An Introduction To Wood Burning Stoves

The past few years have seen something of a Renaissance for wood burning stoves, so much so that manufacturers have struggled at times to keep pace with demand for this, perhaps surprising, must-have item of modern life.

But when you look at it, there are in fact quite a few very good reasons to consider installing a wood burner – assuming, that is, that you’re fortunate enough to be able to do so. These include in no particular order: aesthetic value; efficiency; money saving; green energy; low carbon and other environmental considerations; increase the value of your property even.

Not everyone is able to take advantage of a wood burning stove though. Common impediments are an unsuitable property, air pollution regulations, lack of access to or storage space for fuel (principally seasoned wood or specially manufactured pellets). And anyway, there isn’t at present the capacity or infrastructure to support everyone converting to wood burners.

But if, having weighed up the advantages and disadvantages of wood burning stoves, you find that you are one of those who can install a wood burner, then check this out. The cost of running a boiler using electricity works out at 8.8 pence (UK data) per Kilowatt hour output, whereas a wood burning boiler comes in at 1p per kWh – nearly nine times cheaper!

To be fair, heating a boiler using electricity is not what most people do, but even gas and oil work out at over three times the cost of a wood burning boiler (figures taken from the UK Forestry Commission report A Woodfuel Strategy for England).

With gas and oil set to rise remorselessly in the years ahead, installing wood burners starts to look like a very cost effective move, especially when you also factor in the possibility of grants available for switching to bio-fuels (in the UK grants for installing a whole variety of renewable energy technologies are administered by the Low Carbon Buildings Programme).

Obviously, these figures vary according to your own location. The UK is a densely populated island with accordingly sparse areas of forest and other woodland suitable for sustaining bio-mass energy production. The USA on the other hand, although it has a larger population also has vastly more natural resources to sustain a greater bio-mass industry, and Canada even more so. By contrast, for those living in one of the Gulf States in the Middle East where oil is abundant but trees almost non-existent then buying into wood burners is clearly going to represent a lifestyle choice rather than a sound economic decision.

But whatever your take on it, wood burning (or to use modern parlance, biomass energy) has not only been with us in one form or another since we lived in caves, it’s not about to disappear any time soon either. Quite the opposite in fact; we are caught in a pincer movement between dwindling fossil fuel supplies and the effects on the climate likely brought about by burning all that fossil fuel in the first place.

So we find ourselves in urgent need of viable alternative energy sources that are also non-detrimental as regards carbon emissions. And you can gain a good understanding of the kinds of solutions that the UK government (and indeed most of continental Europe) have in mind when you consider statements such as this from the UK Biomass Strategy document: [from 2007 it became] “a requirement that biomass boilers are installed wherever appropriate in new school buildings and refurbishments.”

Biomass fuel is essentially any type of renewable biological material that combusts well. It obviously has the desirable characteristic that being renewable implies a life cycle, and hence a period of growth which typically ensures it is self-balancing as regards carbon output. The most commonly used biofuels are of course logs, wood pellets and other forms of reclaimed wood, but you can even burn husks from cereal crops and nuts (so long as your particular burner is capable of accepting it).

But before we go much further, let’s rewind and find out how we got here in the first place…

A Brief History Of The Wood Burner

Before 1742 or thereabouts, if you wanted to heat your home, cook food or bathe in hot water your options were pretty much limited to using an open fire. Then Benjamin Franklin, a founding father of the USA and prolific inventor, came up with the freestanding wood stove, which by 1744 was known as the Pennsylvania Fireplace and a huge hit. This was an open stove that resembled a conventional fireplace but enabled more sophisticated control of the air flow to the fire (using metal baffles to direct hot air and a flue to draw exhaust gases) and was effectively the forerunner of the modern wood burning stove.

Except that this isn’t quite true. Franklin may be credited with this invention and given his name to it, but in fact he simply refined an existing design dating back to 1642 in Lynn, Massachusetts. This is believed to be the first cast iron stove and in fact used an enclosed firebox (Franklin’s early models were open, possibly because he was known to have been influenced by the writings of André Dalesme who created an open fronted design in 1686 in France). Also predating Franklin by a good seven years (1735) was another Frenchman, Francois Cuvills with his Castrol stove, better known as the “stew “stove” that sported a hot plate on top for, well, stewing stuff.

But anyway… The motive behind Franklin’s invention appears to have been the increasing scarcity and cost of firewood at the time (the population was expanding rapidly in his home town of Philadelphia). But he also understood that because heat radiates uniformly in all directions, the standard domestic fireplace set into a wall was only delivering usable heat to one of the six sides enclosing it. By devising a metal stove that could stand in the middle of the room (and also retain heat in its cast iron metal casing) he could ensure that all the heat ended up in the room. The new stove gave off twice as much heat for one third the amount of wood, so it was in the order of six times more economical – a major selling point for any heating technology.

These early wood burning stoves were barely 50% to 60% efficient in terms of converting wood into heat, but you do have to consider how that compares with the only alternative at the time; the open fireplace which struggled to achieve 20% efficiency (quite a bit escapes into to the back and side walls, but most of course disappears straight up the chimney). The efficiency of any type of fire not only has a bearing how much heat you can extract from a given amount of fuel, but also directly determines how much smoke, other exhaust gases and particles are emitted. Generally speaking, the more efficient the fire the greater the amount of original fuel that will be consumed and converted to heat and hence the less there is in the way of emissions.

There were of course some enhancements to cater for differing requirements and wood burning stoves become available in many different shapes and sizes, with variations such as adding masonry and/or ceramic cladding to ensure longer, more even heat dispersal. The most significant early modification was by Franklin’s colleague David Rittenhouse, who rectified a fairly serious (and, one would have thought, obvious) flaw in the original design: it vented smoke from the base – yes, into the room! – not good. He added his L shaped air box to vent smoke to the outside (usually by connecting it to the nearest convenient chimney).

But by and large the next three decades of in the story of the wood burning stove were characterized by relative stability in the design. Also, because Franklin refused as a matter of principle to patent his inventions there was no reason to outsmart his design and so the way was open for anyone to set themselves up in the wood stove manufacturing business and add their own little quirks.

A few tweaks quickly became prevalent, sealing off the hitherto open front to ensure a closed firebox for example, but otherwise the technology remained pretty much unaltered for the next couple of centuries. It was a simple, easy to mass produce, heating solution that worked well enough and (like, for instance, Edison’s incandescent light bulb which is only now succumbing to the vastly more efficient LED light bulb) there existed no real pressure to significantly develop it any further and so the underlying technology altered very little.

Not that that really stopped an endless series of incremental enhancements… At the turn of the century Benjamin Thompson (a.k.a Count Rumford) designed the world’s first wood burning kitchen range (it was a bit of a beast though and aimed at large, well staffed kitchens). Anyone who had set their heart on installing this state of the art technology in their own more modest domestic kitchen would have to wait another 35 years until Philo Stewart developed his Oberlin stove.

Meanwhile, over in Europe, and especially Scandinavia (for obvious reasons), the spread of the wood burning stove continued apace, with names that are still pretty big in the business today, such as Jøtul of Norway who setup shop in 1853. It would take forever to list them all, but quite a number of those early makes of wood burning stoves are still in business and doing very well indeed today.

Wood Burners In The Modern Era

But it would be 1946 before signs of significant new development started to appear, with the introduction of the Rayburn (now assimilated by Aga). This ingeneous device combined the features of a kitchen range (with twin hobs and an oven) with a boiler designed to provide central heating and hot water for a decent sized family home. It also dissipates enough heat through the cast iron casing to keep the kitchen nice and warm (I know this for a fact because the kitchen here at Kat Manor was rebuilt around an inherited Rayburn – it somehow seemed easier than moving it to accommodate the kitchen). A sibling also heats her sizable converted barn in the depths of rural France with a Rayburn fed on a mix of logs and junk mail, so this is one very effective and versatile little workhorse that set the tone for the future.

But there the story might have stalled, with the wood burner relegated to the status of either “heating system of last resort” or trendy fashion accessory. but for the antics of OPEC in the 1970′s… In an echo of the wood shortage that sparked the irresistible rise of the original Franklin stove, a series of oil crises led to dramatic rises in fuel prices and people once again looked for a more economical heating technology than the oil and gas fired boilers that had become commonplace by then.

But folk quickly discovered that this was definitely Eighteen Century technology and a poor fit for Twentieth Century homes. Most old styles wood stoves were run at smouldering temperature so as to reduce the ferocity of the heat and prolong the burn. But the down side to this was that less of the fuel was consumed and ended up either belching out the chimney or, in the case of sticky resins, plastering the inside of the flue/chimney with creosote.

This not only caused a fair amount of pollution but also resulted in poor efficiency, which equated to increased fuel consumption and thus cost. And it was a chronic condition, that left unchecked could easily result in very dangerous fires if the creosote lodged in the flue pipe ignited. What really got thing moving towards today’s highly efficient, low pollution wood burners though was the introduction of strict emission regulations, initially by the US Environmental Protection Agency (EPA) Clean Air Act but swiftly followed by most other developed countries.

And so began a three-sided arms race, with the quest for cheaper fuel bills, modern legislation regarding efficiency and pollution, and advances in materials technology and air flow design all locked in a tussle that resulted in the first really significant departures from the original implementations.

Today’s wood burning stove is as close to Franklin’s as a modern automobile is to a horse drawn carriage. Both perform the same basic function and share superficial characteristics, but fundamentally there’s no real comparison. The modern wood stove uses a totally airtight construction and benefits from modern metal alloys.

Modern wood burners also include improvements such as firebrick linings to retain heat better and catalytic converters to burn exhaust fumes, thereby extracting every last bit of heat possible and drastically cutting pollution. Features such as these mean that many wood burners built today are around 80% efficient and thus compare favorably in terms of efficiency and cleanliness with gas and oil boilers, while still typically much cheaper to run.

The original technology behind “clean burn” borrowed an idea adopted by the automotive industry – catalytic converters. These were essentially retrofitted to standard stoves in much the same way that a “cat” is bolted to a car exhaust; the unburnt fuel was thus forced through a ceramic honeycomb containing catalyst compounds that allowed unburnt particles and gases to ignite at much lower temperatures.

The downside to wood stove catalytic converters is much the same as with cars, namely that they wear out within a few years and need to be replaced. More modern “clean burn” wood burners use an ingenious secondary combustion area that burns the flammable gases produced by the primary combustion (of the wood). Unlike fitting a catalytic converter though , the whole stove itself has to be designed around this secondary burn chamber.

This secondary area utilizes fresh oxygen that is pre-heated as it is drawn through channels within the firebox and not only extracts even more heat but considerably reduces emissions to the point where such “clean burn” stoves are often explicitly exempt from smoke control restrictions.

Note, that this is a UK specific list, for the USA you need to consult the List of EPA Certified Wood Stoves. For the record, the current limits are 7.5 grams of particulate emissions per hour for older stoves and 4.1 grams for modern catalytic versions (though most will in fact come in comfortably below that at maybe 1 gram per hour). As a result, most contemporary wood burners emit almost no visible smoke at all.

So what does the future hold for this deceptively simple heating technology? The answer is to be found by considering the “conventional” alternatives based on fossil fuels. The supply of these is steadily diminishing, and unlike wood you cannot easily and fairly quickly grow another oil well or coal mine. They also represent a one way street where CO2 is concerned which does not mesh well the current trend towards a low-carbon society.

With relentless cost and environmental pressures on fossil fuels, it’s hard not to see a bright future for a heating technology based on a genuinely renewable and carbon neutral resource that basically converts sunlight and atmospheric CO2 into solid carbon and gaseous oxygen. In a very real sense, growing trees is simple converting solar energy into a more convenient form.

Already wood burning boilers have started to scale up from purely domestic appliances to being installed in factories, offices and public buildings. Obviously, to support more widespread use of wood burners will require the plantation of more trees, but that’s no bad thing either. And the comparison with Edison’s incandescent light bulb? Well, like the wood burner there was no compelling incentive to improve this particular (and in fact quite staggeringly inefficient) technology until similar external pressures forced a rethink. The result was that a technology formerly regarded as being of limited use and confined to a narrow set of applications suddenly emerged as the future of lighting.

Options For Wood Burners

As already noted, modern wood burning stoves are typically at or above 80% efficient which is on a par with gas and oil fired boilers, which also means that unlike their arguably closest cousin – an open fireplace – they do not generate much in the way of waste cinders and so do not require a great deal of maintenance and cleaning. They can be installed in a variety of situations (though kitchen and lounge are the clear favorites to take advantage of local space heating) and are designed in all sorts of different styles, ranging from traditional looking wood burning stoves and ovens to ultra stylish modern wood burners that are typically installed as a designer space heating center piece for a modern family home.

As you might have guessed from words such as “oven” and “boiler”, wood burning stoves are not confined to the status of glorified living room hearths (without the hassle of lighting and cleaning them). A wood burning boiler – either a standalone wood boiler or a wood burning stove utilizing an integral back boiler – will provide ample domestic hot water and central heating through existing house radiators. Or an integral hot water energy storage “accumulator” tank that stores water at up to 90º C, which ensures a supply of of hot water and heat for radiators regardless of whether the wood burner itself is in use.

Wood burning ovens are, of course, nothing new and the traditional kitchen range oven has long been a favorite, not only for it’s timeless good looks, but also because range cooking is quite different to modern gas and electric ovens and hobs, and there’s nothing quite like the homely background warmth spreading out from the kitchen to the rest of the house. Combination wood burning range cookers such as the Rayburn deliver an all in one solution capable of heating the whole house – full hob and oven cooking facilities plus a boiler for supplying hot water to the main hot water cylinder and also to the central heating radiators, and of course localized space heating and that traditional kitchen range look.

But the story doesn’t end there as far as variation and versatility go; wood burning stoves can be fueled using seasoned wood logs, wood chips and manufactured wood pellets. You can even purchase a dual-fire wood burner that, as the name suggests, will accept either logs or wood pellets and chips as fuel.

The main difference between logs and wood pellets/chips is simply related to size. Wood logs have to be loaded into the burner manually, whereas pellet and wood-chip systems are available that automatically load the fuel into the burner; these are ideal if you want to install a wood burner system as a direct replacement for a standard gas or oil fired heating system.

Many people prefer the aesthetic qualities of a neatly stacked pile of seasoned logs and indeed the look of burning logs, and dual-fire wood burners allow you to have both convenience and visual appeal as the mood takes you. Check out this review of how to select a type and make of wood burning stove for further ideas.

Wood Burners And Climate Change

So wood burners look attractive, saves you a lot of money on your fuel bills, particularly for heating (for saving even more money on lighting and other electrical applications you might want to consider investigating installing domestic solar power), and is extremely versatile. But how about environmental issues? How “Green” exactly are wood heating systems?

Well the news on the environmental front is pretty positive also. Yes, burning wood does release carbon dioxide, but in striking contrast to fossil fuels such as oil, gas and coal, burning wood is a very low carbon fuel source, if not quite totally carbon neutral. It is what can be described as a balanced carbon process, because the carbon released in a wood burning stove is the exact same carbon that the original tree absorbed from the atmosphere.

By sourcing fuel from a renewable and endlessly sustainable resource, in other words by replacing each tree used for fuel with a new tree, the precise amount of CO2 released by your wood burner will be taken from the air and locked up again inside that new tree as it grows. The amount of carbon released into the atmosphere exactly matches that absorbed from the atmosphere in the growth cycle of properly managed forests and woodlands. Which simply means that burning wood in this efficient and controlled way does not contribute to excess CO2 output and thus to global warming and resulting climate change.

Burning fossil fuels is a one way street, but wood burning represents a completely sustainable cycle, if managed correctly. Trees are in fact a form of solar energy – they convert sunlight and carbon dioxide into plant material, namely wood. Whether or not people burn the wood or leave a dead tree to rot naturally makes absolutely no difference as regards CO2 emissions. The carbon dioxide locked up in the wood is completely released back into the atmosphere in both cases.

It is worth noting here though that unlike gas and electricity that are piped directly to your home, wood fuel has to be physically transported and is bulky and heavy. All the low carbon benefits fly out the window if you cannot source your wood logs, pellets or chips from a “local” supplier. You will also find the cost to you increases as you pay extra for higher transport costs.

Save money AND the planet with a wood burner? That’s a bold claim; but as you now see, one that does in fact stand up to further examination and explains why so many people want to install a wood burning stove.