Shunt Flue Boilers in Older Brussels Buildings: The Constraints of Switching to Condensing Models

A flue inherited from another era, still very common in Brussels

If you live in a flat in a Brussels building constructed in the 1960s or 70s, there's a good chance your boiler is connected to a shunt flue. This exhaust system, widespread in buildings from that period, allows several stacked flats to share a single vertical collective flue, with each floor feeding its own combustion fumes into it via a side connection known as a "shunt head" or "shuttle duct".

At the time, this arrangement had an obvious advantage: it avoided the need for multiple individual flues in buildings where space was limited. The problem is that this type of flue was designed for conventional, non-condensing boilers, which release hot, dry fumes. Yet almost all boilers sold today are condensing models, which work in a fundamentally different way.

Why the shunt flue complicates the switch to condensing

A condensing boiler recovers some of the heat contained in the fumes by cooling them significantly, which causes condensation to form inside the flue system itself. The fumes released are therefore much cooler and, crucially, laden with acidic moisture, unlike those from a traditional boiler.

A collective shunt flue is, in principle, not designed to withstand this moisture or acidity over time. The original material, often untreated masonry or concrete, risks gradually deteriorating if the condensed fumes from a new appliance are sent directly into it. There's also another, more insidious issue: draught. A shunt flue works on a principle of shared suction between flats, based on the characteristics of older boilers. If just one occupant in the building installs a condensing boiler without adapting the flue, this can disrupt the draught balance for the other flats connected to the same shunt, with a risk of fumes being pushed back into neighbouring homes.

This is precisely why most boiler manufacturers state in their technical manuals that direct connection of a condensing appliance to an existing shunt flue that has not been specifically adapted is not permitted.

The risks of a poorly adapted installation

Beyond questions of comfort or the durability of the flue, there's a safety issue that allows no room for approximation. A poorly sized or poorly sealed combustion fume extraction system can cause carbon monoxide (CO) to flow back into the home or into a neighbour's flat sharing the same flue. CO is a colourless, odourless gas, and its effects on health can be very serious, even fatal, in cases of prolonged exposure.

The Brussels-Capital Region requires a mandatory EPB periodic inspection (EPB standing for energy performance of buildings) of any gas or heating oil heating installation, carried out by an approved technician, precisely to prevent this type of risk. This inspection includes checking the fume extraction and the ventilation of the room. In a building fitted with a shunt flue, this point deserves particular attention, as a fault can have consequences that extend well beyond the flat concerned.

This is why, at DPS Chauffagiste, we consistently insist on the need to examine the flue before any boiler replacement, rather than after the new appliance has been installed.

The preliminary diagnosis: an essential step

Before proposing any solution whatsoever, we always carry out an inspection of the existing flue. This step helps answer several key questions: is the flue genuinely a collective shunt, or is it in fact an individual flue that has been misidentified? What condition is it structurally in? Is it possible to fit a sealed individual liner inside it without compromising its use by other flats? What is the layout of the roof and access points in the event of works?

This inspection may require an inspection camera, a survey of the flue's internal dimensions, and in some cases discussions with the building manager or other co-owners, since the shunt flue is by definition a communal part of the building. Skipping this step means risking the installation of an appliance that performs well on paper but is unsuited to the reality of the building, potentially leading to premature breakdowns, damp problems in the walls, or even a refusal of the EPB acceptance inspection by the inspection body.

The technical solutions available

Sealed individual lining inside the shunt flue

Where the dimensions of the shunt flue allow it, one solution is to insert a flexible or rigid liner, made of a material resistant to acidic condensation (often stainless steel or a specific synthetic material), creating a sealed individual flue inside the old collective one. This solution allows each flat to keep its own extraction circuit without affecting neighbours, but it requires a sufficiently wide shunt flue and the agreement of the co-ownership, since the work affects a communal part.

The horizontal balanced flue through the façade

In many cases, the simplest and most reliable solution is to abandon the shunt flue altogether and install a balanced-flue boiler, which extracts its fumes directly outside through a concentric duct passing through the façade or roof, without going through the collective flue. This option does, however, require the flat to have an accessible external façade, and the co-ownership regulations or local planning rules must allow this type of opening, particularly for listed buildings or those in a protected area.

A case-by-case approach for flats without a suitable façade

Some flats, particularly those located within an inner courtyard or without direct access to a usable external façade, cannot accommodate either a shunt liner or a conventional balanced flue. In such cases, several options exist depending on the exact layout of the building: creating a dedicated individual flue via an existing service duct, discussing with the co-ownership a collective project to renovate the flues, or exploring an alternative heating solution better suited to the layout of the flat. As each situation is different, these options are discussed on site, following inspection, rather than offered as a standard answer.

Co-ownership consultation: often an unavoidable step

As the shunt flue belongs to the communal parts of the building, any work that modifies it, whether lining it or decommissioning it in favour of a balanced flue, generally requires the agreement of the building manager and sometimes a vote at the general meeting. This point is often underestimated by owners eager to replace an old boiler that has broken down: it's best to start this process several weeks in advance rather than finding yourself stuck at the point of installation.

What Brussels regulations provide for

The Brussels Region strictly regulates heating installations, both in terms of safety and energy performance. Any new central heating appliance must undergo an EPB acceptance inspection carried out by an approved technician, and fume extraction is an integral part of this check. The precise requirements regarding collective flues evolve along with standards, which is why we always recommend checking the current requirements with Brussels Environment before starting any works, rather than relying on information that may be several years old.

In addition, regional grants exist to support the replacement of old boilers with condensing models or heat pumps. As the conditions and amounts are reviewed regularly, it's worth checking the up-to-date information before budgeting for a project, so as not to rely on outdated figures.

Our approach at DPS Chauffagiste

Faced with a shunt flue, we never propose a standard solution without first carrying out a thorough inspection of the building. Our role is to assess the condition of the flue, check its compatibility with the various technologies available, and guide you towards the most durable solution, whether that's a liner, a balanced flue, or another configuration suited to your home. We also support our customers with the steps to take with their building manager when the work affects communal parts.

In summary

A shunt flue doesn't necessarily rule out switching to a condensing boiler, but it does call for careful technical consideration before purchasing any appliance. A preliminary diagnosis of the flue, carried out by a professional, remains the only way to guarantee an installation that is efficient, durable and compliant with the safety requirements specific to older Brussels buildings. If your current boiler is connected to a shunt flue and you're considering replacing it, don't hesitate to contact us for an on-site assessment before making any decisions.