|Building a Paving Slab Kachelofen|
Living here in the damp, well-ventilated West coast of Scotland, I pondered the best way to heat a small super-insulated space. Living amidst hundreds of acres of softwood plantation with an abundance of storm-felled and waste timber left lying after forest operations, wood was obviously a no-brainer. Yet anyone who has relied on a woodburning stove, not merely kept one as a pet, soon realises they have invited a particularly demanding houseguest into their lives. Their unceasing appetite sooner or later tempts even the diehard eco-warrior with guilty dreams of the effortless chainsaw! A closet Luddite myself, I was keen to see if I could provide all my heat using only a bowsaw. That would mean a stove capable of handling large unsplit logs for as little cutting and axework as possible. I also worried that a conventional woodstove in a small highly insulated space could make it unbearably warm.
I had always been intrigued by the kachelofens and masonry stoves of Eastern Europe, with their principle of capturing waste flue heat into stone for slow release. As a fellow devotee of Tony Wrench’s ‘keep it simple, keep it funky’ philosophy, it seemed natural to try and build my own cheap and cheerful version, reasoning that a steady background temperature such as I hoped to get from what is effectively a wood-fired storage radiator would provide the most equitable living conditions in my new home for the least impact, both to the Mother and my own aching back!
20 thistles (£5) produced a stack of paving slabs from the local LETS, a few tubs of fire cement and several hours playing with Lego (a fine design tool!) and I was ready to go. Sizes were determined by using as many whole slabs as possible, and minimising the number of required cuts, since this would entail the hire of a stone cutting disc, or prolonged effort with a concrete handsaw. With hindsight it would be simpler to use the slabs only for the horizontal sections of the stove and bricks or tiles for the verticals. However the cutting list gives every section needed if using slab throughout. And yes I know everyone says they’ll crack in the heat but so far after a year of use they haven’t, and anyway small cracks are easy to fill with fire cement. If you have a choice, go for older slabs with a higher gravel content rather than the modern pure cement type. They are less brittle and prone to flaking when cutting.
This design incorporates an oven, but that space could be used for a longer flue, or even a water heater. The important principle is that the arrangement of the stove core is self-supporting, using the fire cement only for gap sealing, NOT structural strength. In this photo, the complete core stands mortarless, held solely by its own weight. The outer facing slabs make it easier to seal, as well as providing more heat mass and a smooth exterior for decoration.
For the metal front I first made a full-size template from corrugated cardboard, marking all the fixing points etc. before transferring the shapes to 6mm mild steel plate. It incorporates adjustable ring & pin hinges to align the doors to close perfectly. With care these shapes can be cut with a small angle-grinder and drilling for tight curves, or a cutting torch, then finished with files and sandpaper. The doors are each made from two pieces bolted together, sandwiching the woven fibre-glass sealing gasket between them, using the cutouts from the backing plate as the inner sides for each door. Because you are using these cutouts, they are automatically a perfect match with only the width of your cut (3mm) to cover with the door seal. Offsetting the vertical cut in the firebox inner creates an overlap allowing both firebox doors to seal with one catch.
The seal I used is normally for glass panels in stove doors and was actually self-adhesive, though I wouldn't want to rely on that alone long-term. It was a 15mm tape which easily covered the 3mm gap. I also used this all the way round the inside of the backplate, just outside the fixing holes to provide a seal for the backplate against the stove itself. The handles are ordinary alloy room-door handles, dismantled, tapped to screw onto the catchpins, which I threaded myself, with the catches themselves simple flat fingers with ground/filed tapers to wedge against the inside of the backplate. Aligning the handles to close at a chosen angle is acheived by washers between the handle and door which control where the handle lies when it is screwed up snug. Once satisfied fine drill through the handle and catchpin and fit a small bolt or rivet to lock in position and prevent unscrewing when turning.
The firebox can take two logs up to 10”(30 cm) in diameter and 20”(60 cm) long at a time, though I find 18”(55 cm) more manouverable. A simple butterfly damper in the chimney is made from two discs of 2mm scrap clamped to a length of threaded rod with small u-bolts. A wingnut opposite the handle frictions the damper so it stays where you set it. The fixings used to hold the metal plate on the front and to secure the outer slabs together are 6mm non-expanding self-tapping multi-monti type. I drilled the slabs for these with a sharp masonry bit but without the hammer action engaged as the slabs shatter easily, especially when drilled into their edges as many of these fixings are. It is also important to drill oversize holes (8mm) in the backplate to allow it to expand independently of the masonry.
In use the stove takes 3 to 4 hours to heat right through (longer if unlit for several days) but once fully warm keeps on pumping out the heat with only a gentle fire in the box, which translates into one good sized softwood log roughly every hour, and remains warm to the touch for up to eighteen hours after the fire has died. In my home (500 square feet), I tend to light the fire at sundown and feed it for about six hours, which is usually enough to keep the whole house above 15 oC through till the next evening. Hardwood logs should last even longer. As with any natural heat source, cooking is more a matter of timing than control, but fine bread and other dishes (especially stone-baked pizza!) are a delightful bonus. Providing your logs are dry, once well-lit it is also possible to leave the stove doors wide open for a blast of quick heat, or to enjoy the sight of real flames, always a drawback with stoves where you usually sacrifice pleasure for economy and efficiency!
One final but very important consideration is the immense weight of such a stove. Building my own house I was able to install a specially reinforced hearth to carry this weight (approx. 450 Kgs on a 4’ x 3’ footprint). Definitely not a project for the spare upstairs bedroom! Even an existing coal hearth should be investigated for strength before installing such an additional weight. Directly on the ground is safest if you are unsure.
The principle of this stove is simple, make the flue gases travel for as long as possible in contact with the masonry mass before reaching the chimney. This design could be improved in several ways. Heat traps could be formed both in the fire box with a rear lip and by taking the chimney from the back rather than top of the stove. As already mentioned bricks or other modular materials: stone, tile, even concrete could be used for walls making for easier construction, and simple clay could possibly be used for sealing, or even covering and sculpting the entire stove exterior. Doors could possibly be salvaged from a broken cast iron stove and fitted into a moulded clay front, the whole stove could be hand sculpted in stone and clay, but here I was really just trying to create something simple, cheap and useful from unsightly rubbish.
Eli is the artist whose finishing so transforms this stove into a thing of beauty. It seems appropriate she should have the final word!
Well, what’s to be said? Who wants to live with a pile of cement slabs slap bang in their home? It positively cried out for something nice to be done. Keeping the whole project as economical as possible is nearly always a priority, but it doesn’t have to preclude each and every touch of grace. I used water-based paints (avoid oil-based for the smell) which did seem to contain a possibility of earth as well as coal tar pigment, in a rather watery acrylic base. For white I used some left-over water-based undercoat. I had no idea how the colours would stand regular heating, but expect them to darken over time. So far the white holds up least well.
For inspiration let the room and the object in question talk to you. Inspiration will come. Of course, usually the owner has a few wishes, which only manifest after the artist has already developed a design, so it’s best to do so on paper first. I didn’t, which means that one side looks interestingly different from the others. In this case, the design was also to incorporate a fine little ceramic statuette of a bird-headed stone-age goddess.
I intended the design to speak of fire and warmth, and to be relatively impersonal, so chose knot-work, enlivened by a serpent motive, which Steve and I collaborated on. The pattern was transferred with chalk and carbon-paper. I painted while the stove was heating up, and the paints dried fast enough to see quickly how layers of colours overlaid each other and how they worked, creating several new shades from basic primaries and earths. In this case, they all related to the fire-spectrum.
If you have a pattern book, you can enlarge designs with a scanner/photo-copier. If you feel confident, just give your imagination free play! Whatever design you choose, the result will almost certainly improve on what you started out with!