Product Description

TWPro 125mm Twin Wall Adjustable Flue Pipe 250–350mm – Matt Black

Model: 37-125-016

Complete a 125mm insulated chimney route accurately with the TWPro Adjustable Twin Wall Flue Pipe. This telescopic chimney section adjusts between approximately 250mm and 350mm, giving the installer 100mm of adjustment.

The adjustable design helps close a measured gap where standard fixed pipe lengths do not provide the exact chimney height. Therefore, it can simplify the final connection and reduce the need to cut or modify fixed TWPro components.

The section features a 316L stainless-steel inner casing, high-density mineral wool insulation and a matt-black 304-grade stainless-steel outer casing. Once the installer sets the required length, they must secure the section according to the TWPro installation instructions.

Product at a Glance

• 125mm nominal internal flue diameter
• Adjustable length from approximately 250mm to 350mm
• Provides approximately 100mm of adjustment
• Helps complete accurately measured chimney routes
• Useful where fixed lengths leave a small gap
• No need to cut a standard insulated pipe
• Suitable for internal and external TWPro chimney systems
• 316L stainless-steel inner casing
• 304-grade stainless-steel outer casing
• High-temperature matt-black external coating
• 25mm high-density mineral wool insulation
• Compatible with the TWPro FitSystem
• Suitable for compatible solid-fuel, gas and oil appliances
• HETAS approved and CE marked
• T600 temperature classification
• G50 soot-fire resistance classification
• 50mm declared distance to combustible materials
• Product model: 37-125-016

How Does the Adjustable Pipe Work?

The section uses an overlapping telescopic design. During installation, the installer adjusts the pipe until it fills the required space within its permitted 250mm to 350mm range.

Once the pipe reaches the correct length, the installer secures the section and adjoining joints as shown in the TWPro instructions. As a result, the finished component forms a fixed part of the chimney route.

The pipe should not remain loose or move during normal operation. Its adjustment allows accurate installation; it does not act as a flexible or expansion joint.

250mm to 350mm Adjustment Range

The adjustable section provides a working range of approximately 100mm:

• Minimum length: approximately 250mm
• Maximum length: approximately 350mm
• Total adjustment: approximately 100mm

Always keep the pipe within the manufacturer’s stated range. Extending it beyond the maximum can reduce the required overlap and may create an unsafe connection.

Likewise, do not force the section below its stated minimum length. Instead, select another suitable pipe combination if the required space falls outside the adjustment range.

Why Use an Adjustable Twin Wall Pipe?

Fixed TWPro pipes come in standard lengths. However, the measured space between two fixed components may not match those lengths exactly.

This adjustable section allows the installer to complete that final distance accurately. Therefore, it can help when connecting:

• A stove adaptor to the next chimney component
• A tee to a supported vertical chimney
• A pair of bends forming an offset
• An internal chimney route to a roof section
• An external chimney route to a fixed terminal position
• Two fixed sections where the remaining gap varies

Although the section provides useful flexibility, the installer should still plan the complete route before ordering. The adjustable pipe only corrects a limited difference of up to approximately 100mm.

Key Features

• Telescopic design supports accurate length adjustment
• Covers measurements between approximately 250mm and 350mm
• Reduces the need to cut or modify fixed components
• Maintains a continuous 125mm internal flue route
• Suitable for vertical and approved offset arrangements
• High-grade stainless-steel inner and outer casings
• Insulation helps maintain the flue-gas temperature
• Matt-black finish matches other TWPro components
• Designed for use with the TWPro push-fit chimney system
• Suitable for correctly designed internal and external routes

TWPro 125mm Adjustable Pipe Specification

• Download the TWPro Matt Black information sheet
• View the TWPro installation diagram
• Read why we recommend TWPro twin-wall flue
• Product type: adjustable insulated twin-wall flue pipe
• Nominal internal diameter: 125mm
• Minimum adjustable length: approximately 250mm
• Maximum adjustable length: approximately 350mm
• Total adjustment range: approximately 100mm
• Inner casing: 0.4mm 316L-grade stainless steel
• Outer casing: 0.4mm 304-grade stainless steel
• Outer finish: high-temperature matt-black coating
• Insulation: 25mm high-density mineral wool
• Joint system: TWPro FitSystem
• Temperature classification: T600
• Declared distance to combustible materials: 50mm
• System warranty: 25 years, subject to terms and conditions
• Paint-finish warranty: 2 years, subject to terms and conditions
• Product code: 37-125-016

Approvals and Classifications

• HETAS approved
• CE marked
• EN 1856-1 T600 N1 W V2 L50040 G50
• EN 1856-1 T200 P1 W V2 L50040 O20

The T600 designation classifies the relevant chimney arrangement for flue-gas temperatures up to 600°C. In addition, the G50 designation identifies a soot-fire-resistant arrangement with a declared distance of 50mm from combustible materials.

However, the correct classification depends on the appliance, fuel and complete chimney design. Therefore, follow the appliance instructions and TWPro installation guide.

Suitable Applications

This adjustable 125mm TWPro chimney section may suit correctly specified heating appliances such as:

• Wood-burning stoves
• Multifuel stoves
• Compatible solid-fuel appliances
• Gas coal-effect fires
• Gas boilers
• Oil-fired boilers
• Other compatible gas and oil appliances

However, the appliance outlet size alone does not confirm the correct chimney diameter. First, check the appliance instructions. Next, confirm the required chimney classification. Finally, make sure the complete route complies with Building Regulations and TWPro installation requirements.

Measure Before Ordering

Measure the required space between the installed TWPro components rather than relying on the room height or an approximate drawing.

Also, consider the engagement depth of the adjoining joints. The visible gap may not equal the final pipe setting once the components push together.

The required length must fall within the 250mm to 350mm adjustment range. If the measurement sits outside that range, use another combination of fixed and adjustable TWPro sections.

For a complex installation, ask a competent installer to calculate the route before ordering.

Adjustable Pipe or Fixed Pipe?

Choose a fixed pipe where a standard length matches the designed route. Fixed sections provide a straightforward and economical way to build most of the chimney.

Choose the adjustable pipe where the remaining measurement does not match a standard section. It works particularly well as the final piece in an accurately planned route.

Installers will often build most of the chimney with fixed pipes and use one adjustable section to complete the final measured distance.

Do not use several adjustable pipes to replace proper route planning. Instead, select the correct mix of fixed lengths, bends, supports and one adjustable section where needed.

Can the Adjustable Pipe Be Cut?

Avoid cutting the adjustable pipe. Cutting can damage the stainless-steel casings, insulation, protective coating and designed overlap.

The section already provides 100mm of adjustment. Therefore, set it within the approved range rather than altering its construction.

If the required measurement falls outside the range, choose a different TWPro pipe length or revise the combination of components.

How Twin-Wall Insulation Helps

The mineral wool insulation sits between the inner flue and outer casing. This layer helps retain heat as combustion gases pass through the chimney.

Warmer flue gases can support chimney draught and help reduce condensation. In addition, the insulation lowers heat transfer to the outside of the pipe when compared with single-wall stove pipe.

However, the outer casing can still become hot during use. Therefore, maintain the required clearance and prevent accidental contact where necessary.

Strong Stainless-Steel Construction

The inner casing uses corrosion-resistant 316L-grade stainless steel. This surface carries smoke and combustion gases through the chimney.

Meanwhile, the outer casing uses 304-grade stainless steel with a high-temperature matt-black coating. As a result, the pipe matches other components in the TWPro matt-black range.

Handle the section carefully during adjustment. Dents, scratches or excessive force can affect its movement, appearance and final connection.

Installation Guidance

First, install and support the surrounding fixed components. Next, measure the remaining distance and confirm that it falls within the adjustable range.

Adjust the section carefully until it reaches the required length. Maintain the minimum overlap specified by TWPro and keep the inner and outer parts correctly aligned.

Then connect the section fully to the adjoining components and secure every joint according to the manufacturer’s instructions. Finally, check that the finished pipe remains straight, stable and firmly fixed.

Do not apply sealants, screws or fixings unless the TWPro instructions specify them for this component. An unsuitable fixing could damage the casing or affect the tested chimney arrangement.

Not a Structural Support

The adjustable section forms part of the flue-gas route, but it does not provide structural support for the chimney.

Do not use it to carry the unsupported weight of a long vertical chimney. Instead, use the correct base supports, wall supports, ceiling supports and roof supports throughout the route.

The surrounding chimney must remain correctly aligned before the installer secures the adjustable section. Do not use the pipe to pull misaligned components together.

Use Within a Chimney Offset

The adjustable pipe may help set the distance between two bends in an offset. Changing the pipe length also changes both the sideways offset and the vertical rise.

Therefore, calculate the required measurements before adjusting the section. Use matching bend angles and follow the TWPro offset information.

An offset can create extra sideways forces. As a result, the chimney may need additional support above and below the directional change.

Distance to Combustible Materials

Maintain at least 50mm between the TWPro insulated chimney and combustible materials for the relevant T600 G50 system arrangement.

Combustible materials can include timber, roof structures, insulation, furniture and other heat-sensitive products. Furthermore, plasterboard may form part of a timber structure and should not automatically count as non-combustible.

Keep the required air gap clear throughout the pipe’s full adjustment range. Do not fill the space with insulation unless the manufacturer supplies a tested component for that exact purpose.

Can It Pass Through a Wall or Ceiling?

Use only the tested TWPro components and installation details where the chimney passes through walls, ceilings, floors or roofs.

Avoid placing the adjustable overlap or joint inside a concealed penetration. Instead, position it where the installer and future service engineer can inspect the full component.

Use a straight fixed pipe with the correct sleeve, fire-stop, support or flashing component through the building structure wherever the TWPro instructions require it.

Can I Mix TWPro with Another Chimney Brand?

Do not connect this adjustable pipe to another manufacturer’s twin-wall chimney unless both manufacturers provide written approval.

Different systems can use different outer diameters, joint depths, insulation specifications and locking methods. Therefore, components with the same stated internal diameter may not connect safely.

Using matching TWPro components helps preserve the tested system classification and creates a secure, consistent chimney installation.

Maintenance and Inspection

Inspect the adjustable section during routine chimney maintenance. Check the overlap, joints and securing components for movement, corrosion, soot staining or damage.

Once installed, the pipe should remain at its set length. Any unexpected movement may indicate a support or installation problem and requires investigation.

Arrange chimney sweeping at intervals that suit the appliance, fuel and frequency of use. Replace the section if it becomes badly corroded, distorted, damaged or insecure.

Why Choose TWPro?

TWPro combines a durable 316L stainless-steel inner casing with a strong 304-grade stainless-steel outer casing. In addition, the mineral wool insulation helps retain heat and supports a stable flue temperature.

The FitSystem makes compatible sections straightforward to assemble. Furthermore, the range includes matching fixed pipes, adjustable sections, bends, tees, supports, flashings and terminals.

Finally, the smooth matt-black finish creates a professional appearance for modern and traditional properties.

Why Buy from SNH Tradecentre?

• Specialist stove, chimney and flue-system supplier
• Extensive range of matching 125mm TWPro components
• Practical product advice from an experienced team
• Nationwide UK delivery options
• Secure online ordering
• Trade counter in Birchington, Kent
• Support before and after your purchase

Need help calculating the required pipe length or chimney route? Call SNH Tradecentre on 01843 842727 before ordering.

Frequently Asked Questions

What is the adjustment range?

The pipe adjusts from approximately 250mm to 350mm, providing around 100mm of total adjustment.

What diameter is the pipe?

It has a nominal internal flue diameter of 125mm, which is approximately 5 inches.

Is 125mm the outside diameter?

No. The 125mm measurement refers to the internal flue diameter. The insulation and outer casing make the overall outside diameter larger.

Does the pipe move after installation?

No. The installer sets the required length and then secures the section according to the TWPro instructions. It should not remain free to move.

Can I extend it beyond 350mm?

No. Keep the section within the stated adjustment range so it maintains the required overlap and secure connection.

Can I cut the adjustable pipe?

Avoid cutting it. Instead, use its approved adjustment range or choose another combination of TWPro pipe lengths.

Can I use it between two chimney bends?

Yes, it may help set the distance within an offset. However, changing its length will alter both the sideways offset and vertical rise.

Can I use it with a wood-burning stove?

Yes, it may suit compatible wood-burning and multifuel appliances. Check the required diameter and chimney specification in the appliance instructions.

How far must it remain from combustible materials?

The relevant T600 G50 system has a declared distance of 50mm from combustible materials.

Can the adjustable section sit inside a ceiling?

Keep the adjustable overlap accessible for inspection. Use a fixed straight pipe and the correct tested components through building penetrations where the TWPro instructions require them.

Can I connect it to another chimney brand?

Do not mix chimney brands unless both manufacturers provide written approval. Similar-looking components may use incompatible connections.

What is the product model?

The product model is 37-125-016.