You have your single-wall stainless steel flue pipe, your heating appliance, and a wall or roof to cross. The rest is about connecting everything properly. Conical reducers, elbows, drain tees, fittings: each part has a specific role, and an assembly error can compromise the draught, sealing, or compliance of your installation. Here's how to navigate it.

The assembly principle: male at the bottom, female at the top

The basic rule for single-wall flue systems is: always insert the male end into the female end, working from bottom to top. The goal: if condensation forms inside the flue, it flows downwards without leaking at the joints. This is called condensation assembly. Conversely, "smoke" assembly (female at the bottom) is used only for natural draught installations without a risk of condensation — this is rarer in a professional context.

Each joint must be secured by a clamping collar suitable for the diameter. No self-drilling screws in the flue: they create points where soot can accumulate and weaken the seal.

Straight pipes: the backbone of the flue

The single-wall stainless steel straight pipes form the main part of the vertical or horizontal run. Available in lengths of 250, 500, 1,000, and 2,000 mm, they combine to achieve the necessary height without cutting.

Two practical tips: opt for the longest possible lengths to limit the number of joints (fewer joints = less risk of leakage). And always include a telescopic or adjustable pipe in your run to compensate for any measurement discrepancies on site.

Elbows: changing direction without compromising the draught

Elbows allow you to bypass an obstacle (beam, wall, existing flue) or connect an appliance whose outlet is not vertical. They are available in 30°, 45°, and 90°.

Which angle to choose?

The rule: the smallest possible angle. Each change of direction slows down the smoke and reduces the draught. A 30° elbow causes less disturbance than a 45°, which causes less disturbance than a 90°. In practice:

30° Elbow: slight offset to avoid a specific obstacle. Minimal impact on draught.

45° Elbow: the most versatile. Two 45° elbows combined create a clean lateral offset, and this is often preferable to a single 90° elbow.

90° Elbow: to be reserved for horizontal connections (appliance outlet, entry into a masonry flue). Avoid stringing two together on the same run — the draught will suffer significantly.

Regulatory reminder: the NF DTU 24.1 standard generally limits to two changes of direction per flue, with a total cumulative angle not exceeding 180°.

Tees: connection, purge, and attachment

The flue tee is the part that allows you to connect an appliance to an existing vertical flue, or to create a purging point for maintenance. There are two configurations:

90° Tee with purge cap: the most common. The appliance connects horizontally, smoke rises into the vertical flue, and the cap at the bottom allows for inspection and cleaning. Essential at the base of the flue.

135° Tee: used when the connection needs a gentler angle, often to improve draught or adapt to a constrained spatial configuration.

The purge cap (or plug) at the bottom of the tee is not optional: it allows for the evacuation of condensates and access for sweeping. Some installers forget it and end up having to dismantle half the flue to maintain the installation.

Conical reducers: connecting two different diameters

When your appliance's outlet doesn't match the flue's diameter, you need a conical reducer. A classic case: an air heater with a Ø100 outlet connected to a Ø125 or Ø130 vertical flue.

Reducers are in a female (large diameter) / male (small diameter) configuration. They are available in all common combinations: Ø80→125, Ø100→125, Ø110→125, Ø125→130, Ø130→150, Ø150→200, etc.

Pay attention to the direction: in vertical assembly, the reducer must respect the direction of condensate flow — the large diameter at the top. If in doubt, check the male/female direction relative to adjacent parts.

Fixings and supports: what holds everything in place

A vertical stainless steel flue several meters long is heavy. Without correct fixing, the joints will work loose, the connections will separate, and the entire assembly becomes dangerous. Here are the essential fixing parts:

Wall bracket: fixed to the wall, it supports the weight of the flue. One bracket every 2 to 3 meters on a vertical run, and one bracket at each change of direction.

Descent collar: locks the vertical flue to prevent it from sliding down under its own weight.

Guying clamp: for flues extending above the roof. It allows guying wires to be attached to stabilize the flue against wind.

Slab support: when passing through a floor, it supports the flue and ensures safety distances are maintained.

Finishing touches: sealing and roof protection

The roof penetration is the critical point of any installation. Two parts are essential:

Flashing: the part that ensures watertightness between the flue and the roof covering. It is available in stainless steel or lead, and in several pitches (0-15°, 10-30°, 25-45°) to adapt to your roof's inclination. A poorly chosen flashing = guaranteed water infiltration.

Cowling: at the top of the flue, it protects against rain and improves draught in windy conditions. Flat cowling for flues under shelter, classic Chinese hat cowling for exposed flues.

Don't forget the finishing plates at wall and ceiling penetrations: they cover the opening and give a finished look to the installation.

The 5 classic mistakes to avoid

1. Getting the jointing direction wrong. Male at the bottom, female at the top. Always. An inversion and condensates will leak at the joints.

2. Too many 90° elbows. Each elbow reduces the draught. Two 45° elbows are better than a single 90° in most cases.

3. Forgetting the purge cap. Without access at the base of the flue, maintenance becomes a headache. Plan for it during assembly.

4. Under-dimensioning the fixings. A flue that moves will eventually leak. A wall bracket every 2-3 m and a collar at each joint is the minimum.

5. Neglecting the flashing pitch. A 0-15° flashing on a 35° roof will never be watertight. Measure your pitch before ordering.

Need to check the parts list for your installation? Contact us with your configuration (appliance, outlet diameter, flue height, roof type) — we'll provide you with the complete nomenclature.