The Midtherm HTS Extension Length 75–350mm Twin Wall Flue in Black provides a flexible, insulated adjustment section for achieving precise flue heights and configurations. Designed to work alongside standard Midtherm HTS lengths, it enables smooth, accurate extension without compromising stability or system performance. Suitable for residential and commercial installations, it supports solid fuel, multifuel, gas, oil, biomass, and pellet appliances.
Manufactured from 0.5mm 316 2B stainless steel on the inner wall and 0.5mm 304 BA stainless steel on the outer, the extension delivers strong thermal resistance and long-term durability. Sold fully insulated, it maintains safe surface temperatures and optimal operating efficiency. The adjustable mechanism allows extension from 75mm to 350mm, with a minimum of 260mm when paired with a 300mm section and full compatibility starting from 500mm lengths. Its powder-coated black finish offers a clean, modern look suitable for contemporary flue systems.
The component installs via a push-fit socket and spigot joint, secured with a supplied locking band for reliable assembly. Rated for continuous operation at 450°C with a 4-hour fire rating, it meets Class 1 Twin Wall standards and carries CE certification, ISO 9001 manufacturing compliance, and HETAS registration. When installed and maintained correctly, it is backed by a 20-year warranty for dependable long-term service.
Features and Benefits
- Adjustable extension range from 75mm to 350mm
- 316 stainless steel inner and 304 stainless steel outer construction
- Fully insulated for safe, efficient performance
- Locking band installation for secure, consistent assembly
- Powder-coated black finish for modern aesthetics
- High-temperature rating up to 450°C
- Compatible with wood, gas, oil, multifuel, biomass, and pellet systems
- CE-certified, ISO 9001 manufactured, and HETAS registered with 20-year warranty






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