On a room-sealed boiler, the concentric flue brings in combustion air and takes flue gases out through a single assembly, which makes the appliance independent of the air in the room. Two parameters decide the outcome. Equivalent length, the sum of straight runs plus bend and accessory equivalents, is calculated before you order and compared with the manufacturer's maximum for the chosen diameter. Terminal position is governed by the national gas installation rules and the flue-work standard, both backed by manufacturer's instructions that are often stricter. Everything else that causes lockouts comes from assembly direction and condensate fall.
Understanding what a room-sealed appliance changes
The flue does two jobs at once
On a room-sealed appliance, a type C, the combustion circuit does not communicate with the air in the room. The concentric flue is that principle made physical: the inner tube takes the flue gases out, the outer annulus brings fresh air in from outside. The appliance draws nothing from the room, which removes any dependence on room ventilation and the risk of spillage caused by depressurisation, particularly in a very airtight dwelling or one with powerful mechanical extract.
An underrated practical consequence: the incoming air travels around a hot tube and preheats. That helps efficiency, but it also means the assembly works as a thermally coupled whole, and a leak between the two paths recirculates flue gases into the air intake with nothing visible from outside.
Concentric or separate ducts
Concentric wins where the route is short and simple: one wall penetration, one terminal, quick installation. Separate ducts win where the routing forces the air and the flue gases apart, where the concentric length allowance is exceeded, or where the required diameter will not fit the available void. That decision is taken at the equivalent length calculation, not once the hole is cut.
The system designation
A concentric flue is a certified, CE marked system, designated to EN 1443, published in June 2019. The designation reads in order: temperature class, gas-tightness class, condensate suitability, corrosion class, chimney-fire resistance, safety distance to combustible materials. The same reading, applied instead to an existing masonry flue, is set out in our article on chimney flue lining.
| Term | What it states | What to expect on a gas condensing boiler |
|---|---|---|
| T followed by a number | Temperature class in degrees Celsius | A low value is enough, condensing flue gases are lukewarm |
| N, P or H | Tightness under negative, positive or high pressure | P, the appliance is fan-assisted so under positive pressure |
| W or D | Suitability for condensing operation | W, without exception |
| 1, 2 or 3 | Corrosion class according to fuel | 1 for gas, 2 for condensing gas and oil |
| G or O | Chimney-fire resistance | O is acceptable on a gaseous fuel |
| Number in millimetres | Safety distance to combustible materials | To be held at a combustible wall penetration |
Copy that designation onto your commissioning certificate. It is what will later prove that the system installed matched the appliance connected.
Sizing: equivalent length first
The method, and the mistake that ruins it
The manufacturer gives a maximum length per diameter, for example one for 60/100 and another for 80/125. That length is not measured with a tape, it is calculated. Each bend consumes a notional straight length, the equivalent, which differs with the angle. Each accessory, extension, appliance adaptor or special terminal consumes one too.
The calculation is therefore: developed straight lengths, plus the sum of bend equivalents, plus the sum of accessory equivalents. The result is compared with the allowed maximum. The mistake that ruins everything is counting metres of tube and forgetting the bends, because on a three-bend route the overshoot is common while the linear metreage still looks comfortable.
If you exceed it, there are only three ways out: remove bends by reworking the route, move up a diameter if the manufacturer allows it for that appliance, or switch to separate ducts. There is no fourth option, and certainly not the one where you install anyway and see what happens.
Fall and assembly direction
On a concentric flue serving a condensing boiler, fall is not a plumbing convention, it decides where the water goes. The instructions state the expected direction, generally a fall that returns condensate towards the appliance and its trap on horizontal runs. A reversed fall sends water towards the terminal, where it collects and eventually restricts the section, producing a lockout with no apparent cause.
Joints also assemble in a specific direction, the one that makes water run inside the overlap rather than against it. A component fitted the wrong way round creates a leak path that stays invisible until it drips.
The terminal
Terminal position rules come from the national gas installation requirements and the flue-work standard, with values that depend on output and terminal type, and manufacturer's instructions that are frequently stricter. Rather than memorising generic figures, hold on to the logic: the terminal must stop combustion products being drawn back into the dwelling or into a neighbouring one, and must not create a nuisance.
The configurations to avoid are always the same.
- Recesses, walkways and the underside of roof overhangs, where flue gases recirculate.
- Immediate proximity to an opening, an air inlet or an extract terminal.
- Internal facade corners, which create eddies and bring the plume back.
- Circulation routes and boundaries, where the plume becomes a neighbour dispute.
Where a facade is genuinely awkward, a roof terminal is the option that closes the argument.
Installing and handing over
Gas-tightness and penetrations
The seals carry the whole performance of the system. A knife nick on a lip, a seal pinched at a joint, a dry seal fitted without the specified lubricant, and tightness is gone with no external sign. Lock every component, fix without stressing the flue, and treat the wall penetration with the system's own sleeve and collar, holding the designation's safety distance at a combustible wall.
Commissioning
Check the tightness of the joints, the draught and the safety devices, and take the combustion readings the appliance requires. Confirm that condensate from the flue reaches the appliance trap and drains, particularly where the horizontal run is long. If the trap cannot be reached by gravity, deal with it properly using a condensate lifting station rather than improvising a fall.
The handover file
Installation instructions, proof of conformity and CE marking of the system, the full EN 1443 designation, the calculated equivalent length with the bends itemised, the commissioning record with combustion settings, and dated photographs of terminals, penetrations, brackets and fall. The "calculated equivalent length" line is what separates you from an installer who worked by eye, and it is the one almost never found in a file. That file builds itself as the job progresses when Argile ties the photos, the progress and the handover documents to the file that comes from the signed quote.
Diagnosing a concentric flue that misbehaves
Smells, spillage, rapid fouling
Suspect an obstruction at the terminal first, leaves, a nest or ice, then recirculation caused by terminal position, then a leak between the flue path and the air path. That last cause is the sly one: the appliance re-ingests its own flue gases, combustion degrades, carbon monoxide rises, and nothing shows from outside.
Excess condensation, gurgling, whistling
These three symptoms point at sizing or fall. A run that is too long or too heavily bent drops the velocity and lays down condensation. Insufficient or reversed fall holds water. A whistle means the section is too tight for the air flow the appliance demands. Redo the equivalent length calculation before changing any component.
What to correct straight away
Shut the appliance down, then check in this order: assembly direction of the joints, condition and position of the seals, actual fall measured with a level, tightness of the brackets, sealing at the penetration, condition of the terminal. A shortened, better aligned and properly fixed flue resolves most cases without replacement. Finally, remind the customer that the appliance remains subject to an annual boiler service, which includes checking the flue evacuation and measuring carbon monoxide in the room.
