Double skin flue systems

Typical internal flue system

Typical external flue system

General information

Supports

The weight of the flue system must be supported either by a floor support, rafter support or wall support. When attaching the flue to the support make sure that the support component takes the weight not the stove. You may have to raise the flue up a little when fixing the support component. There should be some type of fixing every 2000mm. If you are using bends then there should be a fixing on or near both bends. The fixing can be a wall band or wall fixing bracket (to prevent lateral movement) or one of the load bearing supports as above.

Dimensions and clearances

The flue must be at least 50mm from combustible surfaces, so any holes in combustible floors etc must be cut 100mm larger in diameter than the external diameter of the flue. Thus you should cut a 300mm diameter hole for 150mm double skin flue.
Single skin flue pipe should be at least 3 times its diameter away from combustible materials (ie 150mm pipe should be 450mm from combustible materials). Building Regs do not specify clearances for the stove itself and manufacturers recommendations vary greatly. As a general rule maintain at least 100mm from non-combustible surfaces and at least 400mm from combustible surfaces. You can effectively heat shield materials by using 12mm fireboard with a 12mm air gap behind.

Hearth

If the stove is on a combustible floor then it must sit on a non-combustible hearth 250mm thick and extending 150mm to the sides and back and 300mm to the front of the stove. If the stove has been shown not raise the hearth temperature to above 100 degrees then the hearth may be only 12mm thick.

Cutting single skin flue

Stainless steel single skin flue can be cut to length, cutting from the female end of the pipe. If you need to use a joint clip on the cut end gently bend it out with pliers in four places to give the joint clip something to grip.
When cutting enamel flue pipe with an angle grinder wrap the section you are keeping in a cloth to protect the enamel. Be careful that you cannot catch the cloth in the angle grinder. You cut the plain (male) end but you will lose the swage which normally sits on the flue collar. Double skin flue cannot be cut to length, but adjustable lengths are available.

Spraying stainless steel flue

Make sure that the flue is free from dirt & grease and spray it once it is installed. The paint undergoes it’s final curing once the flue heats up.
Remove and dispose of the rubber seals. All double skin joints are twist-locked together without fire cement, and have a locking band. You should not have joints occurring within a floor or in a flashing.

Double skin flue components

The adjustable lengths have an inner and outer layer which the other length slides between. The rockwool insulation is trimmed to length and goes In the gap between the layers of the first length. The adjustable length requires 305mm clearance to combustible materials and you must never have an adjustable length within a ceiling, floor or roof penetration.

The joint between every piece of double skin flue is secured using a locking band, as well as the joint between the plain adaptor and the first section of double skin flue and a locking band also goes between the cowl and the last section of flue.

The plain adaptor connects the double skin flue to the stove or to the single skin flue pipe. The top twist locks into the bottom of the first length of double skin flue (and is secured with a locking band) and the tail fits into the stove flue collar or into the single skin flue pipe & is sealed with fire cement.

The Firestop spacer goes on the ceiling and covers up the hole around the double skin flue.

A Floor support is a support component consists of a flat plate, that is screwed to the floor, through which the flue goes and the clamp is then tightened around the pipe and rests on the plate, carrying the weight of the flue system. The self_tapping screws can be put through the collar into the flue wall if desired.

A wall support bracket is a supporting component. The triangular plates can be fixed above or below the plate, which has a twist lock end that fits into the bottom of a T or pipe length.

A rafter support bracket is a supporting component. 2 L shaped brackets attach to the rafters either side of the flue pipe and the light plates are bent around the pipe and self tapped to it. The semi_circular slot lets it accommodate any roof pitch.

A wall band is used to hold the flue to the wall and often to hold the flue to the rafters as the last fixing. Wall bands are adjustable from 50_900mm off the wall. Sometimes packing out behind the band is required. This component is not load bearing.

A wall fixing bracket is used to fix the flue to a wall where the flue is a little further away than a wall band would allow for. Wall fixing brackets are adjustable from 50_300mm.This component is not load bearing.

A storm collar is clamped around the pipe, onto a bead of silicone mastic, just above the flashing plate. A storm collar can also be used on the angled arm of a 135* T where the flue has passed through an outside wall.

A standard cowl is the last component in a double skin flue system. This stops rain entry to the flue system and is secured with a locking band.

On first lighting your stove only light a small wood fire for about 1hr and then let it die out. Once the stove has cooled you can burn as normal. On the first few firings you can expect some smoke/steam to come off the stove and a slight acrid odour. This is completely normal and is part or the curing of the fire cement and stove paint.

Everything should be fitted in accordance with the manufacturer’s recommendations and building regs document J. If in doubt contact us