Product Description

125mm Rigid to 150mm Flexible Flue Liner Increaser – 20-125-MR

The 125mm Rigid to 150mm Flexible Flue Liner Increaser connects compatible 125mm single-wall stove pipe below to a 150mm flexible chimney liner above.

This MI-FLEX bottom adaptor increases the nominal flue diameter from 125mm or 5 inches to 150mm or 6 inches. Therefore, it provides a single transition between two different sizes at the base of a suitable masonry chimney.

MI-Flues makes the adaptor from approximately 0.6mm 316L stainless steel. In addition, its integral drain hole helps direct condensate back into the flue route rather than allowing moisture to collect outside the connection.

125mm to 150mm Increasing Flue Adaptor Features

Rigid pipe to flexible liner increasing adaptor
Manufacturer part number: 20-125-MR
Connects 125mm rigid pipe below
Connects to 150mm flexible liner above
Increases the nominal flue diameter by 25mm
Suitable for compatible MI-FLEX 316 liners
Suitable for compatible MI-FLEX 904 liners
Manufactured from 316L stainless steel
Approximately 0.6mm material thickness
Integral condensate drain hole
Designed for the base of a masonry chimney
Maximum continuous temperature class: 600°C
Suitable for compatible wood-burning appliances
Suitable for compatible smokeless-fuel appliances
Suitable for compatible gas and oil appliances
Natural stainless steel finish
Designed as an MI-FLEX system component

Technical Specification

Brand: MI-FLEX
Manufacturer: MI-Flues
Part number: 20-125-MR
Product type: Increasing rigid-to-flexible liner adaptor
Lower rigid connection: 125mm / 5 inches nominal
Upper flexible connection: 150mm / 6 inches nominal
Nominal increase: 25mm / 1 inch
Material: 316L stainless steel
Material thickness: Approximately 0.6mm
Finish: Natural stainless steel
Temperature classification: T600
Pressure class: N1
Condensate resistance: W
Corrosion class: V2
Product designation: BS EN 1856-2 T600 N1 W V2 L50012 G
Compatible liner grades: MI-FLEX 316 and 904, subject to system approval
Suitable fuels: Wood, smokeless fuel, gas and oil
Installation location: Base of an existing masonry chimney
GTIN / EAN: 8435265435793

What Does This Increasing Adaptor Do?

A heating appliance may use a 125mm rigid connecting pipe while the approved chimney design calls for a 150mm flexible liner. This adaptor joins those two parts in one component.

The smaller connection accepts compatible 125mm rigid pipe below. Meanwhile, the larger connection accepts compatible 150mm MI-FLEX liner above.

However, the adaptor only forms one part of the chimney system. The complete installation may also need a register plate, liner support, top fixing, terminal, cowl and chimney notice plate.

Correct Installation Direction

The 125mm connection faces downwards towards the appliance. Meanwhile, the 150mm connection faces upwards towards the flexible liner and chimney terminal.

This arrangement increases the flue diameter in the direction of the rising combustion gases. It also supports the intended path for soot and condensate inside the chimney system.

Do not install the component upside down as a reducer. Reducing the flue below the required diameter could restrict combustion gases and create a serious safety risk.

Choosing the Correct Flue Diameter

The appliance manufacturer’s instructions should state the required flue diameter. In addition, the complete chimney design must meet current Building Regulations and any limits that apply to the appliance type.

A 150mm liner may be required for some appliances, fireplace arrangements or chimney designs. However, do not increase or reduce the flue size using a general rule alone.

Instead, ask a competent installer to check the appliance outlet, manufacturer’s instructions, chimney height, route, draught and proposed liner before ordering.

Important Wood-Fuel Advice

Only burn fuel that the appliance manufacturer allows. For a wood-burning stove, use suitably dry wood with a moisture content below the limit stated in the appliance instructions.

Wet or unseasoned wood can produce more smoke, tar and deposits. As a result, it can reduce performance and increase the risk of chimney blockage or fire.

Increasing the liner diameter does not make wet wood safe to burn. Correct fuel, operation, sweeping and maintenance remain essential.

Compatible Rigid Flue Pipe

The lower connection suits an approved 125mm single-wall connecting system. Depending on the complete design, this may include compatible vitreous enamel or stainless steel stove pipe.

However, socket and spigot designs vary between manufacturers. Therefore, not every component described as 125mm will connect correctly.

The installer must confirm the actual joint dimensions and approved connection method before fitting the adaptor.

Compatible Flexible Liner

The upper connection is designed for compatible 150mm MI-FLEX flexible chimney liner. Suitable options may include 316 and 904-grade liners.

The correct liner grade depends on the fuel, appliance, chimney and expected use. For example, a more corrosion-resistant grade may suit some demanding solid-fuel applications.

Always use the liner manufacturer’s selection guidance. Furthermore, confirm that the liner carries the correct designation for the proposed fuel and temperature.

Important Flexible Liner Restrictions

Flexible flue liner must remain fully enclosed within a suitable existing masonry chimney. It must not remain exposed within the room or run through a timber or plasterboard enclosure.

The rigid-to-flexible transition should take place inside the confines of the masonry chimney. Therefore, the flexible liner should not remain visible beneath the chimney opening.

A flexible liner does not create a complete chimney by itself. The original masonry chimney must remain structurally suitable and provide the required fire protection.

Gas-Tight Connections

The installer must secure the flexible liner above and rigid pipe below in line with the MI-FLEX instructions. Both connections must remain mechanically secure and gas-tight during normal operation and sweeping.

The approved jointing method may use suitable fire cement, ceramic rope, high-temperature sealant, tape or system-specific fixings. However, only use products that the flue and appliance manufacturers permit.

Do not use sealant to compensate for incompatible or poorly fitting components. The parts must engage correctly before the installer completes the seal.

Integral Condensate Drain Hole

The adaptor includes an integral hole that helps direct condensate back into the flue route. As a result, it reduces the chance of moisture collecting outside the connection.

However, the drain feature only works correctly when the installer fits the adaptor and liner in the intended direction. Always follow the current MI-FLEX instructions.

Preparing the Chimney

A competent person should inspect and sweep the chimney before installing the liner. They should also check the chimney route, dimensions, condition and suitability for the proposed appliance.

Furthermore, the installer should confirm that the 150mm liner can pass through the complete chimney. Tight bends, offsets, damaged masonry or old obstructions may prevent safe installation.

The chimney pot, stack and terminal must also remain suitable. Any work at roof level requires safe access and proper working-at-height controls.

Flexible Liner Installation Direction

The flexible liner normally enters the chimney from the top. The arrows printed on the liner must point upwards in the direction of the flue gases.

Do not reverse the liner. Incorrect orientation may expose its internal layers to condensate and deposits in a way the manufacturer did not intend.

A suitable 150mm nose cone and pulling rope may help guide the liner. However, competent people must control the work safely from the top and bottom of the chimney.

Suitable Appliances and Fuels

Compatible wood-burning stoves
Compatible multi-fuel stoves
Suitable smokeless-fuel appliances
Suitable gas appliances
Suitable oil-fired appliances
Appliances approved for the proposed 125mm-to-150mm flue arrangement

The appliance instructions must permit the proposed chimney design. In addition, the system must provide the correct flue area, height, draught, temperature rating and terminal arrangement.

Why Choose a 316L Stainless Steel Adaptor?

316L stainless steel offers useful resistance to heat, moisture and corrosive combustion products. Therefore, it suits the demanding transition between rigid pipe and flexible chimney liner.

The natural stainless finish may remain visible within the chimney opening. However, the transition itself should sit inside the masonry chimney rather than below it within the room.

Why Buy from SNH Tradecentre?

Competitive prices: We aim to offer excellent value across our chimney-liner range.
Collection available: IN STOCK Collect from our Birchington trade counter.
Delivery options: We offer convenient delivery throughout eligible UK areas.
Helpful advice: Our team can help you compare common liner sizes and bottom fittings.
Clear product information: We provide useful compatibility and installation details.
Genuine products: Buy from an established plumbing and heating merchant.
Trade and public welcome: We supply installers, businesses, landlords and homeowners.
Independent merchant: SNH Tradecentre is based in Birchington, Kent.

Need Help Choosing a Flue Adaptor?

Not sure whether this adaptor suits your stove and liner? Call SNH Tradecentre on 01843 842727. Please have the appliance model, rigid pipe system and flexible liner details available.

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Installation Requirements

A competent installer must design and fit the complete chimney system. For solid-fuel appliances, use a HETAS-registered installer or notify Local Authority Building Control where required.

The installation must follow the appliance instructions, MI-FLEX guidance and current Building Regulations. In England and Wales, the work must also meet the applicable requirements of Approved Document J.

The installer must check the chimney condition, liner grade, flue diameter, ventilation, clearances, supports, terminal position and sweeping access before completing the work.

A carbon monoxide alarm must be installed where current regulations require one. However, an alarm does not replace correct installation, maintenance or chimney sweeping.

Inspection and Maintenance

The chimney system must provide suitable access for inspection and sweeping. Therefore, the connecting pipe may need an access door near the appliance or at a change in direction.

A competent person should inspect and clean the system regularly. Solid-fuel chimneys may require sweeping more than once a year, depending on the fuel, appliance and level of use.

Do not use chemical chimney cleaners or mild-steel tools on stainless steel components unless the manufacturer expressly allows them. Instead, use suitable equipment that will not damage or contaminate the stainless steel.

Frequently Asked Questions

What does this increasing adaptor connect?

It connects compatible 125mm rigid single-wall stove pipe below to compatible 150mm MI-FLEX flexible chimney liner above.

Which end faces downwards?

The smaller 125mm connection faces downwards towards the appliance. The larger 150mm connection faces upwards towards the flexible liner.

Can I use it as a reducer?

No. This product is an increasing adaptor. Do not use it to reduce a 150mm appliance outlet to a 125mm connecting pipe.

Does every 125mm stove need a 150mm liner?

No. The correct liner size depends on the appliance instructions and complete chimney design. Ask a competent installer to confirm the required diameter.

Is this adaptor made from stainless steel?

Yes. MI-Flues makes the adaptor from approximately 0.6mm 316L stainless steel.

Can I use it with 904-grade liner?

It can connect to suitable MI-FLEX 904 liner, subject to system compatibility. Confirm the liner and adaptor specifications before ordering.

Can it connect to every 125mm stove pipe?

Not necessarily. Joint profiles can vary between manufacturers. Therefore, the installer must confirm the actual connection and system compatibility.

Where should the adaptor be installed?

It normally sits at the base of a masonry chimney where the 125mm rigid pipe changes to the 150mm flexible liner.

Can flexible liner remain exposed below the chimney?

No. Flexible liner should remain fully enclosed within the existing masonry chimney. A suitable rigid connecting pipe serves the visible section below.

Which way should the liner arrows point?

The arrows printed on the flexible liner must point upwards towards the chimney terminal in the direction of the flue gases.

What is the maximum temperature rating?

The adaptor carries a T600 designation. Therefore, its maximum continuous operating temperature class is 600°C.

Does this adaptor support the liner?

No. The liner requires suitable support and top fittings. Do not rely on this bottom adaptor as the liner’s only support.

Who should install this adaptor?

A competent installer should fit it as part of a correctly designed chimney system. A HETAS-registered installer can complete and certify eligible solid-fuel work.

Order the 125mm-to-150mm MI-FLEX Adaptor

Choose the 125mm Rigid to 150mm Flexible MI-FLEX Increasing Adaptor 20-125-MR to connect compatible 5-inch stove pipe to a 6-inch flexible liner inside a suitable masonry chimney.

Before ordering, confirm the appliance instructions, rigid pipe connection, required liner diameter and liner grade with your installer.

Manufacturers may update specifications and installation guidance. Therefore, check the latest MI-FLEX instructions and confirm suitability with a competent installer before ordering.