Twin Wall Flue Installation

The planning of Twin Wall flue is essential as the pipe cannot be cut to length to make them fit.

Our Twin Wall flue is a push fit connection that then secures together with a locking band to ensure the combustion gases from the stove are drawn out into the atmosphere.

Twin Wall flue is a 316 grade stainless steel tube with a choice of either 25mm or 30mm compressed rockwool insulation with another 316 grade stainless steel pipe for the outer part of the tube. The insulation is necessary as it keeps the inside air of the pipe hot to help the rise of the gases in the flue. If a non-insulated flue pipe was used, the gases would cool down in the pipe and cause the draw to slow down making a “dam”, which will make the underneath smoke exit through the vents in the stove instead of the chimney. Cooling gases also form condensation which forms on the inner walls of the flue and runs down into the stove.

Twin wall flue can travel internally within a building or externally on an outside wall.

You must notify Building Control prior to installing any chimney (unless you are approved to self- certify, as are HETAS engineers).

Steel vitreous Enamel stove pipe (non-insulated) MUST NOT be used to go through a wall or ceiling, except directly into a chimney. If you need to exit the room, whether out through a wall or through a ceiling then you must use twin wall flue pipe and then stay with twin wall flue pipe to the cowl (unless going into a chimney higher up).

Our Twin Wall flue pipes should be kept at a minimum of 60mm from combustible materials (other flues may differ on distance to combustibles).

A chimney cannot have more than four bends and no bend can be more than 45 degrees (except if you use a 90 degree “T” off the rear of the stove). If you are going out through a wall then you will be using two 45 degree bends straight away and have two left if you need them. Use two more to clear the eaves and that’s your maximum! Note that coming off the back of the stove with a T is classed as, for Building Regulation purposes, equivalent to two bends even though it is actually one 90 degree bend.

With the exception of the T off the back of a stove the maximum angle of bend is 45 degrees. Bends of 15 and 30 degrees are also available.

PLEASE NOTE: ceilings are almost always classed as combustible and any steel vitreous pipe must be kept well away (425mm or 17″ covers all situations likely to be encountered e.g. with 5″ and 6″ diameter steel vitreous pipe). The actual rule is 3x the diameter of the offending pipe.

Please see below some design sheets to help with the installation of your Twin Wall Flue

External Rear – Twin Wall Flue design sheet

External Top – Twin Wall Flue design sheet

Internal Rear – Twin Wall Flue design sheet

Internal Top – Twin Wall Flue design sheet

Flexible liner installation

The most common method of installing a flexible liner is from the top of the chimney.

The chimney MUST be swept prior to the installation as any loose soot in the chimney could fall when the liner is inserted causing soot to come out of the fireplace. Sweeping the chimney from the base of the fire also show you which chimney pot the liner needs to go in when the brush comes out the top of the chimney pot.

Drop a rope down the chimney (with a weight on the end) so a helper can grab it from within the fireplace and the other end attached to the liner using a nose cone.

The liner is then fed down the chimney whilst a helper pulls down with the rope. The liner may get stuck on a bend or other obstructions so you have to pull it back up a bit, then push it down or even maybe spin the whole lot 180 degrees. Please also note that the liner must be inserted into the chimney with the arrows pointing up, there is a correct way up for liners to go so the gases flow up and condensation does not run into the stove

The liner can be placed down the chimney by using several different methods, Scaffolding, cherry picker or cat ladder and harness. Once the liner is in, leave some out of the chimney pot so the pot hanging cowl can be fit on the end of the liner and then once the cowl is on the liner the base of the liner can then be fit to the stove pipe on the stove using a flex adaptor.

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