Can you have bends in a wood burner flue?
To ensure optimal performance, it is recommended that bends in the chimney liner be avoided altogether. This is in line with both the Building Regulations and British Standards, which emphasize the importance of a straight, vertical chimney for effective removal of combustion gases.
However, if bends are unavoidable, it is crucial to limit them to a maximum of four along the length of the chimney. Additionally, each bend should not exceed 45 degrees from the vertical. It's worth noting that factory-made 90-degree bends or tees can be treated as two 45-degree bends.When working with systems like twin wall flue, it is advisable to utilise standard offset components provided by the chimney manufacturer, where available. This ensures a safe and efficient installation.
Key Regulations
A wood burner flue installation must comply with strict requirements. The total flue height must be at least 4.5 meters from the top of the stove to the chimney terminal.
At least 80% of the total flue length must maintain a vertical orientation to ensure proper draft. No more than four bends are permitted in the entire length of the chimney.
Each bend must not exceed 45 degrees from the vertical, though a 90-degree factory-made bend or tee counts as two 45-degree bends. Importantly, a 90-degree tee piece can only be used directly at the rear outlet of the stove.
Offset and Distance Requirements
The distance between bends on stainless steel chimneys must not exceed 20% of the total chimney length. For example, with a 4.5 M chimney, the maximum allowable offset is 900mm. Any horizontal sections must not exceed 150mm in length.
Specific considerations for different types of flue system
Twin Wall Systems
- Always use manufacturer-supplied offset components
- Ensure proper support brackets are installed
- Maintain proper clearance to combustibles
Flexible Liners
- Can be bent to accommodate required angles
- Must not be kinked or compressed
- Should be properly supported throughout the length
The Consequence of bends in a chimney flue
Installing bends in your chimney flue can impact performance in several ways:
- Reduced draft efficiency
- Slower start-up times
- Increased risk of smoke spillage
- More frequent cleaning requirements
With too many bends, you could experience issues such as:
- Poor burning performance
- Smoke entering the room
- Blackening of the stove glass
- Creosote build-up around bends
Other considerations
A soot door must be installed between the second and third bend when using four bends. This allows for proper cleaning and maintenance. Flue diameters must be as follows:
- Minimum 5" (125mm) diameter for most installations
- DEFRA-approved stoves may have specific requirements
- Never reduce the diameter at bends
Testing and Maintenance
Draw Testing
Once installed, be sure to:
- Conduct smoke tests after installation
- Check draft performance in different weather conditions
- Monitor for any smoke leakage around bends
Regular Maintenance
More bends requires more maintenance, so follow these points:
- Clean bends more frequently than straight section
- Inspect for creosote build-up quarterly
- Check all joints and seals annually
Remember that while bends in a chimney are allowed, every bend you add reduces efficiency and increases maintenance requirements. Always aim for the straightest possible route when planning your chimney flue.