Product Description

TWPro 125mm Single Wall to 150mm Twin Wall Flat Increaser – Matt Black

Model: 37-125-009

Connect a 125mm appliance outlet or single-wall stove pipe to a 150mm TWPro insulated chimney with this TWPro 125mm to 150mm Flat Increaser. The compact fitting increases the internal flue diameter by 25mm while creating an immediate transition into the twin-wall chimney system.

The lower connection can fit a compatible 125mm appliance outlet, vitreous enamel stove pipe or another approved single-wall connecting pipe. Meanwhile, the upper connection fits a 150mm internal-diameter TWPro twin-wall chimney section.

Please note: this is a single-wall-to-twin-wall increasing adaptor. It is not designed to join 125mm TWPro twin wall directly to 150mm TWPro twin wall.

Product at a Glance

• Connects 125mm single wall to 150mm TWPro twin wall
• May connect directly to a compatible 125mm appliance outlet
• Increases the internal flue diameter by 25mm
• Flat design creates a short, immediate transition
• Useful where installation height is limited
• Compatible with the TWPro matt-black chimney range
• 316L stainless-steel inner casing
• 304-grade stainless-steel outer casing
• High-temperature matt-black external coating
• High-density mineral wool insulation
• Push-fit TWPro connection with locking band
• 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: 37-125-009

What Does This Flat Increaser Connect?

This component creates a transition between:

• Lower connection: compatible 125mm appliance outlet or single-wall pipe
• Upper connection: 150mm internal-diameter TWPro insulated chimney

Therefore, the fitting changes both the construction and the internal flue diameter. It moves from a 125mm single-wall connection to a 150mm insulated twin-wall chimney.

The 125mm and 150mm measurements refer to the internal flue diameters. Because the TWPro section also contains insulation and an outer casing, its overall external diameter will be larger than 150mm.

Why Use a Flat Increasing Adaptor?

A conventional increasing adaptor may use a longer tapered section. In contrast, this flat increaser changes from 125mm single wall to 150mm twin wall over a much shorter height.

As a result, it can help when the installation has limited vertical space above the appliance. It also reduces the need to use separate adaptor and increasing components.

The compact design creates a clean transition above the stove. Therefore, the completed flue route can look neater and use fewer visible joints.

Key Features

• Combines a single-wall adaptor and 25mm increase in one fitting
• Creates an immediate transition into insulated twin wall
• Helps reduce the overall height of the connecting arrangement
• Reduces the number of separate fittings and joints
• Connects compatible vitreous enamel pipe to TWPro
• May connect directly to a compatible appliance outlet
• Fully welded construction provides strength and durability
• Insulated upper section helps retain flue temperature
• Matt-black finish complements most black stoves and stove pipes
• Locking band secures the TWPro connection

TWPro 125mm to 150mm Flat Increaser Specification

• Download the TWPro Matt Black information sheet
• View the TWPro installation diagram
• Read why we recommend TWPro twin-wall flue
• Product type: flat single-wall-to-twin-wall increaser
• Lower connection: 125mm single wall or compatible appliance outlet
• Upper connection: 150mm internal-diameter TWPro twin wall
• Diameter increase: 25mm
• 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
• Construction: fully welded
• Joint method: push fit with locking band
• 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: 37-125-009

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, always follow the appliance instructions and TWPro installation guide.

Suitable Applications

This 125mm to 150mm flat increaser may suit correctly specified heating appliances such as:

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

However, this list does not confirm suitability for every appliance. First, check the appliance’s required flue diameter. Next, confirm that the manufacturer permits an increase from 125mm to 150mm. Finally, make sure the complete chimney carries the correct classification for the appliance and fuel.

Can It Connect Directly to a Stove?

Yes, the flat increaser can connect directly to some compatible 125mm appliance outlets. This allows the installer to start the 150mm insulated chimney close to the appliance.

However, stove outlets can use different internal diameters, external diameters and socket depths. Therefore, check the appliance spigot and adaptor dimensions before ordering.

The appliance manufacturer must also allow a direct twin-wall connection. If the appliance instructions require a length of single-wall pipe above the stove, follow that requirement.

Can It Connect to Vitreous Enamel Stove Pipe?

Yes. The lower connection can fit compatible 125mm vitreous enamel stove pipe or another approved single-wall connecting pipe.

This allows the installer to use a visible 125mm stove pipe section before changing to a 150mm insulated chimney. However, the actual connection dimensions and joint direction must match.

Do not force together components that do not fit correctly. Instead, check both products and confirm compatibility before installation.

Important Flue-Diameter Check

This component increases the internal flue diameter from 125mm to 150mm. Such an increase may form part of a correctly designed chimney, particularly where the appliance instructions require a larger chimney than the appliance outlet.

However, the presence of a 125mm appliance outlet does not automatically prove that this increaser is suitable. The appliance manual, fuel type, chimney height and complete installation design determine the correct diameter.

Never reduce the chimney below the minimum diameter stated by the appliance manufacturer. An incorrect flue size can affect draught, appliance performance and the safe removal of combustion gases.

Is This a Twin-Wall-to-Twin-Wall Increaser?

No. This product connects a 125mm appliance outlet or single-wall connecting pipe to a 150mm TWPro insulated chimney.

A true twin-wall-to-twin-wall increaser would have a compatible TWPro joint at both ends. This flat increaser has a single-wall or appliance connection at the lower end and a TWPro twin-wall connection at the upper end.

Therefore, do not order model 37-125-009 to join an existing 125mm TWPro chimney directly to a 150mm TWPro chimney.

Flat Increaser or Tapered Increaser?

Choose this flat increaser when you need a compact 125mm single-wall-to-150mm twin-wall transition. Its short design saves vertical space and creates an immediate change above the appliance or connecting pipe.

In contrast, a tapered increaser changes diameter over a longer section. That shape may suit installations where appearance, flow path or connection position makes a gradual transition preferable.

Before ordering, confirm:

• The lower connection needs to fit a 125mm outlet or single-wall pipe
• The upper chimney uses 150mm internal-diameter TWPro
• The appliance instructions allow the diameter increase
• The compact flat design suits the available space

Single Wall and Twin Wall Explained

Single-wall stove pipe has one steel wall and no insulation. Installers normally use it as a visible connecting pipe between the appliance and chimney system.

In contrast, TWPro uses an inner flue, mineral wool insulation and a stainless-steel outer casing. The insulation helps maintain the flue temperature while reducing heat transfer to the outer wall.

Therefore, TWPro can form a chimney route through a building or externally when installers use the correct supports, fire stops, flashings and clearances.

Single-wall stove pipe must not pass through walls, ceilings, floors, roofs or concealed spaces. The transition to insulated twin wall must take place before any building penetration and in the position required by the installation instructions.

Strong Stainless-Steel Construction

The inner casing uses corrosion-resistant 316L-grade stainless steel. Meanwhile, the outer casing uses strong 304-grade stainless steel with a high-temperature matt-black coating.

High-density mineral wool surrounds the inner flue in the insulated section. As a result, the fitting helps retain heat as combustion gases enter the 150mm twin-wall chimney.

The fully welded construction also provides strength and durability. Consequently, the increaser can form a reliable transition when installers fit, support and maintain it correctly.

Installation Guidance

First, confirm that the 125mm lower connection fits the appliance outlet or single-wall pipe. Next, fit the component in the correct condensate-flow direction. Then connect the upper end to a compatible 150mm TWPro chimney section.

Push the TWPro joint fully together and secure it with the correct locking band. Afterwards, check that the joint remains straight, stable and fully engaged.

The appliance and connecting pipe must not support the weight of the complete chimney. Therefore, use the correct wall supports, ceiling supports, roof supports and other structural components throughout the installation.

Finally, follow the appliance instructions, TWPro installation guide, Building Regulations and all other relevant standards. We recommend installation by a suitably competent person.

Which Way Up Should It Be Installed?

Fit the 125mm single-wall connection at the bottom, towards the appliance. The 150mm insulated TWPro connection sits above it and continues into the chimney.

Correct orientation helps keep condensate and combustion deposits inside the flue. Therefore, make sure every component follows the joint direction stated in the manufacturer’s instructions.

Distance to Combustible Materials

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

The appliance outlet or single-wall pipe will normally require a greater clearance than the insulated TWPro section. Therefore, apply the correct clearance to each part and follow the greatest requirement where clearances overlap.

Locking Band and Chimney Support

The locking band secures the TWPro push-fit connection. Fully engage the chimney joint before closing the band. In addition, check that the band remains correctly positioned after installation.

However, a locking band does not replace structural chimney support. Use the correct TWPro wall bands and support components to carry the chimney’s weight and control sideways movement.

Can I Mix TWPro with Another Chimney Brand?

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

Different chimney systems can use different joint depths, outer diameters, insulation types 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 installation.

Maintenance and Inspection

Inspect the increaser and joints during routine chimney maintenance. Check the locking band, matt-black surface and lower connection for movement, damage or corrosion.

Also, arrange chimney sweeping at intervals that suit the appliance, fuel and frequency of use. Because the fitting changes the flue diameter, the complete system must provide suitable access for inspection and cleaning.

Replace any component that becomes badly corroded, distorted, damaged or insecure. Furthermore, arrange a professional inspection if you notice smoke leakage, staining or a change in chimney performance.

Why Choose TWPro?

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

The push-fit joint system makes assembly straightforward, while the locking band secures each connection. Furthermore, the extensive range of matching pipes, bends, tees, supports, flashings and terminals allows installers to build a complete chimney from compatible components.

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

Why Buy from SNH Tradecentre?

• Specialist stove, chimney and flue-system supplier
• Extensive range of matching TWPro components
• Practical 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 checking the correct connection or chimney diameter? Call SNH Tradecentre on 01843 842727 before ordering.

Frequently Asked Questions

What sizes does this flat increaser connect?

It connects a compatible 125mm appliance outlet or single-wall pipe to a 150mm internal-diameter TWPro insulated chimney.

Is this a twin-wall-to-twin-wall increaser?

No. It changes from a 125mm single-wall or appliance connection to 150mm TWPro twin wall.

How much does it increase the flue diameter?

It increases the nominal internal flue diameter by 25mm, from 125mm to 150mm.

Why is it called a flat increaser?

Its compact design creates an immediate transition rather than using a longer tapered section. Therefore, it requires less vertical space.

Can I connect it directly to a stove?

Yes, it may connect directly to a compatible 125mm appliance outlet. However, check the spigot dimensions and appliance instructions before ordering.

Can it connect to vitreous enamel stove pipe?

Yes, it can connect to compatible 125mm vitreous enamel or another approved single-wall connecting pipe.

Is 150mm the outside diameter of the TWPro section?

No. The 150mm measurement refers to the internal flue diameter. The insulation and outer casing make the overall external diameter larger.

Can the single-wall connection pass through a ceiling?

No. Single-wall pipe must not pass through walls, ceilings, floors, roofs or concealed spaces. The transition to insulated chimney must occur before the building penetration.

How far must the insulated chimney remain from combustible materials?

The relevant T600 G50 arrangement has a declared distance of 50mm from combustible materials. However, the single-wall pipe or appliance may require a greater clearance.

Can I connect it to another twin-wall chimney brand?

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

What is the product model?

The product model is 37-125-009.