Vaillant S3 - flue extension kit Ø80/130 mm for the 3rd and 4th boiler in cascade

Product detailed description

Vaillant S3 - flue gas duct extension set Ø80/130 mm for the 3rd and 4th boiler in a cascade

The S3 extension set is an original Vaillant component designed for connecting the third or fourth boiler into a cascade arrangement of condensing boilers connected to a common flue gas collector duct Ø130 mm. The set reduces the outlet diameter of an individual boiler (Ø80 mm) to the common diameter Ø130 mm, on which the collector duct for the whole cascade is built. This is a key element when building larger boiler rooms, where several smaller condensing units connected in parallel are used instead of one large boiler.

Why a cascade connection and why the S3 set specifically

Cascade connection of boilers brings the boiler room operator several advantages - better output control according to the current heat demand (by modulating the number of running boilers instead of one large boiler running at partial output), a backup in case of failure or servicing of one of the boilers, as well as the possibility of gradually increasing the installed output according to the developing needs of the building. The system of Vaillant sets (S1, S2, S3...) precisely defines how individual boilers in a cascade are connected to a common flue duct: basic sets (S1/S2) serve to connect the first boiler, while extension sets such as S3 are designed to connect further boilers (in this case the 3rd and 4th in order) to an already existing collector duct. This makes it possible to gradually extend the cascade without having to rebuild the already completed part of the flue gas path.

Technical characteristics

• Inlet diameter (boiler side): Ø80 mm
• Outlet diameter (collector duct): Ø130 mm
• Material: polypropylene (PP), resistant to condensate and to the temperatures of flue gases from condensing operation
• Purpose: connecting the 3rd and/or 4th boiler in a cascade to a common flue duct Ø130 mm
• Operating type: positive pressure (type C), with forced flue gas discharge by the boiler's fan
• Recommended additional equipment: for cascades of three and four boilers, the manufacturer recommends adding non-return flaps, which prevent flue gases from flowing back from active boilers into boilers that are currently not in operation
• Joint type: push-fit (socket) with rubber seal
• Manufacturer: Vaillant Group
• Warranty: 2 years

Why non-return flaps are needed in larger cascades

In a cascade connection of three or four boilers to a common collector duct, a situation arises where not all boilers run simultaneously - depending on the current heat demand, the control system switches on only the necessary number of units. However, boilers that are not in operation are still physically connected to the common duct through which flue gases from the running boilers flow. Without a non-return flap, there is a risk that part of the flue gases (or, in a worse case, even a reverse draught) could enter the idle boiler, which can cause problems with corrosion of the heat exchanger, incorrect boiler start-up at its next activation, or, in an extreme case, even a safety risk. Therefore, for cascades with three and four boilers, the manufacturer explicitly recommends installing non-return flaps on the connection of each individual boiler to the collector duct - this recommendation is stated directly in the technical documentation for the S3 set.

Installation and placement in the system

The S3 set is installed on the outlet spigot of the third or fourth boiler in the cascade order, creating a connection between this boiler (Ø80 mm) and the common collector duct built from Ø130 mm components (straight extensions, elbows, T-pieces and other accessories of this range). Installation includes:

1. Checking the correct order and layout of boilers in the cascade according to the boiler room's project documentation.
2. Installing the S3 set on the outlet of the relevant boiler, ensuring joint tightness using rubber sealing rings.
3. Connecting to the collector duct Ø130 mm, including any necessary elbows or straight sections according to the actual distance and direction of connection to the common duct.
4. Considering and, if necessary, adding a non-return flap on the boiler side, especially for cascades of three or more units.
5. Checking the tightness and mechanical fastening of the entire assembly after completing the installation.

Installation should always be carried out by a professionally competent person experienced with cascade connections of condensing boilers, since an incorrectly designed collector duct (e.g. insufficient slope for condensate drainage, missing non-return flaps) can negatively affect the operation of the entire boiler room, not just one boiler.

Compatibility

The S3 set is part of the complete range of Ø130 mm flue gas duct accessories from Vaillant, which also includes basic connection sets (S1, S2), straight extensions (1.0 m and 2.0 m), elbows at angles of 87°, 45°, 30° and 15°, an inspection T-piece with an inspection/cleaning opening, and spacer brackets for securing the pipe. It is designed for Vaillant condensing boilers with an outlet spigot Ø80 mm connected in a cascade to a common collector duct Ø130 mm - typically smaller units grouped in a cascade to achieve higher overall boiler room output.

Condensate drainage in the cascade collector duct

In a cascade connection of several boilers to a common collector duct, it is particularly important to properly solve condensate drainage, since the duct combines condensate from several sources at once. The collector duct must have sufficient slope (usually at least 3°) towards the lowest point, where the condensate drain is located (often implemented via the inspection T-piece from this range of accessories). Incorrect slope design or an insufficient number of drainage points in larger cascades can lead to condensate accumulating in the duct and subsequent problems with the tightness or functionality of the flue gas discharge.

Why an original Vaillant part

Extension sets such as S3 are directly tested by the manufacturer in combination with a specific boiler range and verified also in terms of behaviour in a cascade connection (including the recommendation for non-return flaps). Using a non-original or universal solution instead of this set brings the risk of incorrect sizing of the collector duct, leaks, or problems with condensate drainage during the operation of several boilers at once - which, in a cascade installation, can affect the functioning of the entire boiler room, not just one boiler.

Frequently asked questions

Advantages of a cascade solution over one large boiler

The decision to install a cascade of smaller boilers instead of one large heat source has several practical reasons, which are directly related also to the design of the flue gas path. Firstly, a cascade allows more precise adaptation of output to the current heat demand - the control system switches on only the necessary number of boilers, so each running boiler operates closer to its optimal operating point with higher efficiency, instead of one large boiler running for a long time at low partial load. Secondly, a cascade provides operational backup - if one of the boilers drops out of operation (failure, servicing), the remaining boilers can partially or fully cover the heat demand, which is not possible with a single large boiler. Thirdly, a cascade can be built gradually, which is exactly the case for which the S3 extension set is designed - the boiler room can start with one or two boilers connected via the basic sets S1/S2, and later, according to the growing needs of the building, be extended with further boilers using the S3 set, without having to rebuild the already built part of the collector duct.

Planning a boiler room with a cascade of 3-4 boilers

When designing a boiler room with three or four boilers in a cascade, it is necessary to consider in advance not only the connection of the boilers to the collector duct itself (which is handled by this S3 set together with the basic sets S1/S2), but also:

• sufficient space around the boilers for service access and replacement of individual components,
• appropriate sizing of the common chimney or shaft so that it can handle the simultaneous discharge of flue gases from all boilers at full load,
• placement of condensate drainage points (inspection T-piece) in a sufficient number and at the correct locations of the collector duct,
• installation of non-return flaps on the connection of individual boilers, as recommended by the manufacturer for cascades with three and four units,
• electrical and hydraulic interconnection of the boilers, including a common cascade control system (outside the scope of this component, but a related part of the overall project).

We recommend entrusting the complete design of a cascade boiler room to a qualified designer experienced with multi-boiler Vaillant installations, since correct sizing of the collector flue duct has a fundamental impact on the reliability and service life of the entire system.

Maintenance of the cascade collector duct

A collector duct with several connected boilers requires somewhat more thorough regular inspection than a simple connection of a single boiler, since it combines flue gases and condensate from several sources and contains more joints where leaks could theoretically occur. As part of regular inspections (carried out by an authorised chimney sweep), we recommend checking:

• the tightness of all connection and extension sets (S1, S2, S3) on individual boilers,
• the functionality and condition of any non-return flaps,
• the patency and slope of the collector duct for condensate drainage,
• the mechanical fastening of the entire collector duct (spacer brackets, anchoring elements), especially for longer horizontal sections.

Standards and legislation

As with other components of this range, the installation of the S3 set and the entire cascade collector duct must comply with applicable standards (STN 73 4201, STN EN 1856 and related standards for plastic chimney systems) and legislative requirements for regular inspection of flue gas paths. Before putting the cascade into operation, an inspection by an authorised chimney sweep is required, confirming the satisfactory condition of the entire flue gas path, including all connection and extension sets.

Storage and handling before installation

We recommend storing the polypropylene components of the S3 set before installation in a dry environment protected from direct sunlight and mechanical damage. During transport and handling, impacts on the sealing surfaces of the push-fit joints should be avoided to prevent deformation that could affect the resulting tightness after assembly. For longer storage before installation, we recommend keeping the components in the manufacturer's original packaging.

Availability and ordering further cascade components

If you are planning a cascade with three or four boilers, we recommend calculating the complete list of required components at once before ordering - basic sets S1/S2 for the first (or second) boiler, an extension set S3 for each further boiler (3rd and 4th), a sufficient number of straight extensions and elbows according to the actual layout of the boiler room, an inspection T-piece for condensate drainage, and spacer brackets for securing the collector duct. Ordering all components at once from a single seller simplifies installation logistics and reduces the risk of delays caused by a missing component during installation.

Further frequently asked questions

Summary

The Vaillant S3 extension set Ø80/130 mm is an essential element when building cascade boiler rooms with three or four condensing boilers, ensuring a safe and tight connection of further boilers to the common flue gas collector duct. We recommend combining it with the recommended non-return flaps and other accessories of the Ø130 mm range, and entrusting the design and installation of the entire cascade to professionally competent persons experienced with this type of installation.