The pipes of the radiant system...

Today floor systems (but also wall and ceiling systems) are almost always made with polyethylene pipes. There are various types of polyethylene on the market (cross-linked type a, b, c, with increased thermal resistance, multilayer, etc.), which guarantee more or less performing values ​​from the point of view of duration and resistance to temperature variations. But there is one feature that is often overlooked or underestimated, to which I would like to draw attention:

Permeability to oxygen

What are we talking about? What is this thing?

It is a very curious phenomenon and… perhaps… for some, even unthinkable.

Hot water (or cold, if we are talking about cooling) circulates in the pipes of our underfloor system... and so far everything is normal! This water cannot come out of the pipes, because the material they are made of prevents it... and here too we all agree!

However, the polyethylene walls of our pipe cannot prevent a certain amount of oxygen molecules or other gases (e.g. carbon dioxide) from entering inwards and towards the water. I assure you it is ... it is not science fiction!

This phenomenon is called "PERMEATION FROM OXYGEN" (or other gases) and involves some plastic materials and all those materials that have a molecular structure such that they can be crossed by very small molecules such as oxygen, which make their way through the "molecular interstices". The permeation does not involve metal pipes (iron, copper, steel, etc...) due to their more “dense” molecular structure.

At this point, someone might argue that the pipes are embedded in concrete and therefore it is not possible for this oxygen permeation to take place inwards. Well... that's not quite the case. Let's not forget that in correspondence with the manifold the pipes retain a visible section:

And then we must not forget another factor: the pipes incorporated in the screed are subject to the permeation of other molecules... chlorides and sulphates present in the cement mortar!

In short ... not only the oxygen molecules, but also other small molecules can permeate the walls of the polyethylene pipes and enter the system water.

What are the consequences of permeation?

When the water from the radiant system is particularly enriched with oxygen or other "foreign" molecules (such as chlorides and sulphates), the consequences can be very serious:

• Oxidation and subsequent corrosion of the metal parts of the system (metal pipes, fittings, manifolds, boiler primary exchanger, etc ...). This phenomenon, once started, grows exponentially, up to the perforation of a pipe or, worse still, of the exchanger inside the boiler! Chlorides and sulfates can also attack stainless steel. And no warranty will ever cover corrosion damage.

• Crystallization of polyethylene, by chemical reaction with chlorides and sulphates, and consequent increase in the phenomenon of permeation.
• Activation and proliferation of bacterial flora, with the formation of algae, sludge, mucilage and occlusive agglomerates that obstruct the normal circulation of water and drastically reduce the thermal output of the system. This proliferation is also favored by the fact that a floor system operates at a lower temperature (35° - 45°) than a traditional radiator system (75° - 85°).

How is the permeation problem solved?

The UNI EN 1264 standard, which regulates the design and construction of radiant heating systems, for polyethylene pipes strongly recommends the use of pipes equipped with a special oxygen barrier. The permeability of this barrier must be less than 0,32 milligrams of oxygen per square meter per day with a water temperature of 40 degrees:

Permeability to oxygen < 0,32 mg/(mq x Day)    [T°(H2O)=40°]

What is the oxygen barrier?

It is a layer, inserted in the section of the polyethylene pipe, which has a molecular structure that prevents or drastically reduces the passage of oxygen molecules.

To date, two techniques are used for the creation of the oxygen barrier:

• EVOH film: it is nothing more than a plastic film (EVOH = Ethynele Vinyl Alcohol) resistant to the passage of oxygen (very "dense" molecular structure).

• Metallic layer: the pipe is made by inserting a metallic layer of aluminum between two layers of polyethylene.

What is the best oxygen barrier?

Both solutions allow to respect the limit imposed by the UNI EN 1264 standard:

Permeability to oxygen < 0,32 mg/(mq x Day)    [T°(H2O)=40°]

But if we look at the following figure, we immediately have the answer:

After all... we have already said at the beginning of the article that the phenomenon of "permeation" does not concern metal pipes! On the contrary, polyethylene (the table shows cross-linked polyethylene PE-x) is a very sensitive material to this phenomenon.

Tips for choosing the pipes for our radiant system

• A pipe equipped with an oxygen barrier represented by an intermediate metal layer is certainly the best product that the current market can offer in terms of resistance to permeation.
• If you are looking to buy an EVOH barrier pipe, you must pay close attention to what is offered by the market:

An EVOH barrier applied to the outer surface of the pipe requires easier processing and therefore the pipe has a lower cost. But let's remember two very important factors:

1. The EVOH film applied to the surface of the pipe is easily damaged during the installation phases of the system (laying the pipe and casting the screed).
2. The material with which the EVOH film (Ethynele Vinyl Alcohol) is made is hydrophilic, i.e. it dissolves with humidity. Can you imagine what happens to this layer when the pipes are covered with the cement screed?

What can we say about polybutylene pipes?

Some companies have introduced on the market, for the construction of radiant systems, an additional pipe: the polybutylene one.

These companies emphasize the extreme flexibility of this pipe and the consequent ease of installation, as well as the lower price.

But they forget to highlight what should be the most important characteristic for a tube to be used in a radiant system: thermal conductivity!

As can be seen from the table above, a polybutylene pipe has a much lower ability to transmit the thermal energy carried by water than a polyethylene pipe.

In conclusion

When choosing the pipe for our radiant system we must be aware that there are some characteristics to take into consideration:

• permeability to oxygen
• thermal conductivity
• value for money

Please note. Source of some images: web