Twin Skin Flue Systems

If the building does not have an existing chimney, you can install a twin-wall metal chimney. When setting up a new appliance with this type of chimney, the first choice is whether to route it externally or internally within the building.

The chimney must generate adequate draught to optimise appliance efficiency. The most effective configuration for achieving this is a straight chimney routed internally through the building. This setup maximises temperature buoyancy and minimises chimney resistance.

Opting for an internal chimney not only tends to be the most economical choice but also enhances the building’s exterior aesthetics compared to mounting the chimney on an external wall.

In cases where an internal route through the building is not practical, the chimney can instead be directed through an external wall and run externally.

Air-Tight Houses

For houses built in or after 2008, the chimney must comply with airtight building regulations. This requires an installation that prevents heat loss through any external walls or ceilings it passes through. Poujoulat’s twin-wall chimneys have components designed specifically to increase air-tightness and reduce heat loss

Finishing Touches

For all chimneys, whether renovated or new, it is essential to include a CO alarm and air vents for safe and compliant operation.
Additionally, take into account these useful accessories:

  • Installing a thermometer on the connecting flue pipe provides better appliance control
  • A moisture meter ensures the wood you burn is dry, enhancing efficiency
  • A heat shield can safeguard timber frame houses and contribute aesthetically
  • Adding a boxed-in air grill GVH can increase room heat by up to 1 kW.

Creating an Internal Chimney

Installing a chimney internally through the building provides flexibility to position the stove as desired, ideal for maximising space, especially when paired with a free-standing appliance. Moreover, THERMINOX twin-wall’s requirement of just a 50mm safe distance to combustible materials further enhances flexibility in appliance placement.

By keeping most of the twin wall system indoors and venting through the roof, you create a warmer flue that boosts the stove’s efficiency, facilitating easier lighting and operation, particularly in colder conditions.

One consideration however is that in buildings with two or more storeys, an internal flue system will need to pass through the living spaces above the appliance, as well as the roof space. These areas must be protected by enclosing the flue in a ‘boxed in’ structure and ensuring it is ventilated to maintain safe distances from combustible materials.

Typical internal chimney configuration

New internal chimney with log burner

Creating an External Chimney

If space is limited inside you can install the stove against an outside wall, with the twin-wall flue pipe exiting through the wall at a 45º angle above the appliance, then running the rest of the flue system up the outside wall to a point above the eaves.


While external flues might not be as efficient due to additional bends and the colder air affecting combustion gas temperature, they remain a practical alternative to building a traditional chimney. Plus, they won’t impact your upstairs space. You will need to route the flue system through an external wall, which limits stove placement options. An external twin wall system tends to be more expensive than an internal one because of the extra bends.

Which Flue?

Polished stainless steel and powder-coated black twin-wall chimneys are popular choices when creating external chimneys. Black is particularly common outside the house as it matches other service pipes. It can also be customised with a copper outer casing (Therminox TI) or powder-coated in any RAL colour to order

Poujoulat’s traditional THERMINOX TI twin-wall is a popular choice for external chimneys, offering an extensive range of components to suit every eventuality. For external applications, Therminox TI, is available in polished stainless-steel and powder-coated black finishes. It can also be powder-coated in any RAL colour to order

Passing Through the Wall

The external chimney passes through the wall via a wall sleeve and is finished with a wall plate flashing, painted and sealed to the wall using standard silicone sealant.

The wall penetration can be installed at a high level in the room with the appliance, providing better clearance for the chimney outside.

For airtight or timber frame houses, it can also be fitted with our insulated wall collar “COQUISOL” and a specially adapted airtight wall plate “PDSER”

Typical external chimney configuration