Shaft for direct vent boiler

The gas boiler duct is a key element for safely evacuating the fumes from combustion. In a gas condensing boiler, it also ensures the arrival of the air necessary for proper operation: it’s an "airtight" system designed for controlled evacuation, without depending on the home's natural draft.

At Expert by Net, we assist both individuals and professionals in choosing a duct that is truly compatible with their gas condensing boiler. On Expertbynet.fr, you will find reliable solutions (elbows, extensions, terminals) for a durable installation, both for new setups and replacements.

• Ensure a safe evacuation that complies with the installation
• Optimize the operation of the gas boiler (stable combustion)
• Avoid breakdowns related to improperly sized evacuation

Warning: a condensing duct must adhere to strict rules (compatibility, lengths, accessories). If in doubt, check the manufacturer's instructions or consult the Expert by Net team.

How to choose a gas boiler duct?

A duct for a gas boiler, especially in a condensing setup, should not be chosen “by eye.” The right duct is one that corresponds exactly to your boiler and your configuration (wall exit, roofing, elbows…). A detail can change everything: a diameter, a maximum length exceeded, or an assembly of incompatible parts.

1) Identify the type of boiler

First, check whether your boiler is indeed a condensing (airtight circuit) or on a "chimney" duct. The ducts are not interchangeable. The boiler manual or the nameplate usually provides this information.

2) Check the diameter (and do not "approximate")

The most common diameters in condensing systems are 60/100 or 80/125 (depending on models). This point is non-negotiable: a wrong diameter prevents correct assembly and can generate operational defects.

3) Choose concentric or separate

Depending on the boiler, the air and the fumes pass through a concentric duct (two "tubes" within each other) or through two separate ducts (air + fumes). The configuration depends on the manufacturer and type of device.

4) Plan the right exit configuration

Wall exit (horizontal) or roof exit (vertical): the accessories are not the same. Also consider the bulk, necessary elbows, and accessibility for maintenance.

5) Respect maximum lengths (extensions, elbows, pressure drops)

Each boiler imposes a maximum length of duct, and elbows “count” too (they add pressure drops). The exact rules vary by model: refer to the manual to avoid exceeding what is allowed.

6) Consider condensation and materials

A gas condensing boiler produces moisture. The duct must be designed to withstand condensation and corrosion. If you are replacing part of the duct, ensure the entire system remains consistent.

7) Brand/model compatibility: a true point of vigilance

Even if some parts look alike, there are differences between manufacturers and ranges. At Expert by Net, we recommend relying on the boiler reference and manufacturers' recommendations rather than "mixing" elements.

Common mistakes to avoid

• Choosing without checking the diameter (60/100 and 80/125 are not interchangeable).
• Exceeding the maximum length by adding extensions and elbows without calculation.
• Installing an unsuitable duct for a condensing boiler (humidity/corrosion).
• Mixing incompatible parts (different brands/systems).
• Neglecting the installation (wall/roof exit) and the constraints of the area.

Compatibility questions

A gas boiler duct must correspond to the type of device (condensing or not), the system set by the manufacturer (concentric or separate), and the mode of exit (horizontal or vertical).

In renovations, be careful: an old installation may require an adapter or a complete kit rather than just a "piece" of duct. If you are unsure, compare the boiler reference, diameter, and configuration before ordering on Expertbynet.fr.

Maintenance & lifespan

A condensing duct requires little daily maintenance, but it must be checked during the annual boiler maintenance. A blockage (dust, nests, debris) or a defective assembly can lead to a safety shutdown.

If you notice repeated shutdowns, error codes related to evacuation, or abnormal condensation, checking the whole system (duct + accessories) is often helpful before randomly replacing parts.

When to call a pro?

The evacuation of fumes is one of the sensitive aspects of a gas boiler. If you are in a collective housing, if the exit needs to be modified, or if you have any doubts about compliance, have a heating engineer intervene. They can validate the configuration, acceptable lengths, and compatibility, and secure the installation.

Need a quick reference? The team at Expert by Net can help you identify the right diameter and compatible accessories based on the boiler reference.

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