How to connect a solid fuel boiler to a sealed heating system?

Solid fuel boilers, so - coal boilers, wood boilers etc. after a period of decline, are proving to remain quite popular. A significant number of people, when modernizing their central heating systems—converting them into more efficient sealed systems and often opting for the installation of heat pumps or gas heating—still consider keeping a solid fuel boiler as an emergency heat source. The motivations vary, but it is usually a precaution against the return of harsh winters, which makes sense.

The main issue is the recommendation that a solid fuel boiler should operate in an open system. This is a safety requirement, and as a heat source with high inertia, its installation in a sealed system is not a good idea.

➡️ How to connect a solid fuel boiler to a sealed central heating system?
➡️ What are the basic differences between a sealed system and an open system?
➡️ Why is it better to leave a solid fuel boiler in an open system?
➡️ What is a heat exchanger for a solid fuel boiler, and how to choose one?

What is the difference between an open and sealed system in central heating?

An open system is, in short, the type of central heating system that was standard in the past—where the water circulating in the pipes has contact with the air supply. This air contact occurs via an expansion vessel, which is located at the highest point of the system (usually an attic, loft, etc.). The air supply makes the system more prone to limescale formation, which is why today open systems are being replaced by sealed systems, which are healthier, lose efficiency more slowly over the years, and accumulate limescale more slowly. However, open systems have certain advantages—they are safer for high-inertia heat sources, such as those based on open flames: coal boilers, wood boilers, solid fuel boilers, as well as fireplaces with water jackets, etc. Modernizing heating in a building often means replacing an old open system with a newer sealed system. Modern heat sources, such as gas boilers and heat pumps, can only operate in sealed systems, and their manufacturers do not provide warranties for operation in open installations.

This brings up the problem of how to modernize a system—to have a sealed installation while maintaining the ability to connect a solid fuel boiler or fireplace, i.e., heat sources that require operation in an open system.

A sealed system is healthier for the central heating itself and ages more slowly

A sealed system, sometimes called a pressurized heating system, does not have contact with air and does not have an open-type expansion vessel. This makes it healthier, as the processes of limescale deposition in the installation are minimized. However, a sealed pressurized installation should not support solid fuel boilers—in contrast to open systems, they could overheat, cause dangerous pressure increases (with no outlet), and damage the system. Meanwhile, "gentler," newer heat sources such as condensing boilers or heat pumps can operate in sealed systems without the risk of overheating. These devices are equipped with sensors that protect the system—they shut down once the set temperature is reached.

• ✅☑️ BLOG: More information about the differences between open and sealed systems

Solid fuel boilers can be installed almost exclusively in open systems

A solid fuel boiler, due to its specific characteristics, should remain in an open system with an expansion vessel. Installation in a sealed pressurized system is legally prohibited unless the boiler manufacturer explicitly specifies other possibilities. If the house has a sealed system with radiators or underfloor heating, and we also want to keep a solid fuel boiler for emergency situations, a slightly shorter open system must be created to accommodate the boiler. This segment of the open system must be connected to the sealed system in a way that allows the transfer of heat from the boiler to the radiators or underfloor heating. This is done using a heat exchanger.

Connecting a solid fuel, wood or coal boiler to a sealed system using a heat exchanger - PHE

A plate heat exchanger (or just PHE) is a small device made up of a pack of brazed steel plates with two flow channels. One channel will be directly connected to the open circuit of the solid fuel boiler, wood boiler, coal boiler etc. The other will be connected to the receiving installation—a sealed pressurized system. When the flow is initiated on both sides, the liquids will not mix, but heat will be exchanged between them.

• ✅☑️ BLOG: The principle of operation of a plate heat exchanger

Connecting a solid fuel boiler to a sealed system using a heat exchanger is not complicated, but it needs to be carefully planned. Proper selection of the heat exchanger is also crucial to ensure it is suitable for the solid fuel boiler.

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Diagram of connecting a solid fuel boiler through a heat exchanger

Below, you will find a simplified diagram of such an installation, featuring a plate heat exchanger used to connect two systems: a sealed system and an open system. Connecting the boiler through the exchanger is straightforward—everything located to the left of the heat exchanger is the open system, secured with an expansion vessel, where the solid fuel boiler operates.

Everything located to the right of the heat exchanger constitutes the sealed system, which in this case receives the heat.

The diagram is simplified—it can be significantly expanded. For instance, in the sealed section on the right, an additional heat source can be integrated (designed specifically for operation in a sealed system). This could be a heat pump, a gas boiler, an electric boiler, etc.

Diagram of connecting a solid fuel boiler through a heat exchanger to a sealed system

Elements of the open central heating system from the connection diagram:

1. Solid fuel boiler / Wood boiler / Coal boiler
3. Circulation pump
6. Expansion vessel (open-type)
4. Plate heat exchanger (PHE)

Elements of the sealed central heating system:

4. Plate heat exchanger - PHE
3. Circulation pump
5. Heat distribution system, meaning underfloor heating or radiators

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Selecting a heat exchanger for a solid fuel boiler

The proper operation of such a system requires a correctly chosen heat exchanger for the solid fuel boiler. A common mistake (even made by professionals) is selecting a heat exchanger that is too small, which results in the inability to transfer sufficient heat to the heating system—or even overheating the circuit of the solid fuel boiler.

• ✅☑️ BLOG: How does an undersized heat exchanger manifest in practice

Another frequent error is selecting a heat exchanger with the same power output (expressed in kW) as the boiler. However, the only truly effective parameter determining the performance of a plate heat exchanger is the surface area of its plates.

Nordic Tec, as a supplier of heat exchangers, recommends adhering to a basic guideline: 0.3 m² of plate surface area per 10 kW of boiler output. This essentially guarantees proper operation for many years. Of course, this calculation is specific to Nordic Tec plate heat exchangers and may vary slightly for other brands (as the plates may differ in their heat transfer coefficient).

Is it possible to install a solid fuel boiler in a sealed system?

For the reasons mentioned above, it is advisable to avoid installing solid fuel boilers in sealed systems. The law regulating this matter in many countries permits this option, but it is subject to additional requirements, such as the installation of additional safety features. These include, for example, an additional cooling valve or the installation of an additional heat buffer.

Very often, it turns out that the simpler solution is to install a plate heat exchanger, provided it is appropriately selected.

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