Vaillant auroFLOW plus VPM 15 D - solar module (drainback system)

Product detailed description

Vaillant auroFLOW plus VPM 15 D – solar module with drainback system

The Vaillant auroFLOW plus VPM 15 D solar module (manufacturer order number 0010015638) represents a more modern concept of a solar charging station using the so-called drainback (self-draining) system. Unlike a classic pressurized solar circuit (such as the one used in the auroFLOW exclusive range), where the heat transfer fluid remains permanently in the collector piping under pressure, the drainback system automatically drains the fluid from the collectors and roof piping by gravity back into the collection tank in the module once the circulation pump stops. This means that no fluid remains in the collectors and external piping when the system is not in operation.

This design solves two fundamental problems of classic pressurized solar systems – the risk of overheating (stagnation) under high solar radiation and a fully charged storage tank, when the heat transfer fluid in the collectors would otherwise reach extreme temperatures and degrade, and the risk of freezing during frosty periods, since no fluid remains in the exposed parts of the system (on the roof) when not in operation that could freeze and damage the piping or collectors.

How the drainback principle works

When the module's differential controller detects a sufficient temperature difference between the collectors and the storage tank, the circulation pump starts, pressurizing the heat transfer fluid from the collection tank up into the collector array. The system operates as a closed circuit while the pump is running. As soon as the differential controller determines that it no longer makes sense to continue extracting heat (for example, when it gets dark, becomes cloudy, or the tank is already fully charged), the pump stops and the fluid automatically returns by gravity back into the collection tank in the module – the collectors and roof piping thus remain empty until the next charging cycle.

Thanks to this principle, the drainback system does not require a classic safety group with a safety valve releasing fluid under excess pressure (as in a pressurized system), because excessive pressure or overheating of stagnant fluid in the collectors cannot actually occur – the fluid simply is not present in the collectors when not in use.

VPM 15 D model – purpose and capacity

The designation "15" in the model name VPM 15 D indicates that this module is sized for solar collector arrays with an area of up to approximately 15 m² – suitable for typical family houses with multiple auroTHERM solar collectors, or with higher hot water consumption. For larger installations with a bigger collector area, the more powerful VPM 30 D model is also available in the auroFLOW plus range. The module contains a collection tank with a volume of approximately 20 liters for the heat transfer fluid of the drainback circuit.

Technical characteristics

Advantages of the drainback system over a classic pressurized solution

• Absolute protection against stagnation and overheating – even with a long-term fully charged tank and intense solar radiation, there is no risk of overheating and degradation of the heat transfer fluid in the collectors, since they are empty in this state.
• Absolute protection against freezing – there is no fluid in the exposed outdoor piping (on the roof) when not in operation that could freeze.
• Longer heat transfer fluid lifespan – thanks to the absence of repeated thermal stress during stagnation, the fluid in the system does not degrade as quickly as in pressurized systems exposed to repeated stagnation cycles.
• Simpler maintenance of the solar circuit – there is no need for a classic safety group with a safety valve releasing the medium under excess pressure, as the system's principle prevents such a state by design.
• Suitable also for existing installations – flexible connection allows combination with a regular heating system as well as with a Vaillant heat pump (e.g. ground-water or air-water).

Installation and specific requirements

Since the drainback system requires the fluid to flow by gravity back into the collection tank once the pump stops, it is essential during design and installation to strictly maintain the prescribed pipe slope between the collectors and the module, in order to ensure complete and reliable draining of the collector array in every cycle. This is a fundamental difference compared to installing a classic pressurized system, where pipe slope is not functionally critical. The module can be mounted directly on the storage tank or separately on the wall, depending on the boiler room's space; a power cable and the necessary seals for the solar connection are included in the delivery.

Cascade connection

For larger collector arrays or systems with multiple separately oriented collector branches, it is possible to connect up to four auroFLOW plus modules in a cascade, which allows covering even more extensive solar installations while maintaining all the advantages of the drainback principle on each branch separately.

Compatibility

The auroFLOW plus VPM 15 D module can be combined with storage tanks from the allSTOR exclusive range as well as with other suitably sized heating water tanks, with auroTHERM solar collectors (recommended type VFK), as well as with common Vaillant heat sources including aroTHERM heat pumps – flexible connection allows its use both in new installations and in the modernization of older solar systems, where the drainback solution offers the advantage of simpler long-term operation without the risk of freezing or overheating.

Maintenance

Thanks to the absence of permanent pressure in the collectors and lower thermal stress on the heat transfer fluid, maintenance of the drainback system is generally less demanding than with a classic pressurized solution; nevertheless, during the annual service inspection it is recommended to check:

• the function and fluid volume in the collection tank,
• proper draining of the collector array after the pump stops (checking pipe slope and patency),
• the operation and technical condition of the charging pump,
• the function of the differential controller and the accuracy of the displayed temperatures/solar yield,
• the tightness of the module's hydraulic connections.

Why choose the auroFLOW plus drainback solution

For locations with significant weather fluctuations, a high risk of summer stagnation (for example with a south-facing roof orientation and high year-round solar gain), or a risk of severe frost, the auroFLOW plus VPM 15 D drainback system is a safer and, in the long run, less costly choice than a classic pressurized system, as it eliminates the two most common sources of faults and the need for service interventions in solar systems – stagnation and freezing. The VPM 15 D model is a suitable choice for a typical family house with multiple collectors that is also looking for a reliable solution with low long-term maintenance requirements.

Frequently Asked Questions

Comparison of drainback and pressurized solutions from a long-term operation perspective

When deciding between the drainback auroFLOW plus module and the pressurized auroFLOW exclusive module, it is useful to consider the following differences relevant to long-term operation:

Drainback system commissioning procedure

Commissioning the auroFLOW plus VPM 15 D module requires a different procedure than a pressurized system, as it is also necessary to verify the correct function of self-draining:

1. Checking the slope and routing of the piping between the collectors and the module according to the project documentation, before filling the system.
2. Filling the collection tank with heat transfer fluid to the prescribed level.
3. Trial charging cycle – starting the pump and verifying that the fluid actually rises into the collectors and circulates without air blocks.
4. Self-draining test – stopping the pump and a visual or pressure check that the fluid actually and completely returns to the collection tank.
5. Setting the differential controller and any cascade parameters, if the module is part of a multi-module assembly.
6. User training and handover of system documentation.

Suitability for various climatic conditions

Thanks to its structural resistance to freezing, the auroFLOW plus drainback solution is particularly suitable for mountain and sub-mountain regions of Slovakia with long, frosty winters, where a pressurized system would require increased attention to antifreeze mixture concentration and its regular checking. It is also suitable for locations with a high number of sunny days in summer, where a pressurized system would risk frequently reaching stagnation temperatures with a fully charged tank.

Summary

The Vaillant auroFLOW plus VPM 15 D is a modern solar module using the drainback principle, which by design eliminates the risk of overheating and freezing of the solar circuit without the need to maintain permanent pressure in the collectors. Thanks to its integrated solution with a differential controller, pump, and heat exchanger, it represents a reliable and low-maintenance choice for solar-assisted hot water preparation in family houses.