Installation Of Wood Burning Stoves

Dealing with the risk of fire

The installation of a stove should aim to be as long-lasting as the stove itself. The picture of long runs of metal stovepipe, glowing a cosy red across the classroom ceiling of an American midwest schoolroom at the turn of the century is not on. Apart from being unacceptable to fire officers and building inspectors, the metal pipes will inevitably become a hazard as they corrode unless frequently checked. If such stovepipe is used for a chimney outside it will need replacing every one or two years. Today, the stove should be linked to some type of permanent chimney stack.

The building regulations set out various minimum distances between a combustion ‘appliance’ and the walls and floor surrounding it where these are of combustible material (such as a timber floor); . What this really means is that the stove should be placed on a stone or concrete hearth, and that the wall behind it (or walls surrounding it if it is placed in a recess) should be of brick or plastered brick. Houses are traditionally built of brick in the UK; it is in the USA where wood is burned in houses built of timber that stringent safety precautions are needed. Nevertheless, any fire is dangerous, so bear this in mind when you are installing or using a wood-burning stove; and when you buy the stove get a fire extinguisher as well. Put it in an easily accessible place in the house, say at the bottom of the stairs, near the door of the room containing the stove; and don’t neglect its maintenance for most types need replacing every few years.

In a typical house with plastered brick walls and a concrete floor, fire prevention is not too difficult. Having a mass of masonry around a stove will also help to even out the heat flow. When the stove is burning hot the walls and hearth will heat up; as the stove cools, before refuelling or at night, the walls and hearth will radiate heat into the room. However, do note that wallpaper on a plastered wall is combustible and is not acceptable within the minimum distance between stove and combustible material; the same applies to carpet laid over a concrete floor. In the latter case, not only is there a possibility of the carpet becoming too hot and catching fire, but whenever the stove door is opened for refuelling there is a high chance of hot ash falling to the floor as the bed of ashes within the stove is raked over.

A generous hearth is always an asset rather than an inconvenience; not only does it lessen the chance of smouldering carpets but, especially if it is only slightly raised above floor level, it provides a ‘stove area’ which can be prohibited to children (for even fireguards around a stove can become hot enough to burn fingers), a useful place to keep the ash bucket and stove tools and somewhere for the bread to rise. However careful you are, the day will inevitably come when the draught is carelessly left open too long and you walk in to find the hot plate on top of the stove glowing a dull cherry red; at such times a properly planned installation surrounded by non-combustible materials is very comforting.

Fireplaces – Adapting existing ones

Many houses in the UK, especially those built in Victorian and Edwardian times which relied on coal fires for heating, often have fireplaces in bedrooms as well as downstairs. These houses also have timber floors and you should find that the old stone hearth extends some 600mm into the room, supported underneath by a short arch of brickwork. This would have been adequate for the old type of register grate fireplace, which had a plate to blank off the chimney when not in use, and there are no regulations to stop you from using the old hearth for a new woodstove installation. It would be best to choose an upright stove so that some of the old hearth projects in front of the door to catch any spilt ashes.

However, if you feel the old hearth is inadequate, it will have to be removed and replaced with a new cast concrete hearth. The regulations will not allow you simply to extend the existing hearth over the timber floor: the worry is that wood in contact with a hot hearth stone will itself become hot, dry out and eventually char, until at some point it will ignite. You will also have to make a concrete hearth if you wish to install a woodstove in a building with timber floors where there is not already a hearth.

Building a suitable base

When you have removed the old hearth stone, you should find a trimmer already in position carrying the floor joists. If you are making the hearth new, you will have to put in a trimmer extending the desired width of the hearth. The trimmer should be made with two pieces of wood, each the same size as the floor joists (two pieces 50mm x 125mm if the joists are 50mm x 125mm). Before you saw through them, prop the floor joists up. This can be done with bricks off the earth for a timber ground floor (use bits of slate as packing to make up the required height), or, if the hearth is being formed in an upper timber floor, the joists will have to be supported off the floor below with Acrow props.

Before you cut the joists turn off the electricity at the mains in case you meet a cable. Saw through the joists and nail the trimmers in place, making sure that the top of the trimmers is level with the top of the joists. Once the trimmers are fixed in place you can remove the supports. Nail a 50mm x 50mm length of wood to the bottom of the trimmer to form a support for the hearth. To support the opposite edge you will have to chip out enough brickwork to give a 50mm bearing. Nail two more 50mm x 50mm lengths to each joist at the side of the opening. Set in place a piece of 16 gauge galvanised mild steel sheet to form the base of the hearth, packing up the brick side with bits of slate until the sheet is level. Form a tray of heavy-duty polythene sheet covering the steel; take it up the cut away brickwork and over the joists and trimmer.

Provide some kind of reinforcement to the concrete using 6mm diameter mild steel rods at 300-400mrn centres (lengths of old bed-stead will do) propped up approximately 25mm above the sheet on small wooden blocks. Make a mix of one to three or four parts by volume of cement to fine aggregate and fill until level with the top of the floorboards. Alternatively a raised hearth can be created if you fix extra shuttering around the formed opening and fill to the top of the shuttering with the concrete mix. This shuttering can be of hardwood to form a permanent lipping to the hearth.

The flue and soot door

Having found or made a suitably fireproof base for the stove to stand on, the stove will have to be connected to the flue. The outlet, which is usually a circular metal flange, will be on the top, at the back, or even through the side of the stove, so select a model that is compatible with the chimney. For a typical small fireplace chimney opening, the type that usually comes with a tiled surround, the stove outlet will have to be at the back. Blank off the chimney, leaving a hole the size of the external diameter of the piece of stovepipe that fits over the outlet flange of the stove. This is best done by bricking up the opening and mortaring in the length of stovepipe. Use vitreous enamelled cast iron stovepipe or plain cast iron pipe.

Asbestos cement, apart from being a health hazard to the people who make it, can crack if the stove accidently runs too hot, and if it is cracked flue gases could seep into the room without your knowledge, and these are dangerous. Pressed metal or enamelled steel stovepipe could be used, but its life will be nowhere near that of cast iron, and if you have a stove that looks as if it will last for a hundred years you might as well use stovepipe to match.

When bricking up, incorporate a cast iron soot door (a standard item at a builders’ merchants): this is best placed at the bottom and to the side of the opening. It will provide somewhere for the sweep to put the brush and the flexible vacuum hose to clear away the soot when the chimney is swept.

If your chimney opening is larger, up to the full ingle nook, you may not wish to brick it up but prefer to stand the stove within the fireplace opening. In this case it is best to have the outlet on the top of the stove and to take the stovepipe vertically up through a register plate blocking off the chimney. An outlet from the back of the stove would require a bend in the stovepipe and bends are always best avoided if possible because they reduce the velocity of the flue gases. The only time you might need to use a side outlet to the stove is to prevent a large stove sticking out too far into the room where there is no chimney opening to stand the stove in.

Fitting the register plate

The register plate will have to be fitted in the brickwork of the fireplace opening before the brickwork starts to narrow to form the restrictions for the chimney. Measure the opening at this point, including the diagonals as it is most unlikely that the opening will be a true rectangle, and mark these measurements on a piece of 16 gauge galvanised mild steel sheet. Cut the steel sheet out 12mm beyond the marked lines at back and sides and cut to the line at the front edge. Cut a hole in the registerplate to fit tightly round the cast iron flue pipe from the stove. You could also cut a hole for a soot door. The edges of the soot door opening must be bent back and a door to fit the opening cut from a further piece of galvanised steel sheet with a projection bent to form a handle. The soot door can then be slid open as required.

In the chimney, rake out a mortar joint right the way around the back and sides of the opening. Bend the steel sheet so that it can be eased into the raked recess at the sides of the opening and then pushed up to fit into the recess at the rear. At the front of the opening use a piece of 25mm x 6mm mild steel strip or 25mm x25mm angle (angle can only be used if the top of the fireplace opening is horizontal) cut to be 25mm longer than the opening. Fit this to provide a support for the front of the register plate, which should be sealed to it with fire cement. Pack the plate up to the top of the recesses with small strips of slate and then mortar it in place.

The short length of cast iron stovepipe should be sealed to the stove outlet with fire cement and similarly sealed where it passes through the register plate. The stovepipe should project 100-150mm beyond the register plate up into the flue.

Fitting the cowl

With the register plate installed some type of cowl must be fitted to the chimney to stop the rain coming down and rusting the plate. There is nothing worse than sitting in a room listening to the steady dripping of rain on to thinly galvanised steel. On round chimney pots the adjustable metal cowl is easy to fit by tightening four bolts but may be knocked off by the sweep’s brush. A clay bonnet cowl must be mortared in to the top of the chimney pot but will make a good job. With square or other fancy pots some kind of cowl will have to be devised from sheet metal.

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