Vaillant VDM 20 M mixing pump group

Product detailed description

Vaillant VDM 20 M mixing pump group – precise temperature control for low-temperature circuits

Vaillant VDM 20 M mixing (order number 0020191813) is an original pump group with a three-way mixing valve, designed for heating circuits that require a lower and precisely regulated water temperature than the temperature leaving the boiler – typically underfloor heating, low-temperature radiators or air-handling registers. It is part of the modular VDM pump group range, which together with Vaillant manifolds and hydraulic separators forms a complete modular system for multi-circuit heating with ecoTEC plus, ecoTEC pro and ecoTEC exclusive condensing boilers.

Technical design and package contents

• High-efficiency electronic circulation pump of energy efficiency class A, 230 V, with automatic speed control according to operating mode (constant or proportional pressure)
• Three-way mixing valve with 230 V electric servo motor, Rp 3/4˝ connection, flow coefficient Kv = 6.3 – the valve gradually blends cooler return water from the circuit with the warmer water from the boiler circuit (or vice versa, as needed for cooling), thereby maintaining the set outlet temperature to the secondary circuit
• Two round thermometers for immediate visual monitoring of the flow and return temperature
• Complete moulded EPP insulation covering the pump, valve and fittings
• Shut-off ball valves for easy servicing of the circuit
• Mounting spacing of 120 mm, compatible with WHV manifolds and WH hydraulic separators

How the mixing group works

Unlike the direct group VDM 10, which only ensures water circulation without changing the temperature, the VDM 20 M mixing group actively regulates the temperature of the water entering the secondary circuit. The servo motor of the three-way valve is controlled by a room or weather-compensated controller (for example based on outdoor temperature and a set heating curve), which continuously opens or closes the supply of hot water from the boiler circuit while simultaneously blending in return (cooler) water from the secondary circuit itself. The result is a stable, smoothly regulated temperature at the outlet of the pump group towards underfloor heating or another low-temperature appliance, independent of the current boiler outlet temperature. This control is particularly important for underfloor heating, where the maximum floor surface temperature must be respected (typically 27–29 °C in living rooms) while preventing damage to sensitive floor coverings such as wood or laminate from prolonged exposure to overly hot water in the pipes.

Rp 3/4˝ and Kv = 6.3 – what these parameters mean in practice

The Kv value (flow coefficient) expresses the flow rate of water in m³/h that passes through the valve at a pressure drop of 1 bar with the valve fully open. Kv = 6.3 with an Rp 3/4˝ connection places the VDM 20 M among groups for medium-sized low-temperature circuits – typically suitable for common family houses with one to two underfloor heating zones or a combination of underfloor heating and low-temperature radiators. For circuits requiring higher flow capacity (larger heated area, more parallel zones connected to one manifold), the VDM 25 M variant with an Rp 1˝ connection and Kv = 8.0 is more suitable, as it handles a higher flow rate at the same pressure drop.

Installation and commissioning

The group is connected to a Vaillant manifold or hydraulic separator (WHV 35, WH 27/40/95/160) via a screw connection with 120 mm spacing. During installation it is important to:

• Check the flow direction according to the markings on the pump and valve body
• Connect the servo motor of the three-way valve to the output of the controller (room, weather-compensated or system controller compatible with Vaillant, e.g. sensoCOMFORT or sensoNET)
• Set the heating curve and the maximum circuit outlet temperature directly in the controller according to the type of appliance (e.g. limiting to 40–45 °C for underfloor heating)
• Thoroughly vent the circuit after the first filling, as air in the system causes pump noise and improper operation of the mixing valve
• Check the rotation direction of the valve servo motor at first start-up – incorrect wiring may cause reverse control (closing instead of opening)

Compatibility and use in the system

The VDM 20 M is designed to work with Vaillant ecoTEC plus, ecoTEC pro and ecoTEC exclusive condensing boilers (including newer models with IoniDetect technology) and with sensoCOMFORT, sensoNET and multiMATIC controller ranges. The 120 mm mounting spacing allows combination with other VDM 10 and VDM 25 M groups as well as WHV and WH range manifolds and hydraulic separators, forming assemblies with two or more circuits. For customers who need a complete dual-circuit system solution, we also offer ready-made VDM assemblies containing a manifold with a hydraulic separator and two pump groups in one package.

Maintenance

The mixing group requires slightly more attention during servicing than the direct group due to the presence of a moving three-way valve:

• Check the smooth operation of the servo motor once a year (no sticking or unusual noises)
• Check the tightness of the valve and connections, especially after the first heating season
• Check the pump function and, in case of summer shutdown, briefly run it to prevent the rotor from getting stuck
• Monitor the thermometers – if the outlet temperature does not match the set curve, this may indicate a fault in the servo motor, sensor or controller
• In areas with harder water, consider installing a softening cartridge to protect the valve and pump from limescale deposits

Why choose an original Vaillant group

The mixing group is a more complex component than the direct group, as it also includes a mechanical valve with a servo motor – this is where the advantage of an original, manufacturer-tuned solution shows most. The Kv value, the valve's authority ratio and the servo motor speed are designed to work together with the control logic of Vaillant boilers and controllers without the need for additional adjustment or compromise. The white housing and uniform insulation also ensure a visually consistent installation that looks professional even in a visible utility room.

Future system expansion

The modular concept with 120 mm spacing allows you to gradually expand the heating system even years after the original installation – for example when adding an extension, changing the layout, or adding underfloor heating to a part of the house that originally had only a radiator circuit. Simply choose a manifold or hydraulic separator with a sufficient number of circuits during the initial installation, or leave a spare connection point, and add another VDM 20 M, VDM 25 M or VDM 10 pump group as needed, without having to interfere with the existing circuits.

Frequently asked questions

• What is the difference between VDM 20 M and VDM 25 M? The VDM 20 M has an Rp 3/4˝ connection and Kv = 6.3, while the VDM 25 M has a larger Rp 1˝ connection and a higher flow capacity of Kv = 8.0. The choice depends on the required circuit flow rate – for larger underfloor heating areas or more zones, we recommend the VDM 25 M.
• Can I use the VDM 20 M for a radiator circuit as well? Yes, the mixing group can also be used for radiators if you need a lower temperature than the boiler circuit, for example with low-temperature radiators or when separately controlling individual zones of the house.
• Is a separate controller required to control the valve servo motor? Yes, the servo motor of the three-way valve is connected to the output of a room, weather-compensated or system controller, which controls its position based on the set curve.
• What does the Kv value on the valve mean? Kv expresses the flow coefficient of the valve – the flow rate in m³/h at a pressure drop of 1 bar with the valve fully open. A higher Kv value means the valve can handle a greater flow rate at the same pressure drop.
• Do I need a separate manifold or hydraulic separator for this group? Yes, the VDM 20 M is a standalone pump group without its own manifold. For a multi-circuit system, a manifold or hydraulic separator is required, such as the WHV 35, or a complete assembly that already includes these components.
• What is the warranty on this product? We provide a standard statutory warranty of 2 years from the date of sale for the VDM 20 M mixing pump group.
• Can the VDM 20 M be added later to an existing single-circuit system? Yes, provided a compatible manifold or hydraulic separator with 120 mm spacing is available or added, the pump group can be installed later without interfering with the existing boiler circuit.

Energy savings thanks to the class A electronic pump

Similarly to the direct group VDM 10, the VDM 20 M also uses an electronically controlled wet-rotor pump of energy efficiency class A. In mixing circuits this feature is even more pronounced, as the flow through the secondary circuit changes not only depending on the thermostatic heads but also on the current position of the three-way valve. The electronic pump can smoothly track these changes and adjust its output accordingly, minimising unnecessary electricity consumption and system noise compared to older, unregulated solutions.

Typical use scenarios

In practice, the VDM 20 M is most often used in the following situations:

• Underfloor heating in one or more rooms of a family house, combined with a radiator circuit connected via a direct VDM 10 group
• Renovation of an older house, where a new low-temperature branch (underfloor heating in an extension) is added to an existing radiator system
• Heating of a swimming pool hall or conservatory, where a lower and stable heating water temperature is required
• Combination with a heat pump or solar system, where the mixing group regulates the temperature entering a low-temperature distribution system

Overview table – when to choose which type of VDM pump group

This table serves as a guideline when designing a system – the final sizing should always be based on a heating design for the specific building, taking into account the building's heat losses, area and type of individual heating circuits.

Noise level and operating comfort

One of the common causes of dissatisfaction with a heating system is the noise of pump groups, especially if the utility room is located close to living spaces (for example in an apartment or a smaller house without a separate boiler room). The VDM 20 M addresses this issue on several levels: the class A electronic pump is significantly quieter than older mechanically controlled units, as it avoids running at unnecessarily high speeds against partially closed thermostatic valves; the complete EPP insulation also acts as an acoustic damper, reducing the transmission of vibrations and noise to the surroundings. The servo motor of the three-way valve operates smoothly without abrupt movements, eliminating the typical "knocking" associated with cheaper or older mixing valves.

Impact on the lifespan of the heating system

Precise temperature control via the mixing group also has a direct impact on the lifespan of the underfloor heating itself or low-temperature distribution pipes. An unregulated or poorly set system that repeatedly delivers overly hot water into the pipes accelerates the ageing of plastic pipes (especially PE-RT or PEX types without an oxygen barrier) and increases the risk of oxygen diffusion into the system, which in turn contributes to corrosion of steel components (for example the boiler heat exchanger). A properly sized and set VDM 20 M mixing group eliminates this burden, thereby extending the lifespan of the whole system, including the boiler itself.

Additional ordering information

When ordering the VDM 20 M pump group, we also recommend considering whether your system already has a compatible manifold or hydraulic separator with 120 mm spacing installed. If not, atria.sk also offers, within the same product family, standalone manifolds (e.g. WHV 35), standalone hydraulic separators (WH range), as well as complete ready-made assemblies combining a manifold and two pump groups in one package – these are particularly suitable when building a new system from scratch, as they guarantee mutual compatibility of all components and are usually more cost-effective than purchasing individual parts separately.

Note: we always recommend consulting the number and type of pump groups with a designer or experienced installation technician, who will take into account the building's heat losses, the area of individual zones and the length of the pipe runs.