Isokern pumice flue liners form the lining of a masonry chimney, carrying smoke and combustion gases from a stove or open fire safely to the outside. This guide explains how the system works, how the sizes compare and what else a pumice chimney needs. Isokern is made by Schiedel; we are not the manufacturer, and the technical points below are taken from Schiedel's published Isokern installation information.

What a pumice flue liner is

Each liner is a rebated section of volcanic pumice concrete. Pumice is light, easy to handle and cut, and highly insulating, which helps the flue warm up quickly, draw well and lose less heat. Schiedel certifies the liners to EN 1857 with the designation T600 N2 D 3 G. They are suitable for all fuel types, but not for condensing appliances.

Where pumice liners are used

Pumice liners are designed for masonry chimneys. They are built into the chimney as it rises and must be surrounded by at least 100mm of brick, block or stone. They suit new-build chimneys as well as the relining of existing masonry chimneys.

How the system goes together

• Liners are installed socket uppermost, and every joint is sealed with Isokern lip glue, about 2mm thick. Excess glue is removed to keep the inside of the flue smooth.
• A minimum of 15mm of LECA (light expanded clay aggregate) insulation fills the gap between the liners and the masonry. Schiedel's mix is 20 parts LECA to 1 part OPC cement with a small amount of water.
• Where the chimney changes direction, bends are used with adequate support and backfilled with the LECA mix. Schiedel allows a maximum of two complete offsets (four bends) per chimney, at no more than 45° from vertical.
• Any cut joint needs a steel collar as well as lip glue.
• Combustible materials must be kept 200mm from the inner surface of the liner, or 40mm from the outside of the masonry chimney. Floorboards, skirting boards, dado and picture rails, mantel shelves and architraves are the stated exceptions.
• The chimney must terminate at the correct height under Approved Document J of the Building Regulations, finished with flaunching around the liner or with a suitable chimney pot.
• After completion, the chimney is tested and checked in accordance with Document J, and a chimney notice plate is completed and fixed permanently in the dwelling, ideally near the electricity consumer unit.

Connection to an appliance that is connected to a gas or oil supply must be carried out by a Gas Safe or OFTEC registered installer, and Schiedel recommends HETAS registered installers for solid fuel.

Pumice liner sizes compared

Figures are from the Isokern price list: internal diameter, outside diameter, section length and approximate weight per section.

The flue size is set by the appliance, so always follow the stove or fire manufacturer's instructions and Approved Document J. Our liner calculator works out the lengths, lip glue and LECA for a given chimney height.

Single liners or a liner pack?

Our liner packs are put together by Isokernshop from genuine Schiedel Isokern components, so a standard chimney can be ordered in one go.

First use and care

Schiedel advises leaving a new chimney for at least 72 hours before use, then lighting only small fires for the first week and increasing gradually. Sweep at least twice a year, before and after the heating season, with a medium-density polypropylene brush of the same diameter as the flue. Never use a steel brush on a pumice flue. Burn approved fuels or dry, seasoned wood, and always follow the appliance manufacturer's operating instructions.

Ready to choose? Browse pumice flue liners, pumice liner packs, lip glue and LECA insulation.