A dual-function boiler in a heating system allows for thermal comfort in the home, and additionally also for hot utility water. This is a practical solution that allows for maximum use of the possibilities. How to install a dual-function boiler and what are the differences between an open and closed system?
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Diagram of a central heating installation with a dual-function boiler
A dual-function boiler is a very functional solution that can heat your home and provide hot water. It is most often installed in a boiler room, basement or kitchen. The installation, in addition to the boiler itself, also includes radiators and pipes connecting the entire system. Due to the fact that water constantly circulates in the system, copper, steel or plastic pipes are used to make the heating system. These materials help avoid problems resulting from corrosion. Depending on the specifics of the installation, models with a diameter of 15 mm or 22 mm are most often used. In turn, radiators installed in rooms are steel, cast iron or aluminium models.
The entire installation consists of specific components: a circulation pump, valves, an expansion tank and thermostats, and each of them performs specific tasks. Depending on the installation location, different types of valves are used in the system, including: shut-off and control valves, as well as a safety valve located near the boiler. Thermostats are placed in individual rooms and help to set the desired temperature very precisely. If the installation also provides a demand for domestic hot water from the tank and the pipe for domestic hot water. The tank can be built into the boiler or as a separate device.
In installations with a dual-function boiler, the heating system is powered by the appropriate fuel. Depending on the variant, it can be gas, oil or pellets. In the system, a circulation pump is located on the return from the radiators. It is an element of the heating circuit, which also includes radiators and pipes. These elements are connected to each other, creating a tight system in which water circulates. The hot temperature from the boiler allows the heat to be released through the radiators to the rooms. The water cooled in this way returns to the boiler. At the highest point of the installation, there is a so-called expansion tank, while in the DHW tank - warm water, heated by the boiler.
What is the difference between an open and closed central heating installation?
In practice, the central heating system can be made in two variants, as an open or closed system. The differences between them are observed in the context of the pressure inside the system, the control process and the method of protection against its excessive increase are different.
In an open system, an open expansion tank is used. It is installed at the highest possible point of the installation, usually in the attic. When the water in the system increases in volume, as it does, for example, as a result of heating, it then rises to the expansion tank, from where it can freely pour out, while the water returns via a gravity siphon.
The open system uses natural circulation and gravity, which means there is no need to install a circulation pump. Due to the fact that the expansion tank is open, allowing water to be poured out when the pressure increases significantly, the system is safe. Such solutions are used, among others, in the case of boilers for solid fuels, e.g. wood, which lead to an increase in pressure much more often. In open systems, it is much easier to maintain control over it. The risk of a potential explosion caused by overpressure is therefore negligible.
In a closed system, a so-called expansion tank (hermetically sealed) is used. It is most often located in a boiler room or basement, near the heating boiler. When the water in the system expands, its excess is compensated by a gas, usually nitrogen, located in a flexible membrane in the expansion tank. The system is equipped with a safety valve. When the pressure is too high, they are responsible for automatic release of the liquid to prevent explosion and other failures. Circulation pumps are used in every closed system. Hermetic closure of the system makes it impossible to use the features of gravity. This is the so-called forced circulation of water in the heating system. Due to the fact that the whole is tightly closed, there is no chance for air to get into the system, which in practice eliminates problems that could be caused by corrosion. Closed circulation is most often used in heating systems using a gas, oil or electric double boiler.
Heating system design depending on the installation elements used
In a central heating system, its structure and the elements used depend on, among other things, the type of system (closed/open) and the heat source used, e.g. whether these are gas boilers or a solid fuel furnace. The scheme takes into account many individual factors that can be consulted with the installer before installation. The specialist will help you choose the optimal solution, tailored to your needs.
Depending on the individual situation and possibilities, you can choose gas condensing dual-function boilers or a single-function gas boiler, remembering that central heating should be efficient and effective. It is also worth checking whether the dual-function gas furnace that will be installed can use modern solutions, such as a heat pump. If so, it is worth deciding on modern heating.