Vaillant VDM pump assembly for two-circuit heating (mix/mix)

Product detailed description

Vaillant VDM assembly for two-circuit heating (mix/mix) – two independently controlled low-temperature branches in one package

The high-efficiency electronic VDM pump assembly for two-circuit heating in the mix/mix configuration (order number 0020229419) is a complete, pre-matched package containing a two-circuit header with a built-in hydraulic separator and two mixing pump groups with a three-way control valve. Unlike the combined direct/mix assembly, which is designed for a combination of a radiator circuit and a low-temperature circuit, this version provides independent temperature control on both connected circuits – the ideal solution where both heating branches need their own, mutually independent temperature control.

What exactly the assembly includes

• A two-circuit header with a built-in hydraulic separator, including a separator sensor, mounting brackets and thermal insulation
• Two mixing pump groups VDM 20 M / VDM 25 M – each with a high-efficiency class A electronic pump, a three-way mixing valve with a 230 V servo motor, two round thermometers and complete insulation
• 120 mm mounting spacing between the individual header outlets
• A complete set of connecting and sealing material

When to choose the mix/mix configuration instead of direct/mix

The choice between the direct/mix and mix/mix configurations depends on the nature of both connected heating circuits. Choose the mix/mix configuration when neither circuit can operate directly at the boiler circuit temperature without control – typically in:

• Two separate underfloor heating zones with different temperature requirements (for example, the ground floor and upper floor with different floor areas or different types of floor covering)
• A combination of underfloor heating and low-temperature radiators or convectors, which also require a reduced and controlled temperature compared to the boiler circuit
• Systems with a heat pump or other low-temperature heat source, where it is beneficial to be able to independently control both branches
• Buildings with two functionally different zones (for example, a residential part and a technical/utility area), where each zone requires a different heating curve

Conversely, if one of the circuits (typically radiators) can operate without issue at the same temperature as the boiler circuit, it is more economical to choose the assembly in the direct/mix configuration, which is simpler and generally more cost-effective thanks to a lower number of control components.

Operating principle of both mixing groups

Each of the mixing groups in this assembly operates independently – its own three-way valve with a servo motor, controlled by a separate controller (room, weather-compensated or system controller), gradually blends cooler return water from the respective secondary circuit with warmer water from the separator, thereby maintaining the required temperature of the water entering that circuit. Thanks to independent control, both circuits can have completely different heating curves, time programs and target temperatures, which is particularly important in zoned heating with different comfort requirements in different parts of the building.

Role of the built-in hydraulic separator

As with other assemblies in the VDM range, the built-in hydraulic separator here hydraulically separates the boiler circuit from both connected secondary circuits. This means that simultaneous or independent changes in the position of both three-way valves do not affect the stability of the flow through the boiler, which is even more important with two actively controlled circuits than with a combination involving one direct circuit – without this separation, mutual hydraulic interference could occur between the two mixing circuits, as well as between them and the boiler.

Installation and commissioning

• The header with the built-in hydraulic separator is mounted on the wall of the plant room using the supplied brackets
• The flow and return pipes from the boiler are connected to the inlet side of the separator
• Both mixing groups are connected to the outlet side of the header with a 120 mm spacing
• The separator sensor is connected to the boiler controller; the servo motors of both three-way valves are connected to the respective controllers of the individual circuits (these may be two separate room controllers, or one system controller supporting multiple heating zones)
• After the electrical connection, complete thermal insulation is fitted to all components
• The system is filled with water, thoroughly vented, and the independent control function of both circuits is verified

Compatibility

The assembly 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 controllers that support control of multiple independent heating zones. The 120 mm spacing is consistent with the entire range of Vaillant pump groups and headers.

Maintenance

• Check the function of both pumps and both mixing valve servo motors once a year
• Verify the function of the separator sensor and its connection to the boiler controller
• Check the tightness of all screw connections
• Monitor the thermometers on both circuits – unusual deviations may indicate a fault in a servo motor, sensor, or incorrect controller settings
• In areas with harder water, consider a softening cartridge to protect both valves, pumps and the separator

Why choose this configuration of the assembly

In systems where neither circuit can operate directly at the boiler temperature, the mix/mix configuration is the only technically correct solution – using a direct group on a circuit that requires control would lead to overheating of that circuit and the risk of damage to sensitive components (for example, floor covering). The complete assembly also guarantees mutual compatibility of both mixing groups and the separator, including matching hydraulic parameters and a unified visual design.

Comparison with the direct/mix configuration

Typical deployment scenario

This configuration is often used in larger family homes or multi-generational houses, where there are two separate underfloor heating zones with different comfort and scheduling requirements – for example, a zone occupied all day and a zone used only occasionally, where it is beneficial to have a separate, more economical heating curve. It is also used for combining two different types of low-temperature appliances (for example, underfloor heating and wall heating panels), which may require slightly different operating temperatures.

Frequently Asked Questions

• What is the difference between this assembly and the direct/mix assembly? This assembly contains two mixing groups with independent temperature control on both circuits, while the direct/mix assembly has one circuit without control (direct) and one with control (mixing).
• Can I control both circuits with a single shared controller? Yes, if you use a Vaillant system controller that supports multiple independent heating zones (for example multiMATIC with the appropriate expansion modules), both servo motors can be controlled from a single central system.
• Is this assembly also suitable for combination with a radiator circuit? Technically yes, since the mixing group can supply water at the same or a lower temperature than the boiler circuit. However, if one of the circuits does not require control, it is more economical to choose the direct/mix assembly.
• Can the assembly be expanded with a third circuit? The built-in hydraulic separator has a fixed number of two outlets. For expansion to more circuits, we recommend a separate, larger separator with a header for the required number of circuits.
• What is the warranty on this assembly? We provide a standard statutory warranty of 2 years from the date of sale on the complete VDM pump assembly for two-circuit heating (mix/mix).
• Why is this assembly more expensive than the direct/mix assembly? It contains two full-fledged mixing groups with a three-way valve and servo motor instead of one direct and one mixing group, which is reflected in a higher purchase price due to the additional control component.
• Can one of the mixing groups be replaced with a direct group later? Yes, thanks to the common 120 mm mounting spacing, any of the groups can be replaced in the future with another type (direct or mixing) according to changing requirements of the given circuit, for example when the purpose of the heated zone changes.
• Is a Vaillant system controller required for this assembly? No, it is not a requirement – the system also works with two independent third-party room controllers, as long as they are compatible with 230 V servo motors. However, a Vaillant system controller provides more convenient central control and an overview of both zones from a single location.

Advantages of independent control of both circuits

The main advantage of the mix/mix configuration is the ability to fully independently optimize the operation of each of the two zones according to its actual use and character. A zone occupied all day can have a higher base temperature and a more stable heating curve set, while an occasionally used zone (for example, a guest room, basement or garage) can run in a more economical mode with a lower temperature that is only temporarily increased when actually needed. This flexibility brings real energy savings without compromising comfort where comfort is truly needed.

Impact of independent control on living comfort

In addition to energy savings, independent control of both zones also brings greater living comfort – occupants of different parts of the house (for example, parents and adult children in a separate part of the house) can have different temperature preferences set without disturbing each other. It is also possible to temporarily shut down one zone completely (for example, during a prolonged absence from that part of the house) without affecting the heating of the other zone, which is not possible with simpler systems lacking independent control.

Controller configuration in practice

In practice, independent control of both mixing circuits is most often implemented in one of two ways: either using two separate room thermostats/controllers, each controlling the servo motor of one circuit directly or via a relay, or using a single Vaillant system controller (for example multiMATIC) with expansion modules for multi-zone control, which centrally manages the time programs, heating curves and current temperatures of both zones from a single interface, or even via a mobile app. The choice of the specific solution depends on the customer's requirements for control comfort and the control budget, while both approaches are fully compatible with the three-way valve servo motors supplied as part of this assembly and require no additional modifications to the hydraulic part of the system.

Recommendations for designers

When designing a system with this assembly, we recommend defining separate heating curves and time programs for both zones in advance, as well as choosing the appropriate type and number of controllers (two separate room controllers, or one system controller supporting multiple zones). We also recommend checking the sum of the flow rates of both circuits against the capacity of the built-in hydraulic separator, especially if both zones are of a larger scale, and taking into account the length and routing of the pipework to the individual zones, which also affects the overall heat loss and the system's response to a change in the set temperature.

Material design and durability

The bodies of both mixing groups and the header with the built-in hydraulic separator are made of durable cast brass with a corrosion-resistant surface finish, dimensioned for the typical operating pressures and temperatures of heating systems. Thermal insulation made of shaped expanded polypropylene (EPP) surrounds all components, including the servo motors and fittings, minimizing heat losses and protecting against condensation on cooler sections of low-temperature circuits. With proper maintenance and heating water quality, the service life of such an assembly is comparable to the service life of the boiler itself and the main system pipework.

Summary

The high-efficiency electronic VDM pump assembly for two-circuit heating in the mix/mix configuration is the optimal solution for systems where both connected heating branches need their own, independent temperature control. Thanks to the built-in hydraulic separator, two full-fledged mixing groups, and the option of central or separate control of both zones, it offers maximum flexibility when designing a comfortable and energy-efficient multi-circuit heating system, while maintaining guaranteed compatibility and a unified origin of all key hydraulic components from a single manufacturer.