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Plenum box for Heat Recovery System - Steel vs Plastics

Plenum boxes EPP vs STEEL

What is a plenum box?

The plenum box is a connection element in the ventilation system, on which we install supply and exhaust air diffusers. Plenum boxes are used both in the ventilation systems of large-area buildings, where they often also have a damping function (i.e. they reduce the noise generated in the installation) and in domestic residential houses.

In this article, we will only focus on plenum boxes used in domestic ventilation with heat recovery (MVHR). These plenums are used to connect semi-rigid ducting and air valves (diffusers).

Types of Plenum boxes

Plenum boxes are most commonly made from galvanized steel or plastics. As with many products, either metal or plastic plenum boxes have ardent supporters and fierce opponents.

In this matter, we have to admit we are not neutral. We have been manufacturing plenum boxes made of stainless steel and galvanized metal for over a decade but this doesn't mean we cannot appreciate the advantages of the air distribution system using plastic plenums. Therefore, we are pleased to introduce plastic plenum boxes - made of expanded polypropylene (EPP) - to our offer as an alternative to our metal ones.

Steel plenum boxes
Metal plenum boxes

Most plenum boxes are made from galvanized sheet metal, however, stainless steel (or steel and plastic) versions are also available.

What is their biggest advantage? Non-flammability. There is no plastic material that can compete with steel in this regard. Many national technical requirements specify that ventilation systems should be made of non-flammable materials.


Here are some examples of galvanized steel plenum boxes offered by our company:

plenum box with a gasket on spigots for round ducting:

  • with male connector (FLX-PLO)
  • with female connector for air valve (FLX-PLO-F)

plenum box without gasket (FLX-PRO)
plenum box with internal insulation made of foam rubber (FLX-PLI)
plenum box for ventilation grilles (FLX-PLO-MOV)

Full range of plenum and manifold boxes

There are also plenum boxes FLX-PRV with elliptical spigots for flat, oval ventilation ducts. Their casing is made of galvanized steel, and oval spigots are made of plastic.

EPP plenum boxesEPP Plenum boxes

FLX-PLO-EPP plenum boxes are made of high-quality expanded polypropylene with high mechanical strengths (greater than a sheet metal alternative product). Plenum casings are made of EPP, connectors for semi-rigid ducts are made of ABS, and the TPE gaskets mounted on the connectors.

Why expanded polypropylene?
Expanded polypropylene (EPP) is characterized by a low weight and high flexibility, making it possible to form the material in whatever shape. This allowed us to design a plenum box with an aerodynamic shape, ensuring low flow resistance. EPP also has good thermal properties, and it serves as both the casing and the insulation layer.

Here are the EPP plenum boxes that are available:

  • with 1 spigot for semi-rigid ductwork (1xØ75mm) and a connector for an air valve with a diameter of 100
  • with 2 spigots for semi-rigid ductwork (2xØ75mm) and a connector for an air valve with a diameter of 125mm and Ø 100mm*

* An additional reducer (sold separately) will be needed to change the diameter from 125mm to 100mm


The heat recovery unit's efficiency is drastically reduced by leaky ventilation ductwork, and thus the entire system. Due to leaks in the installation, some air does not reach its destination, preventing the proper air exchange - supply of fresh air and extraction of stale air. That is why it is crucial for each connection to be airtight, including plenum boxes.

Plenum box installation
For plenum boxes without gaskets (FLX-PRO) on the spigots, the airtightness of the system is mostly determined by the skill of the installer. Since it is he who has to ensure that all connections are tightly sealed, with the sealing tape or additional gasket places on the HDPE duct.

Plenum boxes with clips

The plenum boxes with gaskets (FLXPLO) guarantee air tightness class C and eliminate any additional sealants. Installers simply connect the plenum and the semi-rigid duct by pressing the duct onto the connection spigot until it clicks.

For a more secure joint, we offer plenums with clips that prevent the duct from falling out while being assembled (FLX-PLO-75 -...- H).



The semi-rigid duct system based on EPP plenum boxes achieves airtightness class C. There are three design solutions used to make the connection of the EPP plenum box to the 75mm duct strong and durable.

  1. Spigots have gaskets installed.
  2. The elongated shape of the plenum ensures a permanent connection between the duct and the box.
  3. An additional system for fixing the duct that reflects the shape of the notch on the semi-rigid ventilation duct, allowing for an even better connection.

In contrast to steel plenum boxes without seals, the EPP plenum box does NOT require additional sealing in order to connect with a semi-rigid duct.

Pressure drops

Air resistance is greatly reduced by the aerodynamic internal shape of the box. The EPP plenum boxes have significantly lower pressure drops compared to metal equivalents available on the market. In the chart below, we compare the FLX-PLO and FLX-PLO-EPP plenum boxes.
Comparison of EPP and steel plenums

Comparison of a plenum box made of EPP and steel

Thermal insulation

One of the most common assembly mistakes that affect the operation of the mechanical ventilation system is improper insulation. It's recommended to insulate the whole duct systems with plenum boxes and manifolds, especially if they are installed in an unheated area.

In contrast to the metal plenum box, EPP used in FLX-PLO-EPP plenum box acts as a thermal insulator. EPP is an insulating material with λ = 0.039, so the additional insulation layer is not needed. Furthermore, its shape reduces heat loss.


In the case of metal plenum boxes as well as made of expanded polypropylene, the air duct is mounted by a simple push-fit connection. The semi-rigid ducts are pushed onto male connectors with a gasket in FLX-PLO and FLX-PLO-EPP plenum boxes or inserted into female connectors (FLX-PRO) with inner gaskets.

For a more secure and stable fastening of the ducting, the EPP plenum box, in addition to the gaskets on the connectors, also has an EPP casing that "overlaps" the duct, in order to hold it in place and prevent it from slipping out.

EPP plenum boxes

All of our plenum boxes are equipped with mounting brackets for easy installation. Metal plenum boxes have two mounting plates on the top and bottom. The EPP plenum boxes have 3 mounting holes with variable diameter for quick fixing them to the substrate.

Plenums with mounting holes

Installing plenum boxes is also possible using perforated tape. The EPP plenum box has special guides which, after the perforated tape has been screwed on, it guarantees the stable installation of the box in both ceiling and floor position and make it impossible to move the box.

 Summing up, we've got 2 installation methods to choose from:

Plenum installation with perforated tape

Plenum installation with 3 screws and wall plugs (left) or with 17-25 mm-wide PB perforated tape (right).

Installation flexibility

In EPP plenum boxes, it is possible to smoothly adjust the height of the air valve connection by cutting off the excess material until the required height is achieved.

Plenums for air valves

To make height adjustment easier, special tracks have been made on the air valve connection to facilitate cutting off individual sections. In addition, there are auxiliary height markings (“40” and “60”).

Summary - Steel plenum box vs EPP

Final verdict: Plenum box made of steel or EPP? There is no one correct answer, but to help you decide, we have prepared a table that compares the most important factors.

Material galvanized or stainless steel sheet
+ EPDM gasket
casing made of EPP, ABS spigots
+ TPE gasket
Dimensions link to the catalogue link to the catalogue
Diameter of
FLX-HDPE ducting
50,63,75 & 90 75
Corrosion resistance partially 100%
Assembly ceiling, insulation layer,
suspended ceiling 
ceiling, insulation layer,
suspended ceiling 
Airtightness (EN 12237) C C

female (-F) or male duct connector
with a gasket for diffusers;
spigots (male) with a gasket
or without a gasket (female)

female connector for diffusers;
spigots (male) with a gasket

Thermal insulation option YES
Do you need to additionally
insulate the boxes?
Flammability non-flammable flammable
Are there "matching"
distribution boxes?
Work in progress ...
Pressure drops Low Very low

Check a complete range of Air Distribution System Products for HRV systems

Would you like to know more? Read other articles on heat recovery ventilation:

  1. How can You install heat recovery unit in the ready-built house?  - the article about decentralized heat recovery systems.
  2. What is an Electrostatic Precipitator and how does it work? - the article about the highest filtration efficiency solutions.
  3. 14 Things You Need to Know about PremAIR - pretty self-explanatory title about the best HRU for clients looking for "premium" solutions. 
  4. Air distribution system for the MVHR - overview of FLX-REKU system: radial, semi-rigid ducting, plenum and distribution boxes, etc.