Vaillant VDM 25 M mixing pump group

Product detailed description

Vaillant VDM 25 M mixing pump group – higher flow for more demanding low-temperature circuits

The Vaillant VDM 25 M mixing pump group (order number 0020191788) is an original mixing pump group from the VDM range, designed for heating circuits with higher flow requirements than the basic VDM 20 M variant. Thanks to the larger three-way mixing valve connection Rp 1˝ and higher flow capacity Kv = 8.0, it is particularly suitable for larger underfloor heating areas, multiple zones connected in parallel to a single manifold, or buildings with higher heat loss where a larger volume of circulating water is needed while maintaining a low and precisely regulated temperature.

Technical design and package contents

• High-efficiency electronic circulation pump, energy efficiency class A, 230 V power supply, with automatic speed control (constant or proportional pressure depending on the selected mode)
• Three-way mixing valve with electric servo drive, connection Rp 1˝, flow characteristic Kv = 8.0
• Two round thermometers on both the supply and return for instant visual monitoring of operating temperatures
• Complete shaped EPP thermal insulation covering all components including fittings
• Shut-off ball valves for service shutdown without draining the entire system
• 120 mm mounting spacing, compatible with WHV manifolds and WH low-loss headers

Why choose the variant with a larger connection

When designing a multi-circuit heating system, one of the key parameters is the volumetric flow rate that a given circuit must be able to deliver without excessive pressure loss. While the VDM 20 M with a Rp 3/4˝ connection and Kv 6.3 is suitable for most typical family homes with one or two smaller zones, the VDM 25 M with the wider Rp 1˝ connection and higher Kv 8.0 value can easily handle:

• Underfloor heating over a larger area (typically over 100–150 m² of heated area within a single circuit)
• Multiple underfloor heating branches connected in parallel from a single manifold
• Buildings with higher heat loss, requiring a larger flow of heating water while maintaining a low temperature difference
• Combined systems where, in addition to underfloor heating, air handling heating coils with higher power requirements are also supplied

Choosing the right type of group already at the design stage saves both costs and energy in the future – an undersized group with a low Kv value would cause excessive pressure loss, noise and inability to deliver sufficient flow at low temperature in a larger circuit, which the controller would compensate for by raising the water temperature, thereby increasing the system's energy consumption.

Temperature control principle

Similar to the VDM 20 M, the VDM 25 M also regulates the temperature of the water entering the secondary (low-temperature) circuit using a three-way mixing valve controlled by a servo drive. The controller (room, weather-compensated or system controller, e.g. sensoCOMFORT or multiMATIC) controls the valve position based on the set heating curve and current outdoor or room temperature, gradually mixing cooler return water from the secondary circuit with warmer water from the boiler circuit (or from the manifold or low-loss header). The resulting output temperature from the group towards the underfloor heating or other low-temperature appliance is thus independent of the current boiler circuit temperature and remains within a safe, comfortable and energy-efficient range.

Installation and commissioning

The group connects to a Vaillant manifold or low-loss header (e.g. WHV 35, WH 40, WH 95, WH 160) via a screw connection with 120 mm spacing. Recommended installation procedure:

• Check the flow direction according to the marking on the pump and valve body
• Connect the three-way valve servo drive to the controller output and verify the correct control direction at first startup
• Set the maximum output temperature and heating curve of the circuit in the controller according to the type of appliance (typically 35–45 °C for underfloor heating)
• Thoroughly vent the entire circuit after filling the system, as residual air causes pump noise and incorrect control function
• For larger circuits with multiple branches, also check the flow balance of the individual manifold branches (hydraulic balancing) to avoid uneven heating of individual zones

Compatibility

The VDM 25 M is designed to work with Vaillant ecoTEC plus, ecoTEC pro and ecoTEC exclusive condensing boilers, including newer models with IoniDetect technology, as well as with sensoCOMFORT, sensoNET and multiMATIC controllers. The 120 mm mounting spacing ensures compatibility with other pump groups from the VDM range (VDM 10, VDM 20 M) as well as with WHV and WH manifolds and low-loss headers, allowing free combination of circuits with different temperature and flow requirements within a single system.

Maintenance

• Once a year, ideally before the heating season, check the smooth operation of the valve's servo drive and the pump itself
• Check the tightness of the screw connections and valve, especially after the first heating cycle
• Monitor the thermometers – an unusual difference between the set and actual temperature may indicate a servo drive or sensor fault, or improper hydraulic balancing of the circuit
• In areas with harder water, consider a softening cartridge to protect the valve, pump and boiler heat exchanger from deposits
• During summer shutdown, briefly run the pump to prevent the rotor from being blocked by deposits

Why choose the original Vaillant solution

For larger and more demanding circuits, where the VDM 25 M finds its use, the importance of component compatibility is even greater than in smaller systems. An incorrectly sized or incompatible valve from another source may cause inadequate control, increased noise or shortened component lifespan. The original Vaillant group guarantees that the valve's Kv value, the pump's performance curve, and the mechanical dimensions all correspond to the specifications of the entire system, including cooperation with controllers and boilers of the same brand.

Common mistakes in choosing flow capacity

According to the experience of installation companies, the most commonly underestimated aspect is the dimensioning of the mixing group's flow capacity for larger underfloor heating systems. If multiple branches with a combined flow rate exceeding the capacity of the chosen group are connected to a single manifold, the following problems occur:

• Increased pressure loss in the system, requiring higher circulation pump power and thus higher electricity consumption
• Uneven flow distribution between individual manifold branches, resulting in uneven heating of individual rooms or zones
• Increased system noise caused by water flowing through fittings with too narrow a cross-section
• Shortened lifespan of the valve's servo drive, which must operate against higher resistance than it was designed for

This is why atria.sk recommends basing the choice between VDM 20 M and VDM 25 M on an actual heating design that takes into account the sum of flow rates of all connected branches, rather than just a rough estimate based on the building's area.

Relation to the overall heating system concept

The VDM 25 M pump group is part of a broader Vaillant modular system of pump groups, manifolds and hydraulic low-loss headers, allowing a heating system to be tailored precisely to a given building. In practice, this group is often combined with a WH series hydraulic low-loss header, which hydraulically separates the boiler circuit from the secondary circuits and ensures that flow changes on the heating side do not affect boiler operation. For systems with higher flow requirements, where the VDM 25 M is typically used, it is also advisable to choose a low-loss header with sufficient flow capacity, for example the WH 95 with a flow rate of 8.3 m³/h instead of the basic WH 40 with a flow rate of 3.5 m³/h. For even larger buildings with residential or commercial character, the most powerful low-loss header WH 160 is also available, with a flow rate of 12 m³/h and DN 65 connection, capable of handling the simultaneous operation of multiple VDM 25 M pump groups on a single distribution system without risk of hydraulic overload.

Frequently asked questions

• When should I choose VDM 25 M instead of VDM 20 M? Choose the VDM 25 M with Rp 1˝ connection and Kv 8.0 for larger underfloor heating areas, multiple parallel zones on a single manifold, or higher building heat loss requiring a larger flow of heating water.
• Can the VDM 25 M be combined with the smaller VDM 20 M group in one system? Yes, both variants have the same 120 mm mounting spacing and can be freely combined depending on the needs of individual circuits within the same manifold or low-loss header.
• Does a larger valve connection affect the accuracy of temperature control? No, the accuracy of control depends mainly on the quality of the servo drive and controller, not the connection size. A larger connection only allows higher flow at the same pressure loss.
• Do I need a separate manifold or low-loss header for this group? Yes, the VDM 25 M is a standalone pump group without its own manifold. For a multi-circuit system, a manifold or low-loss header is required, for example the WHV 35, or a complete ready-made set.
• What service interval is recommended for the three-way valve? We recommend an annual visual and functional check as part of a regular heating system service inspection, usually carried out together with the boiler inspection.
• What is the warranty on this product? We provide a standard statutory warranty of 2 years from the date of sale on the VDM 25 M mixing pump group.

Comparison table – VDM mixing groups

Energy efficiency and long-term operation

Like other groups in the VDM range, the VDM 25 M uses a wet-rotor electronic circulation pump of class A with automatic speed control. For larger circuits, for which this group is intended, the savings compared to an older non-controlled solution are even more pronounced, as the difference in consumption between constant full-power operation and continuously regulated operation increases with the size and variability of the circuit load. In multi-zone underfloor heating, where individual zones are gradually shut off by thermostatic heads or room thermostats, the electronic pump can continuously reduce its output, saving electricity without loss of heating comfort.

Materials and durability

The bodies of the VDM pump groups are made of durable brass casting resistant to pressures and temperatures common in heating systems, with a surface treatment preventing corrosion. The thermal insulation made of shaped expanded polypropylene (EPP) not only reduces heat losses but, thanks to its closed cell structure, is also resistant to moisture and condensation that may occur on cooler pipe sections in low-temperature circuits. The round thermometers with a metal casing and readable scale remain functional and accurate even after years of operation, without the need for calibration.

Dimensional compatibility with existing installations

The 120 mm mounting spacing, which the VDM 25 M shares with the entire range of Vaillant pump groups, manifolds and low-loss headers, is also a common industrial standard used by other manufacturers of quick-install systems (for example, Meibes-type systems or similar). This means that in most cases this group can be retrofitted into an existing installation with compatible spacing, although we always recommend verifying the type of screw connection and connection size with the technician performing the installation before ordering.

Recommendations for designers and installation companies

When designing a system with the VDM 25 M, we recommend verifying the sum of the flow rates of all connected manifold branches and comparing it with the capacity of the chosen low-loss header or hydraulic balancer (e.g. WH 40 with a flow rate of 3.5 m³/h or WH 95 with a flow rate of 8.3 m³/h) to avoid hydraulic overload and incorrect operation of individual circuits. For larger systems with multiple mixing groups, we also recommend considering the installation of a differential control valve or balancing the flows on individual branches to ensure even heat distribution to all connected spaces.