WOOD AND WOOD STOVES

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Lynette Quinn
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1 Wood the fuel of the future? WOOD AND WOOD STOVES Wood is a carbon neutral fuel, and burning wood does not contribute to global warming. The amount of carbon dioxide released when wood burns equals the amount taken in from the air when the wood grew as part of a tree. Wood is also a renewable source of energy, and it can be considered as stored sunshine. It was probably humankind s first fuel, and it looks like wood will still be in use long after the burning of fossil fuels has become taboo. The use of wood as a fuel is being promoted by the Government. The Forestry Commission s Woodfuel Strategy for England lays out the official guidance. (The other nations of the UK have their own, similar documents.) The strategy envisages that by 2020 the woodlands of England will be yielding annually 2,000,000 more tonnes of wood for fuel than at present a 60% increase. The increase will come not from the Forestry Commission s own forests well managed as they already are! but from neglected, privately owned woodland. Perhaps surprisingly, the Woodfuel Strategy claims that managing woodland rather than neglecting it increases biodiversity so countering a possible excuse for neglect. For many years the UK was able to gorge itself on cheap North Sea gas. There was little incentive to burn wood, nor to extract wood from the forests. But times are changing. So that s the broader picture. What about our more specific concern, the selfbuilt home of the future? Next year, the energy requirements of the building regulations are due to be tightened, with a 25% reduction in energy use that can be met by better insulation. But looking further ahead, the zero carbon requirement due in 2016 will require more radical changes. The use of woodfuel for heating is an obvious and simple way to move towards zero carbon. It is cost effective, and will become even more so if fossil fuel prices escalate again when the world economy recovers. As a fuel, wood comes in three forms: Logs (for the greenies) Pellets (for the techies) Chips (for the big operators). This month, we look at burning logs no pun was intended, but looking at burning logs is one of the pleasures of wintertime! Wood stoves In the Seventies I built my own wood stove and it incorporated the high tech feature of that time, a throat baffle. (The baffle increased the length of the flue path inside the stove and thereby more of the smoke was burnt up.) Technology has moved on since then, and we now have airwashes (to keep the stove glass cleaner), and secondary air inlets to complete the combustion of the smoke or even tertiary air inlets in some stoves. Complete combustion has a double benefit: More heat is released The efficiency of the stove is increased. WOOD AND WOOD STOVES 1 JUNE 2009.

2 Emissions are cleaner Tar deposits in the flue are reduced and there is less pollution of the outside air. The stove I built is still in use today, and that is indicative of one of the virtues of wood stoves, their longevity especially those built of cast iron. (On the other hand, steel stoves are said to be more airtight.) Some stoves cost more than a gas boiler, but a stove will certainly outlast a gas boiler. Perhaps surprisingly, a wood stove fits well with the Passive House concept. In a Passive House, the insulation and the heat recovery ventilation are so good that incidental heat gains from bodies and appliances are generally enough for space heating. But in midwinter extra heat may be required, and a wood stove serves this purpose well or even admirably, as it is carbon neutral. The clever part is that the HRV system distributes the heat throughout the house simple central heating. With an HRV system installed, it is advisable that a wood stove has its own direct air supply to the outside. External air kits are available for a few stoves, and they incorporate an air inlet at the back or bottom of the stove. (The pipe usually has a diameter of 10 cm or 11 cm.) And of course, since airtightness is such an important aspect of a Passive House, the stove needs to be airtight when not in use, with a good seal on the door, etc. (By the way, even a century ago some houses were built with fireplaces that had their own air supply from outside.) More generally, for stoves with a rated output higher than 5kW, building regulations require some permanent ventilation in the room where the stove is situated in all likelihood, this will result in an excess of ventilation and a subsequent loss of heat when the stove is not in use. So in a new, well-insulated house fit only a small stove. It is better to burn logs at full output in a small stove than at partial output in a large stove. Don't fit an oversized stove. There are a great many wood stoves on the market, some of them made in the UK apparently we can still make some things! Many urban areas are smoke control areas (smokeless zones), and in these areas wood (as logs, pellets, or whatever) may not be burnt except in exempt appliances. (The areas are listed on the UK Smoke Control Areas website see Further Info.) In fact, there are many reputable wood stoves which are not listed as being exempt, although they would meet the required standard. Another official body involved is HETAS (an acronym for Heating Equipment Testing and Approval Scheme ). From the HETAS website you can download a list of wood stoves with their gross efficiencies (as well as their exemption status, as above). At present, the highest efficiency, 78%, is given for a couple of Hase stoves from Germany (the Lisboa and the Sarego). Two British stoves are close behind at 77%: Stovax s small Riva Plus and Clearview s Pioneer 400 with small oven. Note that some extra heat can be gained from the flue/chimney where it passes through the living space, so in practice the overall efficiency of a stove can be a few percentage points higher. Of course, efficiency is not the only factor to be considered when choosing a stove. Controllability is another how easy is it to control the burning rate of the fire? And perhaps the most persuasive factor of all is appearance from Scandinavian style to olde worlde charm. Many wood stoves do not have a grate as they produce much less ash than coal, and an air supply from beneath is unnecessary. Rather, the ash from successive fires is WOOD AND WOOD STOVES 2 JUNE 2009.

3 allowed to build up, leaving a bed of ashes for the next fire. (Wood ash is good for the garden and for potters glazes. And ash on a damp cloth is said to be good for cleaning soot off a stove s window.) Although cleaning out of the stove can be left for a week or more, in general a log stove needs more attention than a coal fire, with refuelling every hour or two though with the right logs and the right stove, a log fire can be kept burning overnight. There is a German-made stove, the Xeoos Twinfire, which stands out from the rest. Its grate is halfway up the stove, and the flames burn downwards, below the grate, as well as upwards. It looks remarkable, and an amazing efficiency is claimed for it 90%! (However, the HETAS list shows a gross efficiency of 76%, which is an excellent figure but not amazing. I ll have more to say about efficiency next month.) Boilers stoves Some stoves can have a boiler added as an extra; a few have an integral boiler. A boiler stove can heat the domestic hot water and/or a radiator or two or even a whole central heating system. Using a boiler stove in wintertime for domestic hot water is a carbon-neutral complement to using solar panels in summertime. Boiler is something of a misnomer, and the water in a boiler should not be allowed to boil. Since a wood boiler cannot be switched off like a gas boiler, it has to be plumbed into a circuit with a heat sink. This usually takes the form of a radiator with permanent thermosiphonic circulation with neither an electric pump nor a TVR (Thermostatic Radiator Valve). The boiler stove with the highest efficiency listed by HETAS is Clearview s Solution 500, with a gross efficiency of 75%. Inset and insert stoves Inset and insert stoves are built into the fireplace so only the front of the stove is visible. What s the difference between them? There seems to be a convention that an insert stove stands on its own base, like a free-standing stove, but an inset stove stands on some masonry in the fireplace and looks more completely built-in. ( Cassette stove is another term used, for both types.) Some of these stoves have two fronts, each with a glazed door. Such a stove is designed to be placed between two rooms, giving both rooms the benefit of a log fire. Duct stoves Some stoves allow ducting to be connected so that warm air can be ducted to other rooms. Catalysts Just as emissions from vehicles can be cleaned up by catalytic converters, so too can the emissions from a wood stove. The result is more heat and cleaner smoke. A leading manufacturer, the Norwegian company, Jøtul, did market some models with catalysts in the Nineties, but they do so no longer Were the stoves eventually found to be unsatisfactory? Design improvements have come from better air flows rather than catalytic combustors. WOOD AND WOOD STOVES 3 JUNE 2009.

4 Open fires Open fires? Ugh! Their efficiency is only about 35% at best. Most of the heat goes up the chimney. Worse than that when the fireplace is not in use, the warm air in the room also goes up the chimney. (If your existing house has an open fireplace, prevent the warm air from escaping by fitting a chimney balloon when the fireplace is not in use see Further Info.) Logs In much of modern life, goods are homogeneous but not so logs, which reflect the variety of nature. The key criterion is that logs should be well seasoned so that their moisture content is below 20%. This can be checked by a moisture meter which has two probes that are pressed against the wood. Simpler: if the bark is loose, then the log is probably dry enough. If you purchase logs by weight, then you need to consider how much of what you are buying is water and how much is useful fuel. Freshly felled beech can consist of about 50% water, and softwoods even more. (Actually, the amount of water in freshly felled timber depends greatly upon when the tree is felled: driest after felling in midwinter and wettest after felling in midsummer.) The general advice is to season logs for a couple of years before burning them. And this means keeping your logs in three distinct wood stacks: Seasoned for more than two years (ready to burn). Seasoned for 1 2 years. The new stack, seasoned for less than 1 year. Of course, if you can purchase logs that have already been seasoned and if you can trust your supplier, then you can cut out one, or possibly even two, of these stacks. To season logs, a wood store should have a roof, to keep rain off, and semi-open sides, to allow air to blow through. (For calculating the space required for a wood stack, it s reckoned that air spaces between the logs account for about a third of the overall volume.) Thick logs need to be split. Use an axe to do this, or, for difficult logs with twisted grain (eg, elm), use a couple of steel wedges and a lump hammer. And the logs need to be short enough to fit into your stove. Or to put it another way, the maximum length of log a stove can accommodate is one of the factors to be considered when choosing a stove. Traditionally, ash and beech are regarded as good firewoods, and poplar and willow as poor ones. However, that is for open fires. In today s efficient stoves, any wood burns well as long as it is has low moisture content. The density of the (dry) wood is significant, with hardwoods generally being denser than softwoods. By volume, a dense wood such as beech (650 kg/m 3 ) contains proportionally more energy than spruce (450 kg/m 3 ). By weight, wood with a moisture content of 20% has an energy content of about 3 kwh/kg, irrespective of species. Generally, softwoods burn faster than hardwoods. So softwoods are good for starting a fire, but not so good for keeping it going. Can I assume that readers know the difference between softwoods and hardwoods, I wonder? The distinction is widely used, and relevant to wood used in construction as well as wood used for fuel. Softwoods are derived from coniferous trees (eg, WOOD AND WOOD STOVES 4 JUNE 2009.

5 spruce) and hardwoods from broad-leaved trees (eg, oak.) Not all hardwoods are hard though balsa is a hardwood. Nor are all softwoods soft. Yew is a hard softwood! Next month: More about wood stoves. FURTHER INFO: Home Heating with Wood By Chris Laughton. 8. Woodfuel Strategy for England Published by the Forestry Commission. Free download from: Smoke control areas HETAS For efficiencies of stoves and boilers, download Part 1 of their Official Guide. Anglia Fireplaces Importers of Hase and Xeoos stoves. Stovax Their Riva 76 cassette stove allows a fan assisted ducting system to warm a second room. Clearview Euroheat Distributors of Continental stoves such as Hwam from Denmark, and Nestor Martin from Belgium. Jøtul Pronouncing the ø as er is probably near enough. Catalytic com Combustors need replacing after two to six years of use. Chimney Balloon Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, June, WOOD AND WOOD STOVES 5 JUNE 2009.

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