Round static cowl: a modular system so the chimney draws on its own
A chimney finished with an open mouth collects everything that falls on it: rainwater that carries soot down the flue, dust, leaves and nests. And when wind blows sideways, smoke comes back into the living room. This round static cowl solves it by fitting a small concrete tower onto the flue — base, intermediate pieces and cap — that covers the mouth without closing it and uses the wind itself to draw smoke upward, with no motor, no electricity and not a single moving part.
✅ Key advantages
Venturi effect: wind enters through the open ring between pieces and is forced through a narrower section. As the passage narrows, the air speeds up and its pressure drops; that suction right above the flue mouth pulls smoke outward. The harder it blows, the better it draws.
Three-piece modular system: a base that rests on the flue top, several intermediate pieces that form the suction chamber, and a cap that finishes the assembly. Height and diameter are combined to suit each flue.
Minimum of four intermediate pieces: that's the chamber height needed for the Venturi effect to really work. With fewer, the assembly keeps out the rain but barely improves the draft.
Round geometry, no dead corners: air flows around the assembly with no turbulence in corners, and water runs off the whole circumference instead of pooling.
Drip edges on the cap and the base: the grooved edge cuts off water and lets it fall clear, so it doesn't run down the lower pieces or stain the pillar with soot streaks.
Concrete with no internal reinforcement: there's no steel inside for acidic smoke condensates to corrode. Nothing to paint or replace: no maintenance needed.
🏗️ Recommended applications
Wood-burning fireplaces, stoves and inserts in single-family homes.
Outlets for oil, gas or biomass boilers suffering wind-driven backflow.
Ventilation and extraction ducts finishing at roof level.
Renovating chimneys that get dirty or drip soot from an open mouth.
Homes in windy, coastal or mountain areas, where downdrafts are common.
📋 Technical specifications
Material: concrete, no internal reinforcement.
Caps: Ø 40 × 4.5 cm (11.2 kg) and Ø 50 × 4.5 cm (16.7 kg).
Intermediate pieces: Ø 30 × 8 cm (4.2 kg) and Ø 46 × 9.2 cm (9.3 kg).
Bases: 38 × 38 × 8 cm (9.5 kg), 50 × 50 × 9.5 cm (17.9 kg), 65 × 65 × 9.2 cm (35.6 kg) and 76 × 56 × 9.2 cm (36.6 kg).
Distance between drip edges on the Ø 40 cm cap: 35.5 cm.
Units per pallet for the Ø 40 cm cap: 30.
Recommended set composition: 1 base + at least 4 intermediate pieces + 1 cap.
Colors available: White and Natural Grey.
Use: outdoor, on the roof.
Packaging: single unit or pallet.
🧰 Installation
Measure the internal flue section and choose the base whose opening covers it completely with firm support all around.
Level the top of the flue with mortar until you have a flat, horizontal surface.
Bed the base on fresh mortar, centered over the opening, and check the level in both directions.
Stack the intermediate pieces — at least four — aligning the axes and leaving the air-passage rings between them clear.
Finish with the cap and seal only the support joints. Don't fill in the open rings: they're what create the suction.
Let it cure for 24-48 hours before lighting the appliance.
📦 Available formats
Caps of Ø 40 × 4.5 cm and Ø 50 × 4.5 cm, in White or Natural Grey.
Intermediate pieces of Ø 30 × 8 cm and Ø 46 × 9.2 cm, in White or Natural Grey.
Bases of 38 × 38 × 8 cm, 50 × 50 × 9.5 cm, 65 × 65 × 9.2 cm and 76 × 56 × 9.2 cm.
All formats, in single-unit or pallet packaging.
💡 Professional tip: the costliest mistake with this system is building it too short. People buy the base, the cap and one or two intermediate pieces — the parts that "show" — and the assembly ends up as a rain cover with no real suction. You need at least four intermediate pieces for the chamber to be tall enough for the Venturi effect to work. Also order one extra piece per chimney: these are thin-edged pieces that chip easily going up to the roof, and a second trip for just one piece costs more than the piece itself. And check the mouth height: as good practice in Spain, it should sit 1 m above the highest point within a 2 m radius.
Todo perfecto
Envío rápido, aunque el transportista se hizonun lío con el bulto, al final la entrega fué perfecta.
Combre grava de río, el diámetro ybel color coinciden con lo ofertado. Venía algo sucia, pero nada del otro mundo.
La relación calidad/precio es inmellorable.
