Product Description

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

Model: 37-125-002

Increase the flue diameter while changing from single-wall stove pipe to an insulated chimney system with the TWPro 125mm to 150mm Single Wall to Twin Wall Increaser.

The lower connection accepts compatible 125mm single-wall or vitreous enamel stove pipe. Meanwhile, the upper connection joins to the TWPro twin-wall system with a 150mm internal diameter.

Therefore, this component provides two functions in one fitting. It increases the flue from 125mm to 150mm and creates the transition from single-wall pipe to insulated twin wall.

Furthermore, the matt-black outer finish creates a neat connection between black stove pipe and matching TWPro chimney components.

Product at a Glance

• Product type: single-wall to twin-wall increasing adaptor
• Lower connection: 125mm single-wall pipe
• Upper connection: 150mm TWPro twin-wall chimney
• Increases the internal flue diameter from 125mm to 150mm
• Suitable for compatible vitreous enamel stove pipe
• Tapered design for a smooth change in diameter
• Matt-black outer finish
• 0.4mm 316L stainless-steel inner liner
• 0.4mm 304 stainless-steel outer casing
• 25mm high-density mineral-wool insulation
• Push-fit TWPro connection with locking band
• Locking band included
• Suitable for compatible solid-fuel, gas and oil appliances
• Solid-fuel temperature classification: T600
• TWPro distance from combustible materials: 50mm
• Model: 37-125-002
• System warranty: 25 years, subject to conditions
• Paint-finish warranty: 2 years

Key Features and Benefits

• Two functions in one: Increases the diameter and changes from single wall to twin wall.
• 125mm lower connection: Accepts compatible 5-inch single-wall stove pipe.
• 150mm upper connection: Continues into the 6-inch TWPro insulated chimney range.
• Smooth transition: The tapered design avoids a sudden internal step.
• Matt-black finish: Complements black stove pipe and matching TWPro components.
• Strong inner liner: Uses 0.4mm 316L stainless steel for heat and corrosion resistance.
• Durable outer casing: Uses 0.4mm 304 stainless steel with a black coating.
• Insulated construction: Contains 25mm of high-density mineral wool.
• Secure connection: Uses the TWPro push-fit joint and locking-band system.
• T600 classification: Suits compatible high-temperature solid-fuel installations.
• G50 classification: Allows a 50mm clearance around the certified insulated section.

What Does This Increaser Do?

This component connects two different flue sizes and two different types of pipe:

• 125mm single-wall or vitreous enamel connecting pipe below
• 150mm insulated TWPro twin-wall chimney above

The flue gases move from the smaller 125mm connection into the larger 150mm chimney. As a result, the fitting increases the internal flue area as the system changes to twin wall.

This product does not reduce a 150mm chimney to 125mm. Install it only in the correct direction, with the 125mm single-wall connection below and the 150mm TWPro connection above.

Why Increase from 125mm to 150mm?

Some appliances use a 125mm outlet or 125mm connecting pipe, while the chimney design requires a 150mm internal diameter. For example, the appliance instructions, chimney height, fuel type or Building Regulations may require the larger chimney.

This increaser provides the correct connection between those sizes. However, do not increase or change the flue diameter simply because the component fits.

The appliance manufacturer and installer must confirm:

• The permitted appliance-outlet diameter
• The required chimney diameter
• The correct point for the increase
• The permitted flue route
• The required temperature and pressure classification

Never reduce the flue below the appliance manufacturer’s stated requirement.

Where Should You Use the Increaser?

Use the increaser at the planned transition from 125mm single-wall connecting pipe to a 150mm TWPro insulated chimney.

The exact position will depend on:

• The appliance manufacturer’s instructions
• The stove outlet and connecting-pipe arrangement
• The position of walls, ceilings and combustible materials
• The support points for the twin-wall chimney
• The complete chimney design
• Current Building Regulations

The increaser does not allow single-wall pipe to pass through part of the building structure. Instead, it creates the change to the suitable insulated system before the chimney reaches a wall, ceiling, floor or roof.

Compatible Single-Wall Pipe

The lower connection can accept compatible 125mm single-wall pipe, including suitable vitreous enamel stove pipe. However, manufacturers may use different joint profiles and tolerances.

Therefore, do not select the increaser by diameter alone. Confirm that the chosen single-wall pipe fits the lower connection correctly and follows the required joint direction.

If you plan to connect VIT-Smooth or another stove-pipe range, ask SNH Tradecentre to confirm compatibility before ordering.

TWPro Construction

TWPro uses stainless steel for both the inner and outer casings. The inner flue liner uses 0.4mm 316L stainless steel, while the outer casing uses 0.4mm 304 stainless steel with a matt-black finish.

Between these casings, a 25mm layer of high-density mineral wool reduces heat transfer to the outside surface. Consequently, the certified insulated TWPro section can sit closer to combustible materials than standard single-wall stove pipe.

The TWPro connection uses a formed push-fit joint and locking band. This system provides a secure connection while keeping the outside profile neat.

TWPro 125mm to 150mm Increaser Specification

• Download the TWPro matt-black information sheet
• Download the TWPro installation diagram
• Discover why SNH recommends TWPro twin-wall flue
• Product type: single-wall to twin-wall increaser
• Model and SKU: 37-125-002
• Lower internal connection: 125mm (5 inches)
• Upper internal connection: 150mm (6 inches)
• Size change: 125mm to 150mm
• Design: tapered increasing adaptor
• Inner casing: 0.4mm 316L stainless steel
• Outer casing: 0.4mm 304 stainless steel
• Outer finish: matt-black high-temperature coating
• Insulation: 25mm high-density mineral wool
• Joint method: push fit with locking band
• Locking band: included
• Maximum solid-fuel temperature classification: T600
• Solid-fuel classification: EN 1856-1 T600 N1 W V2 L50040 G50
• Positive-pressure classification: EN 1856-1 T200 P1 W V2 L50040 O20
• TWPro distance from combustible materials at T600: 50mm
• HETAS-approved system
• System warranty: 25 years, subject to conditions
• Paint-finish warranty: 2 years
• Manufacturer: Mi-Flues
• Range: TWPro Matt Black

Understanding the Two Classifications

Classification	Typical Meaning
EN 1856-1 T600 N1 W V2 L50040 G50	High-temperature, natural-draught applications with a 50mm combustible clearance
EN 1856-1 T200 P1 W V2 L50040 O20	Lower-temperature, positive-pressure applications with a 20mm declared clearance

 

The correct classification depends on the appliance, fuel and complete chimney design. Therefore, the installer must use the classification that matches the intended application.

Suitable Appliances

The increaser suits compatible solid-fuel, gas and oil appliances when installed as part of a correctly designed TWPro system. These may include:

• Wood-burning stoves
• Multifuel stoves
• Suitable gas appliances
• Gas coal-effect fires
• Compatible oil-fired appliances
• Other appliances approved for the relevant TWPro classification

Always check the appliance instructions before ordering. The manufacturer will state the permitted outlet diameter, required chimney diameter, temperature class and pressure class.

Installation Direction

Install the increaser with the 125mm single-wall connection below and the 150mm TWPro connection above. This arrangement increases the flue diameter in the direction of the rising combustion gases.

Push the first 150mm TWPro component fully into the upper connection. Next, fit and secure the supplied locking band around the formed joint.

Make sure the increaser remains vertical and correctly aligned. In addition, support the chimney independently so the appliance outlet and increaser do not carry its full weight.

Important Clearance Information

The T600 classification gives the insulated TWPro section a minimum distance of 50mm from combustible materials. Measure this clearance from the outer surface of the twin-wall component.

However, the lower connection joins to single-wall stove pipe. Single-wall pipe normally requires a much greater clearance. Therefore, maintain the clearance required by each component and follow the greatest distance where their clearance zones overlap.

Combustible materials may include:

• Timber studs and roof structures
• Wooden beams, shelves and mantels
• Furniture and curtains
• Decorative wall coverings
• Timber hidden behind plasterboard

Plasterboard does not automatically make a wall non-combustible. For example, it may cover a timber frame. Check the complete construction before confirming the transition position.

Passing Through Walls and Ceilings

Do not pass the single-wall pipe or uninsulated lower connection through a wall, ceiling, floor, roof or concealed space.

Change to the suitable insulated TWPro system before the chimney reaches the building penetration. The distance between the increaser and the penetration will depend on the manufacturer’s instructions and the surrounding materials.

Use the correct TWPro fire stops, finishing plates, supports and weathering components for the chosen route.

Locking Band and Support

The upper TWPro connection uses a push-fit joint with a locking band. Push the components fully together before positioning the band around the joint.

Next, secure the locking band as the manufacturer directs. Check that the joint cannot separate and that the flue remains correctly aligned.

The locking band secures the connection, but it does not replace the required structural supports. Therefore, fit every wall bracket, ceiling support and roof support shown in the chimney design.

Can the Increaser Be Cut?

Do not cut or alter the increaser unless the manufacturer specifically permits the modification. Cutting could damage the tapered flue path, insulation, coating or formed connection ends.

Instead, use compatible straight or adjustable TWPro components to achieve the required chimney height. This approach keeps the tested joints and factory finish intact.

Inspection and Maintenance

Inspect the increaser, locking band and adjoining pipework regularly. Check for:

• Loose or incorrectly fitted joints
• Movement in the chimney
• Damage to the black coating
• Corrosion or staining
• Signs of overheating
• Signs of escaping combustion products

Arrange chimney sweeping and appliance servicing at intervals that suit the appliance, fuel and frequency of use. Furthermore, make sure the complete flue route provides suitable inspection and cleaning access.

Important Safety Information

• Use the increaser only for the specified 125mm-to-150mm transition.
• Install the 125mm single-wall connection below.
• Continue with 150mm TWPro twin wall above.
• Never use the fitting to reduce a required 150mm flue to 125mm.
• Confirm single-wall pipe compatibility before installation.
• Secure the upper TWPro joint with the supplied locking band.
• Maintain at least 50mm around the certified T600 twin-wall section.
• Follow the greater clearance required around the single-wall pipe.
• Do not pass single-wall pipe through the building structure.
• Support the chimney independently from the appliance.
• Follow the appliance and TWPro installation instructions.
• Use a suitably competent installer.

Why Choose TWPro?

TWPro provides a high-quality insulated stainless-steel chimney system for solid-fuel, gas and oil installations. Its push-fit joints and locking bands help installers create a secure chimney route with a clean outer profile.

Furthermore, the T600 G50 classification allows a 50mm clearance around the correctly installed insulated system. This compact clearance can help when planning both internal and external chimney routes.

The matt-black version complements black stoves and connecting pipes. Therefore, it works particularly well where the twin-wall chimney remains visible inside the room.

Why Buy from SNH Tradecentre?

• Specialist stove, chimney and flue-system supplier
• Extensive range of 125mm and 150mm TWPro components
• Matching vitreous enamel stove-pipe options
• 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 confirming whether your installation requires a 125mm-to-150mm increaser? Call the SNH Tradecentre team on 01843 842727 before ordering.

You can also read SNH Tradecentre customer reviews on Google.

Frequently Asked Questions

What sizes does this increaser connect?

It connects compatible 125mm single-wall stove pipe to a TWPro insulated chimney with a 150mm internal diameter.

Which connection goes at the bottom?

The 125mm single-wall connection goes at the bottom. The 150mm TWPro connection continues upwards towards the chimney.

Does this fitting reduce a 150mm flue to 125mm?

No. It increases the internal flue diameter from 125mm to 150mm in the direction of the rising combustion gases.

Why might I need to increase the flue to 150mm?

The appliance instructions, chimney design or relevant regulations may require a 150mm chimney above a 125mm appliance connection. Ask a competent installer to confirm the required size.

Can I connect any brand of 125mm stove pipe?

Not necessarily. Products with the same nominal diameter may use different joint profiles. Confirm compatibility before ordering.

Does it include a locking band?

Yes. The increaser comes with the locking band needed to secure the upper TWPro joint.

Can the increaser pass through a wall or ceiling?

No. Position the transition so the suitable insulated TWPro system passes through the building structure. Do not place the single-wall connection inside a wall, ceiling, floor, roof or concealed space.

What clearance does the TWPro section require?

The T600 G50 classification requires at least 50mm between the outside of the insulated TWPro section and combustible materials.

Does the 50mm clearance apply to the single-wall pipe?

No. The adjoining single-wall pipe may need a much greater clearance. Follow its classification and the appliance instructions.

What does T600 mean?

T600 means the system has a nominal maximum continuous flue-gas temperature classification of 600°C for the relevant installation class.

Can I use it with a wood-burning stove?

Yes, it suits compatible wood-burning and multifuel appliances when the approved chimney design requires a 125mm-to-150mm transition.

What warranty does it include?

SNH lists a 25-year system warranty and a two-year warranty on the paint finish. The manufacturer’s installation, fuel and maintenance conditions will apply.