Buckfastleigh Transition Town – for a sustainable future for Buckfastleigh, Devon
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  • 22nd December – Wood-burners

    Posted on December 23rd, 2009 andy No comments

    Wood-Burners

    I had been asked by the retrofit team to look into solid-fuel burners that might be suitable for fitting in the retro-fitted house, so here are some of my thoughts…

    We burn wood to provide a large amount of the heating in our house, but in really cold weather like we are having at the moment (with temperatures down to -5 deg C at night), we are having to resort to using our old 35 KW gas boiler to drive the central heating radiators, and I am really aware how inefficient a process this is – I can see the steam being vented out of the back, and feel the cold air and draughts around the windows and front door.

    Most of the time however, we use our small (6-7 KW) wood-burner in the front room, and an old-fashioned open fire in the dining room. The woodburner is very basic and the fireplace is extremely inefficient for burning wood – it tends to burn too fast, since our wood is stored in a dry shed and therefore is bone dry – a good thing for the woodburner where the burning is controlled by the air intake, but not so good for an open fire, where a percentage of water is desirable, as this is the only way to control the burn speed.

    Both us and our neighbours occasionally use coal in the fireplace, but we would all like to stop doing this, as we recognise that apart from being a fossil fuel adding to greenhouse gases, this is extremely polluting and has an increasing amount of embodied transport energy, as it is mainly imported from Russia, South Africa & even South America & Australia now, instead of being mined in England.

    With a normally ventilated wood-burner, the air is sucked into the room from elsewhere, creating draughts. Since the airflow into the completed retrofit house will be so low, it will probably be necessary to vent air in from outside the house to feed the wood-burner – we can’t rely on there being enough air in the house to keep it burning well.

    Size Matters!

    The engineers say that one very small (around 5 KW) wood-burner will be far more than is necessary to heat the entire house, when the retrofit is completed. Wood-burners with external air intake/duct kits tend to be higher-end models costing £1200 or more, such as the Jotul F400, which is an 8 KW model – of course we wouldn’t have to run it at full power!

    I’m interested to see how they propose to circulate the heat around the house though – some kind of vents between rooms? Obviously we won’t need to keep internal doors closed to contain heat so much, but we often want to shut them for reasons of privacy – or more often to stop cats or rabbits having free run of the house (especially the rabbits who like nothing better than an electric cable for dinner!).

    Choosing a wood-burner is not just a simple matter of deciding what heat power you require though – some compact burners like the Aarrow Ecoburn 7, whilst being very efficient and specified as a 5 KW unit, is only 46cm wide by 37cm deep – and that’s the external measurements – the internal space for burning wood will be somewhat less. This means that logs have to be sawn up very small and chopped thin – lots more work for me!

    It was while living in an old 16th Century farmhouse on Exmoor, that I realised why the fireplaces in those days were so big – unlike the small fireplaces in the house I live in now, which were built in the nineteenth century for burning coal. Splitting wide logs is not a problem if you have a decent maul or axe, but if you had to saw every tree up by hand into logs, you’d soon make sure you built your fireplace as big as you could possibly fit in your house!

    Secondary airflow

    Wood-burning technology has come a long way since our little wood-burner was built – with just one (primary) air inlet at the bottom front of the unit. It has a small glass window in the front door which is virtually useless, since it is always covered in soot.

    Modern wood-burners usually incorporate ‘secondary airflow’. These are inlets above the window at the front, which create a down-draught over the window, keeping it clean. This also creates a more efficient burn, ensuring that more of the exhaust gasses are burnt off, instead of escaping up the flue.

    Even better are units fitted with ‘cleanflow’ secondary airflow – ducts at the top front, which release pre-heated air that has been sucked from the bottom of the unit, which burn still more efficiently.

    Newer burners that are fitted with secondary airflow vents tend to have much bigger windows, that look almost as good as an open fire, allowing the fire to be better seen and enjoyed (and monitored).

    Tertiary airflow

    At the top end of the range, more expensive and powerful burners can incorporate ‘tertiary airflow’ – air-jets into the back of the burner, which ensure even more of the exhaust gases are burnt off.

    All these innovations ensure that you get the most heat out of your wood – having more than 75% efficiency in most cases.

    When you compare this to the fact that open fires burn with an efficiency that is probably less than 20% in most cases, this becomes a no-brainer (especially when it is me that collects, saws and chops most of the wood!).

    Standards

    It is also important to consider the level of polluting emissions that are vented into the atmosphere. These include the gases Nitrous Oxide (NO), Sulphur Dioxide (SO2) (both causes of acid rain) and Carbon Monoxide (CO), as well as particulates – bits of solid material. Inefficient wood-burners can actually be more polluting than open fires – emitting 3 times as much Carbon Monoxide for example, so an efficient system with secondary (and better still, tertiary) airflow is desirable for this reason too.

    The basic European standard EN 13240/A2 only sets the minimum efficiency of a burner to 50%, and the maximum emission levels to 1%. This is pretty useless, as any modern wood-burner should excede these levels – most of the burners I looked at had efficiency of over 75%. The German DIN Plus standard is much more stringent, it requires a minimum of 78% efficiency and emission levels below 0.12%.