Midtherm

Midtherm Flue & Chimney Systems

Midtherm is a British manufacturer of domestic, commercial and industrial flue systems, manufacturing in the West Midlands since 1968.

Bonfire has supplied Midtherm chimney components for over 20 years. Our current range includes insulated twin-wall chimney systems, flexible liners, single-wall connecting pipe, flashings, cowls, brackets, supports and accessories for building a complete flue installation.

Unlike an appliance category, the correct Midtherm product cannot normally be chosen from appearance alone. Flue diameter, fuel, chimney route, appliance requirements, clearances, support and termination all need to be considered together.

Which Midtherm Flue System Do You Need?

Midtherm HT-S Twin-Wall Flue

HT-S is Midtherm’s insulated Class 1 twin-wall chimney system for medium and high-temperature applications.

The system uses a 316L stainless-steel inner liner, 304 stainless-steel outer casing and 25mm high-density mineral-wool insulation. Components connect using a socket-and-spigot joint secured by locking bands.

For domestic stove installations, Bonfire stocks HT-S in popular 5″ and 6″ diameters, with stainless-steel and matt-black finishes available across the range.

Typical components include:

• 150mm, 300mm, 500mm and 1000mm straight lengths
• Adjustable extension lengths
• 15°, 30° and 45° elbows
• Tee sections and inspection components
• Wall bands and intermediate supports
• Rafter supports
• Guy-wire and telescopic bracing
• Flashings and storm collars
• Terminals and cowls

This allows a complete chimney to be built through an internal route, externally up a wall or through a suitable combination of vertical and offset sections.

When Is Twin-Wall Flue Used?

Insulated twin-wall chimney is particularly useful where there is no suitable existing masonry chimney.

Common applications include:

• Modern houses without chimneys
• Extensions
• Open-plan rooms
• Barn conversions
• Freestanding stove installations
• External chimney routes

The system can therefore make a wood-burning stove possible in properties where installing a traditional masonry chimney would be impractical.

The complete route should still be designed before installation so that offsets, wall distances, roof penetration, terminal position and structural support are established correctly.

Midflex Flexible Chimney Liners

Where a suitable masonry chimney already exists, a Midtherm Midflex flexible liner may be more appropriate than constructing a new twin-wall chimney.

Bonfire offers solid-fuel liners in both 316 and 904 grade stainless steel, including popular 125mm and 150mm diameters.

Midtherm’s current Midflex 316 is a twin-wall flexible stainless-steel liner designed for Class 1 chimney applications, with manufacturer specifications allowing operating temperatures up to 600°C.

316 or 904 Chimney Liner?

Both are stainless-steel flexible chimney liners, but 904 grade provides greater corrosion resistance than 316.

The correct choice depends on the appliance, fuel, how frequently it will be used and the conditions within the chimney.

A 904 liner can therefore be worth considering for more demanding installations, while 316 remains widely used for suitable domestic wood-burning stove installations.

The stove manufacturer’s minimum flue diameter must always be followed. The liner should not simply be reduced because a smaller size is easier to install.

Midtherm Single-Wall Stove Pipe

Single-wall stove pipe is normally used for the visible connection between the appliance and the chimney or insulated flue system.

Bonfire stocks Midtherm straight lengths, elbows and associated components in stainless steel and Vitrelux vitreous black enamel.

This creates a clean connection above the stove while allowing the system to transition into flexible liner or insulated twin-wall chimney at the appropriate point.

Single-wall pipe is not a substitute for insulated chimney where the flue passes through combustible walls, floors, ceilings or roof spaces.

Black or Stainless Twin Wall?

The choice is primarily visual.

Stainless steel provides the traditional exposed-metal chimney appearance and is commonly used externally.

Matt black can visually continue the finish of a black stove and connecting pipe, making it particularly effective where the flue remains visible inside the room.

The chimney specification and clearances remain more important than colour.

Flue Supports, Flashings & Ancillary Components

A chimney system is more than straight lengths and bends.

Depending on the route, an installation may require:

• Load-bearing supports
• Wall brackets
• Rafter supports
• Guy wires or bracing
• Roof flashings
• Storm collars
• Ceiling trim plates
• Adapters
• Inspection sections
• Rain caps or terminals

The support arrangement is particularly important on tall external chimneys because the system must remain structurally secure as well as gas-tight.

Why Flue Diameter Matters

The correct chimney diameter is determined primarily by the appliance and installation.

A common modern wood-burning stove may require a 125mm or 150mm flue, but this should never be assumed across every appliance. DEFRA-exempt operation, manufacturer instructions and appliance outlet size can all affect the permitted diameter.

Changing flue diameter unnecessarily can alter chimney velocity and draught performance.

Midtherm Flue Design & Installation by Bonfire

Bonfire can supply individual Midtherm components or provide a complete chimney and flue installation service.

This can include:

• Home and site surveys
• Appliance and flue specification
• Flexible chimney liners
• Internal or external twin-wall systems
• Roof and wall penetrations
• Flashings and structural supports
• Chimney termination design
• Professional HETAS installation
• Testing and commissioning

Why Buy Midtherm from Bonfire?

• Over 20 years supplying Midtherm products
• Large range of individual components available online
• Black and stainless twin-wall systems
• 316 and 904 flexible liners
• Single-wall stove pipe and accessories
• Supply-only or complete installation
• Technical support for public and trade customers

Midtherm FAQs

Where is Midtherm manufactured?

Midtherm is a British manufacturer based in Dudley in the West Midlands and has manufactured flue and chimney products since 1968.

What is Midtherm HT-S?

HT-S is an insulated Class 1 twin-wall stainless-steel chimney system suitable for a wide variety of domestic and commercial applications including solid fuel.

Do I need twin-wall flue if I already have a chimney?

Not necessarily. A suitable existing masonry chimney may instead be lined with an appropriate flexible chimney liner following inspection and specification.

What is the difference between 316 and 904 flexible liner?

904-grade stainless steel provides greater corrosion resistance than 316 and can be considered for more demanding installations. The appropriate grade depends on the appliance, fuel and operating conditions.

Can Midtherm twin wall be installed externally?

Yes. Insulated twin-wall chimney can be routed externally where the correct supports, clearances, termination height and weatherproofing components are used.

Can I use single-wall stove pipe through a ceiling?

No. Single-wall connecting pipe is intended for suitable visible appliance connections and should not pass through combustible floors, ceilings or similar structures where an approved insulated chimney system is required.

Can Bonfire design my Midtherm flue system?

Yes. Bonfire can survey suitable projects, specify the required chimney components and provide professional installation and commissioning.