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Masonry stoves: how to make one from recycled paving slabs

Steve James & Eli Benedetti
Tuesday, 21st June 2011

Reader, Steve James, sent in this brilliant design and build for a Kachelofen, a fast-burn efficient masonry stove

Masonry stove

Warm and cosy...

Whilst building my small strawbale home in the damp, well-ventilated west coast of Scotland, I pondered the best way to heat such 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 wood-burning stove, not merely kept one as a pet, soon realises they have invited a particularly demanding house guest 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 axe work as possible. I also worried that a conventional woodstove in a small highly insulated space could make it unbearably warm.

Wood stove physics

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!

Twenty 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 minimising the number of required cuts to the slabs, 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 (see envisioneer.net) 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.

Back burners or stove oven

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 stove core is self-supporting, using the fire cement only for gap sealing, NOT structural strength. In the photos below, the complete core stands mortarless, held solely by its own weight. The facing slabs make it easier to seal, as well as providing more 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 (in) mild steel plate. It incorporates adjustable ring and pin hinges to align the doors to close perfectly. With care these shapes can be cut with a small angle-grinder, or a cutting torch, then finished with files and sandpaper. The doors are each made from two pieces bolted together, sandwiching the woven fibreglass sealing gasket between them, using the cutouts from the backing plate as the inner sides for each door. Offsetting the rear vertical cut creates an overlap allowing both firebox doors to seal with a single catch. The handles are ordinary alloy room-door handles, tapped to screw onto the catch pins, the catches themselves simple fingers with ground/filed tapers to wedge against the inside of the backplate.

The firebox can take two logs up to 25cm (10in) diameter and 50cm (20in) long at a time, though I find 40cm (16in) more practical. A simple butterfly damper in the chimney is made from two discs of 2mm (1/16in) scrap steel 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 (7/32in) 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 (1/3in) in the backplate to allow it to expand independently of the masonry.

Using the stove

In use the stove takes 3 to 4 hours to heat right through (longer if unlit for several days) but once fully warm it keeps pumping out the heat with only a gentle fire in the box, which translates into one good sized soft-wood log roughly every hour, and remains warm to the touch for up to 18 hours after the fire has died. In my home 46.5m2 (500ft2), 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 15ºC (59ºF) until 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 – approximately 450kg on a 60 x 60cm (990lbs on a 2 x 2ft 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.

Retaining stove heat

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 transformed this stove. 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 econ-omical as possible is nearly always a priority, but it doesn't have to preclude each and every touch of grace. I used water-based acrylic paints (avoid oil-based because of the smell). For white I used water-based undercoat. I had no idea how the colours would stand regular heating, but I expect they will darken over time.

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 the design on paper first. I didn't, which means that one side is different. This design also incorporates a fine little ceramic goddess statuette.

I intended the design to speak of fire and warmth, so I chose knot-work, enlivened by a serpent motif. The pattern was transferred with chalk, or 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/photocopier. Feel confident – just give your imagination free play! The result will certainly improve on what you started out with!

Steve James originally trained as a church organ builder, but has also worked as a sewerman, piano tuner, antique restorer, millwright, software engineer, and owned and run an organic box scheme employing ten people. He now designs and builds low impact homes and associated services, runs a 12volt recording studio and is studying to become a practising shaman.

Eli Benedetti is an all-round artist crafter and Jill-of-all-trades, who studied at Glasgow School of Art among others. She wants to design and build gorgeous houses for people who appreciate beauty, originality and natural materials.

Trouble circulating the air from your stove? Why not try the EcoFan 800 - powered solely by the heat of the stove?

Help spread the permaculture word...

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