Flue & Extraction Installation

The flue decides whether an oven works. A perfectly built dome with bad extraction will smoke into your kitchen, burn inefficiently, and never reach the temperature it should. It is also the part of an installation with the least room for improvisation, because the physics does not negotiate.

How draw works, and why it fails

Hot gas rises, and the column of hot air in the flue pulls fresh air in through the oven mouth. That pull is the draw, and it feeds the fire as much as it removes smoke.

It depends on how tall the flue is and how hot the gases stay on the way up — which is why insulated twin-wall matters. Single-skin pipe cools the gases, the column loses buoyancy, draw drops, and smoke takes the easier path out of the mouth into your face. It is cheaper and it is a false economy you will notice at every service.

Sizing

To the oven and the fuel, not to what is convenient to route:

• Gas ovens — 150 to 180 mm.
• Wood ovens to 110 cm — 180 to 200 mm.
• Wood ovens 120 cm and above — 250 mm.

Undersized and the fire starves and smoke backs up. Oversized and the gases cool too much on the way, which also weakens draw. This is a specification rather than a preference. The full reasoning is here.

Termination and weathering

The flue must terminate above the roofline and clear of anything causing turbulence — parapets, adjacent taller buildings, large trees. Downdraught from a nearby structure will push smoke back down a perfectly sized flue on a windy evening, and it is maddening to diagnose afterwards.

A rain cap is essential. Water down a flue reaches the dome, and water in hot refractory is how you crack an oven.

Clearance and fire-stopping

Insulated flue still runs hot. It needs specified clearance to timber, plasterboard studwork, ceiling joists, and anything else that will burn, with proper fire-stopping where it passes through a floor or roof, and correct flashing where it exits.

This is a safety and insurance matter rather than a detail. Get it wrong and a claim can be refused, which turns a fire into a business-ending event rather than an insured one.

Working with an existing kitchen canopy

Kitchens with canopy extraction often assume the oven can vent into it. Usually it cannot. A canopy is designed for cooking fumes at moderate temperature, not combustion gases — pushing oven exhaust into it can overwhelm the system, exceed its temperature rating, and load the grease filters with soot.

The normal solution is a dedicated flue for the oven, with the canopy capturing what escapes the mouth during loading. We survey the existing extraction and design around it rather than assuming it can be shared.

Diagnosing an oven that smokes back

If you already have an oven with poor draw, we work through it in order — cheapest first:

• Blockage. Creosote, a bird's nest, a collapsed liner. Sweep before diagnosing anything else.
• Fire position. A cold flue does not draw. Establish the fire near the mouth, then move it back.
• Makeup air. A sealed kitchen with extraction running leaves the oven competing for air. Open a door — if the smoking stops, that is your answer.
• Wind. If it only happens in certain conditions, suspect downdraught. An anti-downdraught cowl often resolves it.
• Sizing or insulation. The expensive answer, and the one we eliminate last.

The decision to make early

If there is any chance of converting from gas to wood later, size the flue for wood at the outset. Retrofitting a larger flue means opening up structure, redoing fire-stopping and flashing, and occasionally discovering no viable route exists.

It costs nothing to specify correctly at survey. It is the most common regret we encounter, and it is entirely avoidable — which is what our design and consultation service exists to prevent.