Proper flue design by commercial flue installers is absolutely critical to getting the best out of a wood-fired pizza oven. Whether you’re installing a small domestic oven in a garden kitchen or a large commercial oven in a restaurant, the pizza oven flue extraction system will affect:

  • How quickly the oven gets up to temperature
  • How well smoke and fumes are removed
  • Fuel efficiency and ongoing running costs
  • Overall safety and compliance with building regulations

This guide explains what a twin wall flue system is, why it’s usually the best choice for pizza ovens, and what you need to consider when sizing, installing and maintaining it.

What Is a Twin Wall Flue System?

A twin wall flue system is a prefabricated, insulated metal chimney made from two layers of stainless steel with high-temperature insulation in between. It is designed to safely carry hot combustion gases from the pizza oven to outside the building or cooking area.

Typical features include:

  • Inner liner: Stainless steel tube that is directly exposed to the hot gases from the oven.
  • Insulation layer: Usually mineral wool or ceramic fibre between the inner and outer walls, reducing heat loss and protecting nearby materials.
  • Outer casing: Stainless steel shell that remains significantly cooler than the inner liner.
  • Locking/jointing system: Twist-lock or clamp bands to join each section securely.
  • Support components: Brackets, plates and adaptors that allow the system to be safely fixed to the structure.

The insulation is what distinguishes twin wall flue from basic single-wall pipe. For pizza ovens, that insulation is key to maintaining flue gas temperature, improving draft and ensuring safe clearances to nearby combustible materials.

Why Pizza Ovens Often Require Twin Wall Flue Systems

1. Temperature Management

Wood-fired pizza ovens frequently operate at internal temperatures of 350–500 °C or more. The flue must cope with these high temperatures without failing or causing excessive heat transfer to surrounding structures.

  • Insulation keeps the flue gases hot for longer, which improves the natural draft.
  • Hotter flue gases help evacuate smoke quickly and reduce the tendency for smoke to spill from the mouth of the oven.
  • Stabilised temperatures help the oven reach and maintain its ideal cooking range more efficiently.

2. Safety Requirements

Because the flue often passes close to combustible materials (timber roofs, plasterboard ceilings, cladding, etc.), a twin wall system significantly improves safety:

  • Lower external surface temperatures reduce the chance of heat damage or fire.
  • Properly designed components provide tested clearances to combustibles.
  • Secure joints help prevent fumes or smoke escaping into the building.

3. Performance and Cooking Quality

A well-designed flue system is not just about safety – it has a direct impact on the way the oven cooks:

  • Correct draft enables a strong but controllable fire, improving flame pattern and heat distribution.
  • Better extraction means less smoke in the cooking area and a more pleasant environment.
  • Consistent combustion can enhance the characteristic wood-fired flavour without over-smoking the food.

Materials & Construction Standards

Stainless Steel Grades

For wood-burning pizza ovens, the inner liner of a twin wall flue is usually:

  • 316 grade stainless steel for the inner wall – offers good resistance to the acidic condensates and high temperatures associated with wood combustion.
  • 304 grade stainless steel for the outer wall – provides a durable, attractive outer finish.

Cheaper systems or unapproved single-wall components can corrode rapidly, deform under heat or fail to provide safe clearances. Using an appropriately rated twin wall system is essential for longevity and safety.

Temperature & Soot Fire Rating

In Europe and the UK, metal chimney systems are tested and certified under standards such as EN 1856-1. For pizza ovens, you will typically look for:

  • Temperature class T600 – suitable for continuous high-temperature operation.
  • Soot fire resistance “G” – means the system has been tested to withstand a soot fire under specific conditions.

Always check the product data plate and documentation to ensure the flue is correctly rated for solid fuel appliances operating at high temperatures.

Regulatory Context (UK)

In the UK, chimney and flue installations usually fall under Building Regulations Approved Document J (Combustion Appliances and Fuel Storage Systems). For indoor ovens, and many commercial projects, compliance with Part J is essential. It covers:

  • Flue sizing and design principles
  • Clearances from combustibles
  • Termination height and location
  • Ventilation and combustion air supply

For commercial installations, planning permission, environmental health and ventilation requirements may also apply. Using competent installers (e.g. HETAS-registered for solid fuel) helps demonstrate compliance.

Key Components of a Pizza Oven Twin Wall Flue System

A typical twin wall system for a pizza oven may include:

Straight Lengths

  • Standard sections (for example 250 mm, 500 mm and 1000 mm lengths).
  • Twist-lock or clamp-band joints to connect each length securely.

Bends & Offsets

  • 15°, 30° and 45° bends are commonly available.
  • Used to navigate roof structures, eaves or obstacles, but kept to a minimum to maintain draft.

Support Components

  • Wall brackets to fix vertical runs to a wall.
  • Roof support plates and guy brackets for tall external chimneys.
  • Ceiling and wall sleeves to provide safe passage through structures.

Terminals and Cowls

  • Rain cap or basic terminal to prevent water ingress.
  • Anti-downdraft cowls in exposed or windy locations to help stabilise the draw.
  • Spark arrestor mesh where required – but only if it can be cleaned easily, as it can clog with soot.

Adaptors & Connectors

  • Special adaptors to connect the pizza oven outlet to the first flue section.
  • Increasers/decreasers where a change in diameter is necessary (subject to design rules).

How to Size the Flue for a Pizza Oven

Flue Diameter

The correct flue diameter depends on the oven design, outlet size and manufacturer recommendations. As a rough guide:

  • Smaller domestic pizza ovens often use flues in the region of 125–150 mm diameter.
  • Larger domestic and small commercial ovens may use 150–200 mm.
  • Large commercial ovens may require 200–350 mm or more, depending on their capacity and design.

Always check the oven manufacturer’s technical guidance first. Oversizing or undersizing can cause performance problems.

Matching Flue Size to Oven Volume

If the flue is too small you may see:

  • Smoke spillage from the mouth of the oven.
  • Slow heat-up times and poor combustion.
  • Excessive soot and creosote buildup.

If the flue is too large you may see:

  • Weak draft because the gases cool too quickly.
  • Difficulty maintaining a stable fire.
  • Unnecessary cost for overspecified components.

Height Requirements

A minimum effective flue height of around 3–4 metres is often recommended for reliable draft, though this can vary by oven design and installation type.

  • Where possible, keep the flue run as vertical and straight as you can.
  • Limit the number of bends – each bend adds resistance and can reduce draft.
  • For indoor installations, ensure the terminal height and position meet local building regulations and avoid nearby obstructions.

Installation Guide for Pizza Oven Twin Wall Flue Systems

Important: The following is a general overview. Always follow the oven and flue manufacturer’s installation instructions and relevant building regulations, and use a competent installer where required.

Pre-Installation Checks

  • Confirm whether the oven is indoor or outdoor, domestic or commercial.
  • Check the oven outlet size and recommended flue diameter.
  • Plan the flue route: straight up, or with offsets to avoid structural elements.
  • Check clearances to combustible materials and space for required fire-stopping and support components.

Step-by-Step Overview

  1. Fit the oven adaptor
    Secure the appropriate adaptor to the oven outlet, ensuring a gas-tight, mechanically sound connection.
  2. Install the first flue section
    Attach the first length of twin wall flue to the adaptor. Ensure it is plumb and properly locked or clamped into place.
  3. Build the vertical run
    Add further lengths, locking each joint and fitting support brackets as you go. Check alignment regularly.
  4. Use bends only where necessary
    If the route requires offsets, install the appropriate bends and connectors, supporting them as specified.
  5. Pass through structures safely
    Where the flue passes through a ceiling, roof or wall, use the manufacturer’s fire-safe sleeves, flashings and trim plates. Maintain the correct clearance to combustibles.
  6. Terminate correctly
    Fit the terminal or cowl and ensure it is securely fixed. Check that the termination height complies with local regulations and is clear of obstructions.
  7. Test the system
    Carry out a smoke or draw test according to guidance. Check for leaks at joints and confirm that the oven draws correctly under normal operating conditions.

Common Installation Mistakes

  • Using single-wall pipe externally, leading to major heat loss and poor draft.
  • Installing too many bends or sharp offsets.
  • Inadequate support, allowing the flue to move as it heats and cools.
  • Failing to respect clearance distances or not using the correct penetration components.
  • Mixing incompatible brands or unapproved components.

Indoor vs Outdoor Pizza Oven Flue Considerations

Indoor Installations

Indoor pizza ovens must be designed and installed with particular care:

  • Compliance with Building Regulations (e.g. Approved Document J in the UK).
  • Provision of adequate permanent ventilation for combustion air.
  • Correct flue termination height above the roof and away from openings.
  • Use of tested components where the flue passes through floors, ceilings or walls.
  • In many cases, work should be carried out or signed off by a competent person (e.g. HETAS-registered for solid fuel).

Outdoor Installations

Outdoor ovens offer a bit more flexibility, but there are still important considerations:

  • Wind exposure can create downdrafts – an anti-downdraft cowl may be beneficial.
  • External flues should be properly supported to withstand weather and thermal movement.
  • Rain protection is essential to reduce water ingress and extend the life of the flue.
  • Consider the impact of smoke on neighbouring properties and seating areas.

Common Problems with Pizza Oven Flue Systems

Smoke Leakage and Poor Draft

Symptoms include smoke spilling from the oven mouth or lingering in the cooking area. Causes can include:

  • Flue too short or with too many bends.
  • Undersized diameter or constrictions in the route.
  • Cold flue (especially with uninsulated external sections).
  • Adverse wind conditions or poor terminal location.

Creosote and Soot Buildup

Excessive tar and soot can accumulate if:

  • You burn damp or unseasoned wood.
  • The oven is frequently run at low temperatures.
  • The flue is oversized and gases cool too quickly.

A heavy buildup increases the risk of chimney fires and can restrict the flue, reducing performance.

Corrosion and Component Failure

Corrosion can result from aggressive condensates, poor-quality materials or continuous exposure to moisture. Signs include:

  • Discolouration or rusting of the outer casing.
  • Staining or leaks at joints.
  • Distortion or warping of sections.

Regular inspection helps identify these issues early before they become serious safety risks.

Maintenance & Operational Tips

Looking after the flue system is essential for safety and performance.

  • Sweeping: Have the flue swept regularly by a competent chimney sweep. Frequency depends on usage – for heavy commercial use, this may be several times per year.
  • Fuel quality: Burn only dry, seasoned hardwoods. Avoid treated timber or rubbish, which can damage the flue and create excess deposits.
  • Visual checks: Inspect brackets, locking bands and joints periodically for movement, corrosion or damage.
  • Terminal maintenance: Clean any mesh or spark arrestors to prevent blockage.
  • Record keeping: Keep a log of maintenance, sweeping and any repairs for safety and insurance purposes.

Cost Considerations

Costs vary depending on the size and complexity of the system, the flue brand, and whether the installation is domestic or commercial. As a broad guide:

  • Domestic outdoor systems: A basic twin wall flue for a small garden pizza oven may range from a few hundred pounds upwards, depending on height and accessories.
  • Indoor or commercial systems: More complex runs with multiple penetrations, support structures and higher flue heights will typically be significantly more expensive.

Key cost drivers include:

  • Flue diameter and overall height
  • Number of bends and special components
  • Roof/wall penetration kits and flashings
  • Brand, certification and material quality
  • Labour, access and any associated building works

While it can be tempting to cut costs with cheaper or improvised components, a properly rated, professionally installed twin wall flue system is a long-term investment in safety, performance and regulatory compliance.

Conclusion

A well-designed twin wall flue system is a vital part of any wood-fired pizza oven installation. It helps your oven reach and maintain the high temperatures it needs, keeps smoke and fumes under control, protects your building or outdoor structure, and supports compliance with relevant standards and regulations.

If you are planning a new pizza oven or upgrading an existing installation, consider:

  • Choosing a properly rated twin wall system with suitable temperature and soot-fire classifications.
  • Correctly sizing the flue diameter and height for the specific oven.
  • Minimising bends and designing a route that supports good draft.
  • Using experienced installers and following manufacturer guidance and building regulations.

Done correctly, your twin wall flue system will provide years of safe, efficient service and help you get the very best performance – and pizzas – from your oven.