Hydraulic clutch in the central heating system - the most important information

It turns out that many people are not quite sure what a hydraulic coupling is and what its role is in central heating installations. We often receive emails or calls like: "The plumber said I need to install a coupling in my system, can you tell me what this coupling is for?"...

What is a Hydraulic Coupling for Central Heating? What Do You Need to Know?

A hydraulic coupling for central heating is a simple device used in modern heating systems, often in installations with gas boilers (condensing) or, for example, with heat pumps. Why with these devices? Because condensing boilers and heat pumps - especially require balanced pressures in the heating system to operate most efficiently and economically - in other words, the best. In these systems, the coupling divides the heating system into 2 smaller sub-systems, thereby balancing the pressure in the central heating system and ensuring the most efficient operation of these devices. It is particularly useful to divide the system in large buildings, where it becomes quite extensive, causing uneven flow rates. In this way, the hydraulic coupling definitely influences long-term savings, but let's be honest - it should be installed in systems where there are technical indications; it is not a way to reduce heating costs in every installation.

Balance of systems in terms of flows and pressures is not the only function of the coupling. As we mentioned, it divides the installation into two parts, creating something like a separate heating circuit for the heat source (such as a boiler). The second part constitutes the heated system. Therefore, the coupling ensures that water does not cool down so quickly in the short circuit - there is no so-called cold return, allowing boilers to operate better. As a result, it also balances temperatures. This means that it is sometimes desirable in installations with solid fuel boilers or pellet boilers. By preventing the cold return, hydraulic couplings also prolong the lifespan of boilers and are often recommended by manufacturers of heating equipment for this purpose.

So How Does a Hydraulic Coupling Differ from a Plate Heat Exchanger?

In the latter application, the coupling actually functions as a heat exchanger, but it is not. The difference lies in the fact that in the case of the coupling, the fluids of both sub-systems actually come into contact with each other and can mix. In the heat exchanger, there are not two sub-systems, but two fully separate systems (usually closed/open), as the heating system water does not mix with the water from the heated system in the exchanger. Therefore, the exchanger can connect two independent heat sources - which require separate systems (e.g., a gas boiler in a closed system with a fireplace with a water jacket in an open system). The exchanger thus has certain advantages over the coupling, although both devices have their proponents and advantages.

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How is a Hydraulic Coupling Constructed?

Most often, it takes the form of a steel tube, cylindrical or sometimes cuboid. It is installed in the boiler room, on the wall, to have a stable hanging position. There are couplings for horizontal mounting, although vertical ones are becoming increasingly popular, which also have better air venting capabilities - and separate accumulated impurities from the installation, which can be removed (vertical couplings usually have a drain valve at the bottom).

What Does a Hydraulic Coupling Have Inside?

Actually, nothing, it is only about making the coupling a place where water entering the coupling partially mixes with water from the so-called return. In this way, the pressure should balance. In many manufacturers, there is a metal mesh inside the coupling to better balance the flows. However, it is needed only if the coupling outlets for both sub-systems are directly opposite each other, and the water could flow through them freely. Couplings without a mesh - rely on a different solution - they have connections on both sides not opposite each other, so that supply and return water can mix independently of the situation. Therefore, the metal mesh inside the coupling is not always necessary, although its presence certainly does not hurt.

How Many Independent Heating Circuits Can Be Connected to a Hydraulic Coupling in Central Heating?

It depends on the type of coupling. Classic couplings have inputs for the heating circuit and the heated circuit, but there are also couplings to which several circuits can be connected, even several.

An important element of the coupling nowadays is also its insulation. The coupling is a long cylinder with a fairly large external surface. If we do not want it to function in the boiler room as an additional radiator (which we certainly do not need in the boiler room), the coupling should be insulated.

Below is a technical drawing (without dimensions) of a classic hydraulic coupling designed for vertical installation by NORDIC TEC. The drawing also shows an air vent valve at the top and a drain valve at the bottom for better orientation. As can be seen, the connections for the boiler circuit and the heating circuit are not opposite each other so that the water can mix and balance.

Below is the same central heating coupling with the possibility of connecting 2 heated circuits and the heating circuit.

We recommend Nordic Tec hydraulic couplings. We ensure good quality, thanks to the construction of solid steel, couplings from this brand are a purchase for many years.

PUH LEDNET / Nordic Tec Team