Please note: The solutions presented here are options for consideration, as every building has its own specific requirements. Before taking any action, always consult a qualified professional to determine the right option for your situation.
Your old boiler has just given up the ghost and you're planning to replace it with a new condensing model, more energy efficient and effective. Great idea! But before you go ahead and sign the order form, don't forget about your chimney. In an apartment, you need to contend with the apartment building's technical constraints and the rules of the co-ownership.
Can I connect a new condensing boiler to a collective chimney? Yes, but not without a prior technical inspection.
A condensing appliance has specific requirements regarding flue gases. For boilers connected to a shared chimney, a collective solution designed by a specialist in the field should be considered.
Depending on the type of flue, relining, a new separate exhaust or a global renovation of the system may be necessary. In a co-ownership, the decision also depends on the status of the flue, the rules of the apartment building and, sometimes, authorisation from the general meeting.
Mixed connections are a no go!
Trying to connect an ultra-modern boiler to an old flue is a bit like plugging an induction hob into old copper wiring: it simply wasn't designed for it. It's risky, and regulations prohibit it.
It's a physical issue : a condensing boiler ejects much colder byproducts, filled with water vapour.
- Corrosion and watertightness: The water vapour is transformed into condensates (acidic water) which attack the walls of stone flues or old aluminium flues. To solve this issue, you can use lining: Slide a new, watertight flue inside the old one. You should choose a rigid, polypropene lining if the flue is perfectly straight, or a flexible lining if there are bends in the chimney.
- Backflow: Condensing boilers work with a fan (forced draught). Injecting these vapours under pressure into a flue shared with devices using a natural draught may cause a malfunction or lead to the risk of vapour backflow.
- Removal of condensates: The condensation "makes" acidic water. It is essential that these condensates are removed to the wastewater network. If your old boiler was far from a drain, you will need to install a condensate pump or create new connections.
Do I need to reline my chimney if I have a condensing boiler?
With a condensing boiler, your chimney needs adapting. Discover your options and whether this is the moment to look beyond gas.
Collective solutions
Whether it involves a flue passing through common areas or a shaft jointly owned by neighbours, any intervention on a vertical stack strictly requires collective approval (either a vote at the general meeting or the unanimous agreement of the households connected to that same stack).
As every building has its own unique technical requirements, it is essential to explore different options with a qualified professional. Here are the most common solutions:
- Individual relining within the existing flue: A new, sealed independent liner is inserted for each new boiler as needed, while maintaining separate vents for the remaining older appliances. This approach allows each co-owner to switch to a condensing boiler at their own pace, although it often proves more expensive in the long run than a complete, building-wide overhaul.
- Installing an external flue: If the original chimney is too narrow, a double-walled stainless steel flue can be installed on the rear facade. While highly efficient, this option alters the building's exterior appearance and therefore strictly requires formal approval from the co-owners' general meeting (GM).
- A new collective relining system: This is the ultimate shared solution. A single, sealed collective liner is inserted into the existing chimney, which requires all atmospheric boilers connected to that specific vertical stack to be replaced with condensing models at the same time.
Are non-condensing boilers still permitted?
B1 type atmospheric boilers have been on borrowed time since the tightening of Ecodesign requirements. In some apartment buildings, fitted with collective chimneys, they may still exist, but the feasibility of replacing it should be assessed on a case-by-case basis, depending on the region and the type of flue.
Individual solutions subject to conditions
Even if you decide to become technically independent of the collective flue, you are still tied into the life of the apartment building. In a co-ownership, any modifications that affect the building envelope or the acoustic comfort require a prior agreement. If you choose to separate yourself, you could consider:
- Concentric facade vent (horizontal flue): A hole is drilled through the exterior wall to evacuate the flue gases. Please note that in dense urban areas, planning regulations and minimum clearance distances from neighbouring windows are so strict that this option is often difficult to implement in an apartment.
- A heat pump: Replacing your heating system with a heat pump featuring an outdoor unit also requires the agreement of the co-owners (due to potential noise nuisance and visual impact) and must comply with local police regulations on noise.
Is it possible to heat an apartment with a heat pump?
What if you decide to move up a gear?
Even if it means having to carry out some expensive lining work, why not suggest to the property management that you all stop tinkering with your own sections and instead build a collective solution that's actually sustainable? If the building allows, there are a few possible solutions, such as:
- Cogeneration (CHP): Here, the system feeds the building's heating network, while the electricity produced is used for the common areas. By turning the boiler room into a small local power plant, the building self-generates the energy needed for lifts, lighting, and pumps.
- Collective heat pump: Cascades of heat pumps are installed on the roof or in an inner courtyard to feed a centralised network. This is a future-proof solution, but take note: many older buildings often require insulation to be reinforced beforehand.
- Collective hybrid: An interesting compromise for existing buildings. A heat pump covers most of the year's needs, while a high-performance gas boiler takes over during cold spells. This allows the building to decarbonise without having to replace all the private radiators.
Which route should you take?
Given the variety of possible situations, there is no 'silver bullet' for the chimney issue. Here are a few paths to consider:
- Looking for a long-term solution for the whole building? A new collective relining, with the replacement of all boilers, could be the best choice. By sharing the relining costs, the building enhances its property value and allows everyone to modernise their system without spoiling the façade.
- Your boiler's just given out and you need a solution quickly? A ventilation outlet could be a replacement option, under strict conditions. It requires approval from the general meeting and strict compliance with emission zones to avoid any conflicts with neighbours.
- Keen to get ahead of the energy transition? An individual heat pump can offer autonomy. However, you need to allow time to consult property management for them to approve the location and acoustics of the external unit.
Ask a specialist
Whatever solution you're considering, you should always ask a specialist. Simply "tinkering" and trying to achieve an approximate solution should be out of the question: This is about the safety of all the building's occupants. Only an expert in these issues can take into account all the technical aspects. Not all heating engineers have this expertise and the best thing to do is refer to a specialist consultancy.