Flue and Siting: Getting Extraction Right

More brick ovens disappoint because of the flue than because of the oven. 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 the installation with the least room for improvisation, because the physics does not negotiate.

How draw actually works

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 two things: how tall the flue is, and how hot the gases stay on the way up. That second point is why insulated twin-wall flue 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.

A flue that does not draw properly is not merely unpleasant. The fire starves, combustion is incomplete, and you get more soot, more creosote, and less heat from the same wood.

Diameter

Sized to the oven and the fuel, not to what is convenient:

Oven	Flue diameter
Gas ovens	150–180 mm
Wood ovens to 110 cm	180–200 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, not a preference.

The decision you cannot reverse

If there is any chance of converting from gas to wood later, size the flue for wood now.

A larger flue on a gas oven is a minor, manageable inefficiency. Retrofitting a 250 mm flue into a building piped for 150 mm means opening up structure, re-doing fire-stopping and flashing, and sometimes discovering there is no viable route at all.

This costs nothing to get right at survey and is among the most expensive things to fix afterwards. It is the single most common regret we encounter.

Height and termination

The flue must terminate above the roofline and clear of anything that causes 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 after the fact.

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

Clearance to combustibles

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.

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 simply vent into it. Usually it cannot.

A canopy is designed for cooking fumes at moderate temperature, not for 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 existing extraction and design around it as part of flue and extraction installation.

Siting the oven itself

Beyond the flue, three things decide where an oven can go:

• Floor loading. One to two and a half tonnes concentrated in a couple of square metres.
• Working space. Room to stand in front with a long peel, and somewhere to set pizzas down.
• Fuel access. For wood, somewhere dry to store it and a route that does not cross the service line.

Our siting guide works through each in detail.

Diagnosing a flue that will not draw

If an existing oven smokes back, work through these in order:

• Is the flue blocked? Creosote, a bird's nest, or a collapsed liner. Sweep first — it is the cheapest possibility.
• Is the fire too small or too far back? A cold flue does not draw. Establish the fire near the mouth first, then move it back.
• Is the room starved of air? Modern sealed rooms with extraction running can leave the oven competing for makeup air. Open a door and see if the smoking stops — that is diagnostic.
• Is it wind? If it only happens in certain conditions, suspect downdraught from a nearby structure. An anti-downdraught cowl often resolves it.
• Is it undersized or uninsulated? The expensive answer, and the one to eliminate last.

Get it surveyed first

Flue routing is the most common reason a quote comes back higher than expected, and the most common reason a chosen oven position turns out to be unusable.

It is a half-day survey that saves a great deal of money and disappointment. We do it free across Kenya before quoting anything.

Flue components, and what each is for

• Anchor plate or starter section — the transition from the oven's flue gather into the flue proper. Where leaks and poor draw commonly originate.
• Insulated twin-wall sections — the run itself. Keeps gases hot so the column stays buoyant.
• Fire-stop plates — at every floor or ceiling penetration. A code and insurance requirement, not an optional extra.
• Flashing and storm collar — where the flue exits the roof. The commonest source of water ingress, and water reaching a hot dome cracks it.
• Rain cap or anti-downdraught cowl — keeps water out and, in the cowl version, resists wind pushing smoke back down.

A quote that lists these individually is from someone who has installed flues before. One that says "flue" as a single line is worth questioning.

Bends cost you draw

Every change of direction slows the gas column. A straight vertical run draws best; each 45-degree offset costs a little, and 90-degree bends cost a great deal. Where a route needs offsets, they should be as few and as shallow as the structure allows, and the overall height increased to compensate.

This is why a flue route that looks merely inconvenient on a drawing can be genuinely unworkable in practice — three offsets around structure may leave you without enough draw regardless of diameter.

Sweeping access

Design the flue so it can be swept. That sounds obvious and is regularly forgotten: a run with no access point and no removable section means a sweep involves dismantling, which means it does not happen annually as it should.

We fit an access point as standard. If you are inheriting an oven, find out where yours is before you need it — and if there isn't one, factor that into your maintenance planning.