Flueet — Chimney flue sizing, draught and liner choice | flueet.com
Choose a liner by four numbers: diameter, EN 1856 temperature class, clearance to combustibles and sweep interval.
Three families of liner cover almost every domestic job: clay and pumice liners built into masonry, and stainless twinwall, an insulated metal system that can run inside or outside the building. The right choice is made on four numbers, not on price.
First, diameter. Twinwall of 125 mm internal diameter covers appliances of about 5-8 kW and 150 mm covers 8-12 kW; open fires normally need 175-200 mm. Whatever the material, the liner must stay at or above the appliance outlet — 100-130% of outlet area for a sealed stove.
On narrow screens, swipe or scroll the plate sideways.
Second, the temperature class. Under EN 1856-1, class T400 rates a system for 400 °C continuous flue gas with a 600 °C soot-fire peak, and the N1 in the designation means it is rated for 20 Pa of negative pressure. For wood service, T400 N1 is the rating to recognise.
Third, clearance to combustibles. Single-wall stove pipe needs three times its diameter — 375 mm for a 125 mm pipe — while insulated twinwall is usually stated at 50-65 mm by manufacturers. In a tight stud wall or near a ceiling, that difference often decides the system.
Fourth, the upkeep you are signing up for: two sweeps a year when burning wood, one when burning smokeless coal. Whichever liner you choose, the diameter still has to be proven by a draught reading at the outlet — the draught guide on this site shows how.
Further reading