[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

In apartment projects the room-sealed combi boilers are usually placed in kitchens or on balconies, and the horizontal flue terminal goes through the external wall. Facade designers ask how close the terminal may be to windows, balconies, corners and neighbouring buildings.

• What are the principles behind flue terminal clearances?
• Which documents set the actual distances?
• What should be coordinated with the architect early?

I do not need a table of distances; I will use the local code and the manufacturer’s instructions.

[MEP Center Engineering]

Terminal clearances exist so that flue products cannot re-enter the building or the appliance, cannot harm people, and cannot damage the facade. The binding distances come from the local gas code (or the utility’s technical specification) and the boiler manufacturer’s installation instructions; where they differ, the stricter one applies.

The principles behind each clearance

1. Re-entry into the building: distance from openable windows, doors, air bricks and ventilation intakes, measured horizontally, above and below the terminal. Openings above the terminal are critical because flue gas rises.
2. Re-circulation into the appliance: corners, recesses, balconies with solid parapets and areas under eaves trap the plume, and a room-sealed boiler can draw its own flue gas back in, leading to poor combustion.
3. People and property: minimum height above ground, balconies and walkways where people stand; terminal guards where the terminal is within reach; distance from boundaries facing a neighbour’s window.
4. Plume nuisance and damage: condensing boilers produce a visible plume and acidic condensate that stains facades, freezes on paving and damages some finishes. Distances from facing walls, car ports and soffits address this.
5. Other openings and services: distance from other terminals, gas meters, electrical boxes and drain pipes.

Why condensing boilers need extra attention

Burning methane produces about 1.6 kg of water vapour per m³ of gas (2 m³ of vapour per m³ of methane). A 24 kW boiler using 24 / 9.5 = 2.53 m³/h of gas therefore releases up to about 2.53 x 1.6 = 4 kg/h of water vapour. Part of it condenses inside the boiler and drains away; the rest leaves as a plume. This is why terminal position, plume kits and condensate drainage are part of the design, not site decisions.

Design and coordination steps

• Fix boiler locations with the architect at concept stage, with flue routes shown on plans and elevations.
• Mark the terminal on facade elevations with the clearance zones from the applicable code drawn around it, and check windows, balconies and adjacent terminals on all floors.
• Check maximum flue length and number of bends against the manufacturer’s limits; horizontal flues of condensing boilers are normally laid falling back towards the boiler (commonly about 3 degrees, roughly 50 mm per metre) so that condensate drains to the appliance.
• Where clearances cannot be met, consider a vertical flue, a plume management kit, or a shared flue system approved for the appliance and permitted by the local code.
• Show the condensate drain route to a trapped drain.

Common mistakes

• Terminal directly below an openable window of the floor above.
• Balcony later glazed in by the occupant, turning an outdoor terminal into an indoor one; many codes prohibit flue terminals inside enclosed balconies.
• Terminals in light wells or narrow passages where the plume cannot disperse.
• Using distances from another country’s guide or another manufacturer.

Related: Natural gas pipe sizing basics. The gas utility’s approval and the local gas code govern the final terminal positions.

Check the edition of the code that applies to your project.

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