from the point of view of physics. Namely, what happens to this supposedly significant amount of thermal energy when the exchanger miraculously cuts 10°C? Where does it go, since, as we know, nothing disappears in nature? How could a heat exchanger weighing, for example, 3–4 kg consume as much thermal energy as half of our house? It hardly needs to be explained that this is an unreasonable claim – which can be compared to the persistent myth that homes equipped only with underfloor heating, without radiators, are cold.

Installing the heat exchanger – what to pay attention to

Installing a heat exchanger is a relatively simple task, but a few issues must be addressed, and attention should be paid to the following:

• First and foremost, the plate heat exchanger should be installed vertically. If it operates horizontally, it will become air-locked, and it should especially not work with its connections pointing upward.
• The plate heat exchanger is generally a counterflow device and must be connected as such – in counterflow. This means that the liquid on one side of the exchanger flows from top to bottom, while the liquid on the other side flows in the opposite direction. If the counterflow setup is forgotten (which happens to installers), the exchanger will lose 30–40% of its efficiency – enough to prevent the system from functioning satisfactorily.
• The heat exchanger should be installed so that it is accessible and can be disconnected from the system. It is advisable to install shut-off valves (ball valves) on both sides, before and after the exchanger, so that it can be isolated and removed, even years later, for cleaning purposes. The ability to isolate it means that you will not need to drain the water from the entire system.

It is also important that the heat exchanger is installed securely, in a way that protects it from vibrations. It is worth considering the use of mounting kits for heat exchangers offered by Nordic Tec:

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• ▶️ BLOG: Detailed description of plate heat exchanger installation

Does a gas boiler work better with a heat exchanger?

That depends on many factors – but in many configurations, yes. When we say that a gas boiler can work better with a heat exchanger, we mean that by dividing the system into two circuits, a relatively short loop is created for the gas boiler. This eliminates the pressure problem in the boiler circuit, which occurs when this loop is too extensive. Pressure issues cause frequent shutdowns of the gas boiler, as well as cooler return water (not to mention insufficient heating in certain rooms of the house). This is why longer boiler loops sometimes require the additional use of a hydraulic separator (also called a low loss header) for central heating.

• ▶️ If you want to learn more about hydraulic separators, visit our BLOG: What is a hydraulic separator (low loss header), and how does it work?

When using a heat exchanger, it takes over the function of the hydraulic separator – meaning the gas boiler automatically gains a short, pressure-stable pump loop without a cold return. In practice, this means that the boiler can operate in its most economical mode, where it heats the medium only by a few degrees Celsius, but does so in a more stable and continuous manner. This often results in noticeably lower gas consumption, which translates to lower bills. It is therefore a solution worth considering if, at this stage, we are not able to undertake a thorough renovation of the installation and convert it into a closed system.

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