difference between non-mixing and mixing boiler sets

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Choosing the correct boiler assembly is a key decision when installing a new heating system, which will affect not only its functionality, but also long-term maintenance and safety. The difference between an unmixed and a mixed assembly lies in the presence or absence of control elements that change the temperature of the heat carrier before it enters the radiators or floor heating.

Operating principle of an unmixed assembly

An unmixed boiler assembly functions as a simple connecting body, where the water temperature from the boiler flows directly into the heating units without any mixing with return water. This configuration is ideal for systems designed to operate at high temperatures, for example with traditional cast iron boilers or with radiator heating that requires a temperature of 70 to 80 °C. The system operates on a single circuit principle, which means that the pump in the assembly must provide sufficient pressure to overcome the resistance of the entire system at maximum supply temperature.

The simplicity of this solution is reflected in the lower cost and fewer components that could potentially fail in the future. For common domestic applications, where it is not necessary to limit the supply temperature, models such as Boiler module HEAT BLOC K31 unmixed DN 32 without pump are often selected, which provide a solid basis for the installation of a pump according to precise calculations. However, such assemblies are sensitive to excessively low return water temperatures, which can lead to condensation in the boiler if the boiler is not designed for low-temperature operating mode.

Construction and function of a mixed assembly

A mixed boiler assembly includes a three-way valve or mixing valve that actively regulates the temperature of the water entering the heating system. The valve mixes hot water directly from the boiler with cooler return water coming back from the radiators, thus achieving an optimal and constant supply temperature that is lower than the temperature generated by the boiler. This system is essential for modern floor heating, which typically operates at temperatures of 35 to 45 °C, or for systems with thermostatic heads, which require a stable temperature for proper function.

The main advantage of this technology is the protection of the boiler from condensation and increased efficiency of the entire system, as the boiler can operate at its most efficient temperature, while the system receives water at a set temperature. When selecting such an assembly, it is necessary to pay attention to the correct pipe size and pump capacity, so complete packages such as Boiler module HEAT BLOC K31 unmixed with Grundfos pump are often used, although for mixed assemblies, specific wiring with a bypass valve or an integrated three-way valve is required.

Criteria for selecting the type of assembly

When deciding between these two types, the decisive factor is the type of heating system and its temperature requirements. If you have a classic system with a cast iron boiler and standard panel radiators, an unmixed assembly is the most economically advantageous choice, as it eliminates the need for expensive control valves and more complex electronics. On the contrary, when combining a condensing gas boiler with floor heating, an unmixed assembly is technically unsuitable, as it would cause damage to the boiler and non-functionality of the floor due to excessively high temperature.

Necessity of temperature regulation for floor heating (max. 55 °C).
Protection of the condensing boiler from excessively low return water temperature.
Ability to independently regulate temperature in different zones of the house.
Lower cost and simpler installation for unmixed versions.
Smaller space requirements and higher reliability when removing valves.

In practice, we often encounter situations where the customer wants to save money at the beginning, but then has to return due to insufficient performance or damage to the equipment. Therefore, it is always advisable to consult the project with a specialist who takes into account the heat losses of the building and the type of heat sources, so that you can choose the correct model, for example Unmixed boiler module with DAB EVOSTA 3 pump.

Differences in installation and operation

The installation of an unmixed assembly is technologically simpler and requires fewer steps, which shortens the installation time and reduces the risk of error during pipe connection. The pump is installed on the return pipe and draws water from the system, which it then pumps back into the boiler, with the entire assembly functioning as one closed loop without the need for external control. This means that even in the event of a power outage, the system remains in passive mode, which can be critical in winter months if the boiler does not have an emergency circulation.

Mixed assemblies require precise setting of the three-way valve, which must respond to signals from the thermostat or control unit. Installation includes connecting the actuator, calibrating the valve position, and often also installing a bypass valve that ensures bypassing if the valve is fully open. It is important to follow the manufacturer's instructions, for example with boiler assembly with bypass valve, to avoid hydraulic problems and excessive pump load when the valve is fully open.

Maintenance and Common Faults

Unmixed circuits have a minimal number of moving parts, which means their maintenance usually reduces to regular pump inspection and filter cleanliness. The only place where a problem may occur is the pump itself or the shut-off valves, which may leak due to worn seals. Due to the absence of complex mechanics, this type of circuit is considered very reliable and suitable for systems where dynamic temperature change is not required.

Mixed circuits require more regular inspection, especially of the three-way valve and the actuator. A common fault is the valve jamming due to lime deposits or wear of the valve stem, which causes the valve to stop changing the temperature and the system either fails to operate or delivers water that is too hot. Another problem may be the incorrect function of the bypass valve, which should ensure the bypass when the three-way valve is fully open, which is critical for protecting the pump from overheating. Regular inspection of these components is essential for the long life of the entire circuit.

Economic Return and Energy Efficiency

From the point of view of energy efficiency, the mixed circuit is of crucial importance, especially for condensing boilers, which achieve maximum efficiency at low return water temperatures. If you were to use an unmixed circuit with a condensing boiler and radiators, the boiler would operate in a less efficient mode and condensation could occur, which would shorten its lifespan. A mixed circuit allows the boiler to operate at an optimal temperature and at the same time delivers water at an ideal temperature to the system, which leads to gas savings and a longer lifespan of the equipment.

On the other hand, unmixed circuits are cheaper in the investment phase and have no additional electrical accessories that consume energy. For older systems where it is not possible to change the temperature regime, an unmixed circuit is often the only possible solution. The choice depends on the specific project, but in 90 % of new builds and renovations with floor heating, a mixed circuit is technically and economically more advantageous in the long run.

Hydraulic Separation and Boiler Safety

Mixed circuits with a bypass valve (so-called bypass) solve the problem when the three-way valve completely closes the flow to the heating units – in such a case, the water returns directly to the boiler, thus preventing the pump from overloading and ensuring continuous heat circulation.

Impact on Hydraulic Stability of the System

The presence of a mixing valve and the associated pumps creates two hydraulically separated circuits in the system, which eliminates the risk of mutual pressure influence when thermal loads change in different parts of the house.

Common Questions

Can I use an unmixed circuit for floor heating?

No, a mixed circuit is essential for floor heating, which is able to reduce the water temperature to 35 to 45 °C, because an unmixed circuit would deliver water at boiler temperature, which would damage the floor.

How often should the three-way valve be checked?

The three-way valve should be checked at least once a year, especially before the start of the heating season, to prevent it from jamming or the actuator from failing.

Is it better to buy a pre-assembled unit or a modular one?

Pre-assembled units are often cheaper and quicker to install, but modular units allow for better adjustment according to the exact parameters of your system, which is important for specific requirements on pump performance.

The summary of all the above facts indicates that the correct choice between a non-mixing and a mixing boiler set is a crucial step for the efficient and safe operation of your heating system. The right choice not only ensures comfort in your home, but also extends the life of your expensive boiler and minimizes maintenance and energy costs over the coming years.

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