Chimneys For Wood Burning Stoves

Choosing the best position

To make the most of any stove installation it is best to use a chimney placed in the middle of your house. Any brick or stone chimney will become warm as the hot flue gases pass up it, especially if the stove is run continuously throughout the winter. The warmed bricks will then radiate heat to the upper rooms through which they pass and this contributes considerably to the distribution of heat around the house. Such a chimney might increase the 50 per cent stove efficiency by between 5 and 15 per cent. If your existing chimney is on an external wall most of the heat of the flue gases will be lost and you may therefore decide to put in a new chimney somewhere in the middle of the building.

Prefabricated chimneys

Building a chimney is not the best thing to start on if you have not yet put trowel to mortar, as a chimney must be airtight to prevent poisonous flue gases leaking through into the rooms.

However, there are prefabricated chimneys available on the market which will take two people an afternoon to install. These chimneys are in fact elaborate versions of the old American midwest stovepipe chimney. An inner ceramic or metal circular stovepipe is surrounded by mineral fibre insulation and the whole is covered with an outer metal casing. The inner stovepipe must be insulated in this way to prevent it becoming too hot and therefore a fire hazard.

The firms who make prefabricated chimneys will advise you what parts you will need. You should supply them with a sketch of the position of the chimney, showing floor to ceiling heights of the rooms it is to pass through, the pitch of the roof, the presence of other roofs nearby (which will affect how far the chimney should stick up above the roof to work properly), and the size of the outlet of the appliance to be connected.

The materials

You will be able to buy all the parts necessary to make up the chimney including sufficient lengths of the insulated flue; a loadbearing plate to fix to the top of the first floor joists to carry the weight of the chimney (this can be covered with the floorboards of the room afterwards); firestop plates which are fixed to the top of the upper floor ceiling joists and the underside of the rafters; dust stop plates which are fixed to the underside of the first floor and ceiling joists (these will be covered with plaster afterwards); a flashing to weather-proof where the chimney comes through the roof; an outer weather sleeve to protect the part of the insulated chimney that projects beyond the roof; and some type of coping cap that fits over this. The type with the ceramic liner has to have this liner fixed together with fire-cement whilst the all-metal insulated chimneys simply twist-lock together.

Assembly

Start putting the sections together, beginning with the support plate, until you reach a point below the roof. Remove the tiles and slates directly above the chimney and, if necessary, make a hole in the roofing felt. Do not come directly underneath a rafter; if during the installation you come out directly under a tiling batten, this can be cut out between the rafters. Now drop in the last section of the chimney from above and fix it into place; then fit the flashing, the weathersleeve, and lastly the terminating cap.

The most difficult part of putting up the prefabricated chimney is making and flashing the hole in the roof. In our own case we had to deal only with a flat roof covered with white rubber material. The hole was made in the chipboard roof with a padsaw and after the chimney had been fixed extra white rubber flashing pieces were heat-welded to the chimney and the roof covering. If you are happy on ladders and have an idea of what is involved in repairing a slate roof have a go at making and flashing the hole. If you have any doubts, however, it would be wise to employ builders to do this part of the work for you as they will have both the equipment and the experience necessary.

We have always found that sharing the work with builders has been a great help when it comes to jobs where you do not want to make any mistakes. It is easy enough to take over the labouring jobs and save yourself money, employing people to do skilled work only when it is really necessary. We have also found it instructive to practise increasing our building skills by altering and constructing barns and sheds where the success or failure of the work is not so critical. Perhaps you know of a cow that would appreciate a woodstove connected to a prefabricated chimney?

Selecting the appropriate model

The choice of a particular prefabricated chimney depends on the external appearance, stainless steel or painted metal, but the type with a ceramic flue liner will have a longer life than that with a stainless steel liner. There is one other type of prefabricated chimney which consists of a special building block with a ceramic flue liner moulded in. Although these would be a quick way of installing a chimney in a new building or an extension they would be difficult to install in an existing building. The

metal prefabricated chimney, although it will be warm to the touch, will not radiate as much heat as an existing internal masonry chimney just because it is insulated. However, the type with a ceramic liner will have some heat storage if the stove is used intermittently, and any internal chimney will put more heat into the house than a chimney on an external wall.

Checking existing chimneys

If there is a suitable chimney in the building, before installing a stove it would be wise to check it over first, especially if you do not know when it was last in use. From the ground I-t is possible to see if the stack protruding through the roof is sound or crumbling and this will be an indication of the state of the whole chimney. Inside, damp patches around chimney stacks indicate that the fleshings are either faulty or missing, and these should be repaired either by yourself or a builder. To test the whole chimney temporarily seal the pot at the top, perhaps with a large plug of glass fibre, and light a smoky fire with corrugated cardboard or real sacking in the fireplace. Now spend a few happy minutes, felt tip pen in hand, inspecting the stack in all the rooms it passes through including the attic, looking for leaking smoke and marking the holes.

If there are no leaks and you are satisfied with the exterior of the chimney, have it swept and put your stove in. If there are one or two leaks these can be mortared up although it would be wise to check the repairs with a second smoky fire. It may not be possible to detect all the leaks by this method, if for example the stack leaks within the thickness of the floor, but it will give an indication of the relative soundness of the construction.

If the chimney stack is badly cracked or full of tiny leaks then you have two choices. If the old chimney is on an exterior wall forget it and connect your stove up to a new prefabricated chimney in the centre of the house. Alternatively, Rentokil have a method of providing a new chimney within an old stack. An inflated rubber bag is put down the chimney and vermiculite concrete is poured round it. The bag is removed after the concrete has set, leaving a smooth-walled insulated flue, ideal for solid fuel appliances. This is not, however, a DIY technique. Metal stove liners, such as are used in old chimneys for central heating boilers, are not robust enough to stand the higher temperature and more corrosive flue gases of a woodstove.

It is important to check the condition of the chimney as without it the stove cannot function. As a safeguard the chimney should be capable of withstanding a chimney fire. Any leaks in the flue could bring fire into the house with tragic consequences. The exterior of the chimney should be well pointed and the mortar cap around the chimney pot should not be cracked so that rain cannot penetrate the stack. This should be seen to when the stove is installed. In the UK, where coal ;fires are traditional in brick buildings, many houses come provided with chimneys suitable for wood-stoves. Providing these are kept structurally sound and swept at yearly intervals to remove the potentially dangerous creosote, then the stack and stove together should provide a robust method of heating the building, which will require relatively little maintenance.

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