Condensing boilers are the only boilers that satisfy the new European efficiency regulations. That is why they are becoming the new standard on the market.

However, you cannot simply switch to a condensing boiler without first modifying your chimney to the specific operation of the new appliance. The chimney, for example, must be able to withstand the water vapour that is emitted by the boiler. If this is not the case, the moisture can cause damage to the building.

Solution 1 : install a moisture-resistant chimney 

Moisture-resistant flue ducts are concentric, rigid or stainless steel pipes that are installed inside or outside the building. These are more commonly known as double-walled pipes.

• The outer pipe provides for air circulation. This ensures that the temperature of the flue gases - necessary for maintaining the condensation process - is kept low.
• The inner tube is the actual chimney that discharges the combustion gases.

Both tubes are equipped with hermetically sealed seals.

Solution 2 : intubate the existing chimney 

In the case of renovation, a flexible single-walled pipe made from stainless steel or a synthetic material can usually be inserted into the old chimney.

Concentric or single-wall flue? 

The choice is entirely determined by the configuration of the building. If the masonry flue changes direction, the installation of a single-walled tube is often the only practical solution. If there is no flue or the masonry flue is straight and wide enough, a concentric tube can of course be installed.

And in an apartment building? 

If each apartment has its own flue, then the solution is straightforward. In this case it is sufficient to modify the chimney in question.

However, if the apartment building has a communal flue, then only an atmospheric boiler (type B1) can be installed. This flue must therefore be completely replaced when switching to a more efficient condensing boiler. Such decisions are obviously made in close consultation with the other owners in the building.