How do I size a common flue for multiple boilers?

Sum connected gas input, apply a preliminary velocity limit by draft type (natural, fan-assisted, condensing), and select a circular flue diameter.

Is this sufficient for Building Regulations sign-off?

No. This is a rule-of-thumb screen. Final compliant sizing requires BS EN 13384-2 with manufacturer flue data and the approved chimney system.

What velocity limits apply to condensing flues?

Condensing appliances typically allow higher flue gas velocities. Always use the appliance manufacturer's published flue guidance.

BuildFlo Design Guide · Thermal · BS EN 13384-2

Common flue sizing calculator

Preliminary common flue diameter from connected gas input. Velocity check per draft type — final design per BS EN 13384-2.

What this calculator does

Flue sizing helps UK building services engineers with preliminary common flue diameter from connected gas input - velocity check per draft type. The calculation follows BS EN 13384-2 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "flue sizing", "flue sizing calculator", or "how to flue size", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: mm.

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about flue sizing under UK standards.

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Frequently asked questions

How do I size a common flue for multiple boilers?: Sum connected gas input, apply a preliminary velocity limit by draft type (natural, fan-assisted, condensing), and select a circular flue diameter.
Is this sufficient for Building Regulations sign-off?: No. This is a rule-of-thumb screen. Final compliant sizing requires BS EN 13384-2 with manufacturer flue data and the approved chimney system.
What velocity limits apply to condensing flues?: Condensing appliances typically allow higher flue gas velocities. Always use the appliance manufacturer's published flue guidance.