FLX-REKU - Air Distribution System for the MVHR

Air distribution system with semi-rigid ducting

Spiral, metal, rigid ducts next to flexible ducting (flex) are the most typical solutions for air distribution ductwork in the MVHR system – both have the pros and cons which we discuss later in the article – however, there is also the third option – modular air distribution ductwork using semi-rigid non-metallic ducting – radial or oval. In our offer this type of system is called FLX-REKU – if You want to find out:

  1. What IS FLX-REKU and what are its the components?
  2. What are the advantages of this solution?
  3. The comparison of 3 types of ductwork - rigid vs. flexible vs semi-rigid?

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What is FLX-REKU and what are its components?

FLX-REKU system is an air distribution ductwork that consists of radial semi-rigid, non-metallic ducts, metal plenum boxes and distribution boxes, couplings, bends, reducers, silencers, dampers, sealing rings and mounting accessories that make for a full installation which may be connected to the heat recovery unit.


Semi-rigid ducting for the MVHR

FLX-HDPE Non-metallic, semi-rigid ducting for the MVHR systems

FLX-HDPE semi-rigid ducts are made of high-impact polyethylene, available in two versions - FLX-HDPE standard version and FLX-HDPE-A duct ( the "A" stands for antibacterial) is treated with antibacterial additives - with silver ions to limit bacteria development inside the duct. Semi-rigid ducting is used whenever flexibility and durability are important - owing to their high diametral stiffness, the ducts may be embedded within concrete floors.

An antibacterial coating used in the FLX-HDPE-A ducts prevents the growth of microorganisms  – Escherichia coli (ATCC 11229), Staphylococcus ureus (ATCC 6538). The test was performed by the Institute of Biopolymers and Chemical, according to PN-EN ISO 22196:2011 Plastics – Measurement of antibacterial activity on plastics and other non-porous surfaces

See Results of the Testing of Antibacterial Properties of Semi-rigid Ducting


Galvanized steel plenum box for air valvesMetal plenum box for SHR air grille

FLX-PLO Plenum boxes for air valves and FLX-PLO-MOV plenum boxes for ventilation grilles

Plenum boxes can be used for the installation of supply and exhaust air valves, They are very compact and low profile which means they can fit in the space between the layers of attics, suspended ceilings, walls, slabs and screeds. FLX-PLO-MOV plenum boxes can be installed directly onto the ventilation grille – suitable for SHR grille.

Looking for more "aesthetic" diffusers?  See our bestselling linear slot diffusers

Distribution box for semi-rigid ductingManifold for semi-rigid ducting

FLX-PLO-R Distribution boxes (manifolds)

Distribution boxes (manifolds) gather the ducts with supply and extract air and moves the air to and from the heat recovery unit. Distribution boxes with multiple duct connection spigots are excellent for buildings in which the ventilation system can only be lead from a single point. Distribution boxes are made of galvanized steel, but there is a stainless steel option available upon request.

Semi-rigid ducting for air distributionMetal plenum box with end cap

End caps for semi-rigid ducting and plenum boxes

Plastic end caps are used for blanking unused duct connection spigots in the distribution box and semi-rigid ducting. End caps can be also used to protect from building dust and other impurities at the construction site.

Pressed bend for semi-rigid ductingGalvanized steel offsets for ducts

FLX-BP bends and FLX-ODSQL offsets plenum boxes

Bends are used whenever a sharp bend is required right next to the floor, where bending the duct section would not be possible.

What are the benefits of Flexible/ Modular Air Distribution System?

There is no such thing as a perfect solution for every building design. However, there are several arguments in favour of the FLX-REKU Air Distribution System which should be taken into account while deciding on the type of ducting we're going to use. A few most important are presented below.


Owing to the small size and durable construction, FLX semi-rigid ducting may be installed on roofs, walls and floors. The use of highly durable materials, resistant to mechanical damage, allows placing the installation under concrete levelling or in roof ceilings. Semi-rigid ducting on the one hand has higher tensile strength and rigidity and on the other hand is flexible enough to work around „obstacles” e.g. joists (due to the small bending radii for sharp bends, FLX-BP elbows must be used). Low profile plenum boxes do not take up a lot of space, which means that no structural modifications are necessary – even when two ducts must cross (FLX-ODSOL offsets are used when few ducts must cross), floors or ceilings retain their structural integrity.


The FLX-REKU system uses a limited number of components to make a ventilation system that is tailored to your individual requirements. System installation does not require special tools. Components are joined together, special sealing rings allow for an exact fit with a perfect seal.

Flexible duct structure allows leading the installation along the shortest possible route. Smooth internal coating of the FLX ducts, the minimum number of connectors and fittings as well as a slight bending of the distributed ducts allow to reduce airflow and limit pressure loss inside the installation.


FLX-REKU air distribution system is compact, with its low-profile plenum and distribution boxes and small size ducts it is designed for installation in attics, suspended ceilings, walls, slabs and screeds, suits narrow joists and low ceiling voids.

Flexibility and durability of the system allow it o be poured in a concrete layer of the bare ceiling, cavity walls and double ceilings within the thermally insulated building envelope. Non-metallic semi-rigid ducts can be used in many situations where traditional ducts cannot be used, due to the confined space and narrow ducting routes.


To make the connections as airtight, it's recommended to use plenum and distribution boxes with factory-installed seals. The airtight connection means that no glues or sealants are required on-site. Semi-rigid ducting is supplied in 50 m long rolls meaning far fewer connections.


FLX semi-rigid ducts have smooth inner walls with antibacterial lining to limit bacteria development inside the duct. Antibacterial coating guarantees very good installation hygiene. Moreover, ducts are supplied capped to protect from impurities that can occur during transport, storage and assembly work. Since there are no protruding screws used during the installation, it is possible to clean the system easily and thoroughly.

Rigid vs. Flexible vs Semi-rigid Ductwork

For sure each system has its advantages. But in most cases, they’re used simultaneously. As long as you can design and create the whole installation based on rigid, metallic SPIRAL ducts in a traditional manner, it’s pretty impossible to use only plastic, non-rigid ducts exclusively. For connecting a RAHU with manifolds diameters 125-250mm are typically used, which ultimately leads to spiral ducts. On the other hand, there is no easier way to connect RAHU spigots by means of flexible ducts – you have the comfort of positioning the unit with a warning about a margin of error. However, it is not recommended to use only flexible ducts – they aren’t much resistant to damage, create much more pressure drop and are nearly impossible to clean.

For more information about FLX-REKU Air Distribution System please check technical sheets

FLX-REKU leaflet
FLX-REKU system brochure 
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Alnor Ventilation Systems
Krakowska 10 Avenue
05-552 Wola Mrokowska

tel. +48 22 737 40 00