BABY, IT’S COLD OUTSIDE The nature of the chimney you have – or are proposing to build – is critical to the success of your wood burning stove.
If you are lucky enough to have a chimney that is completely enclosed within the house, then you have a potentially ideal situation. Not only will heat radiate around the house more effectively, the heat itself will be maximised because the chimney should not develop any cold spots.
Your stove relies on the creation of a balanced flue, where the heat up the chimney gradually and steadily reduces up to the chimney top. The higher pressure hot air is drawn up to the lower pressure ambient atmosphere. As nature abhors a vacuum, cool air flows downwards to equalise the pressure below and thus fans the flame.
Of course if your chimney is on an external wall, there’s little you can do to change the fact but you should then pay even more attention to the insulation of your flue.
Whether you are converting a brick or stone chimney, or using a new prefabricated lightweight structure like Versachimney (which can be very useful for installing stoves in difficult new builds or conversions), you cannot rely upon the chimney itself as the outlet for the stove.
A large aperture reduces the pressure of the upward gases. And there are myriad problems of tar build up, broken linings and blockages.
The correct installation uses a circular flue lining, matched to the stove outlet. Normally this is the same diameter as the stove outlet, from 6″ cases upwards. In most cases this will be made of stainless steel. The easiest to install, especially where there is any deflection from the vertical, is the flexible double-walled liner pack. The outer wall is corrugated to flex more easily and to resist crushing, and the inner skin is smooth for easy cleaning and to make moisture run down. Flexible systems come in the most popular sizes from 5″ to 10″ and the rigid smooth types run from 5″ to 8″.
Other parts are needed for the installation, including a rain cap in the form of some sort of ‘suspending cowl’ which in the best cases inhibits excessive downward airwash.
Where the flue passes through the roof there will be a top plate or some sort of flashing in heat-resisting rubber (avoid the low-temperature ones) plus a fixing kit to ensure there is no rain ingress.
A ventilation pack may be required in the chimney. This is made of heat-resistant high-impact UV stabilised polystyrene.
And down at the point where the chimney meets the room, there will be a stainless steel register plate which apart from finishing off the installation, holds in any insulation in the void of the chimney and prevents unwanted draughts.
So what is in the chimney apart from the flue itself, and any internal supports?
Some installers maintain that modern double-wall flues are so efficient that they do not need further insulation. This is a moot point and the decision will be influenced by whether you have to use an outside wall chimney, and the size of the chimney cross-section.
On larger chimneys you can use a 30mm-section Rockwool-filled, aluminium-skinned sleeve which encircles the flue and is held together at joints with aluminium tape.
For smaller stacks the preferred material is the ‘Micafil’ brand, or generically Vermiculite, which can be in poured loose or used 1:1 as an aggregate with Portland cement to make an ultra-lightweight rigid fill. This amazing stuff does not burn, resists high temperatures and is an excellent insulator, but it would be expensive and wasteful to pour it into, say, a huge inglenook stack.
Those who have no existing chimneys should do their utmost to route the flue through the interior of the house, suitably shrouded, otherwise to create a new exterior casing for it wherever possible to avoid the benighted cold spot problem.
Hey, Good Lookin’ But what, you may ask, of the lovely vitreous enamel stove pipe that you have artfully matched to your freestanding stove?
This is the show-off of the flue world. It is pointless for built-in ‘cassette’ stoves where nothing is on display. As a single-skin tube that gets very hot, it is actively discouraged from being routed through a floor that might catch light and in practice it must only be used up to the point where it disappears from view. An adapter is then used to connect it to the double-walled stainless steel flue.
Having read thus far, you are probably concluding that this is the province of a professional, and indeed if you did it yourself or used an unqualified builder you would have to pay an application fee and go through the inspections of Buildings Control.
Whereas if you enlist a HETAS-certified engineer they will know how to deal with the above issues. At least now you can maybe have a more informed conversation with them.
Once the work is completed, ask your engineer for their Certificate of Completion so that you can wave it at any passing official; and make sure you know where the legally required statutory notice has been placed, telling anyone who asks (unlikely but who knows?) the technical details of your stove installation.