Wood stoves: introduction

What are they?

They are stoves for burning wood to heat your space, your water, and even to cook on. A basic stove will be of cast iron or steel (lined with fire-brick to retain heat), usually with a door at the front for loading, lighting and ash removal – but sometimes on top in small stoves.

A basic woodstove with logs stacked next to it

A basic woodstove with logs stacked next to it, plus waste paper for lighting the fire, and a copper kettle on top for making tea

Open fires are pretty, but most of the heat disappears up the chimney. Also, updraughts from below pass through the logs and draw off volatile gases (which provide most of the heat) and suck them up the chimney unburnt, which wastes heat and causes tar build-up in the chimney. The casing of a stove heats up and radiates heat out into the room. You can have a simple stove for heating, or one with a back boiler to provide hot water / central heating.

Ceramic stoves or kakkelovns (popular in Scandinavia and Eastern Europe) have a ceramic casing that acts as a heat store to radiate heat into the room for hours after the fire is out. These are much more expensive than standard log burners. Aga / Rayburn-type cookers are also much more expensive than a basic stove, and nowadays tend to be fuelled by gas, oil or electricity.

Pellet stoves burn pellets made of compressed organic material such as sawdust or agricultural plant waste, and can be automatically fed from a hopper. Larger automatic-feed wood or pellet boilers can be installed in schools, hospitals etc.

A home-made saw-horse

A home-made saw-horse, on which you can chainsaw cords (lengths of timber) to the right length to fit into your wood stove.

What are the benefits?

CO2 is the most important greenhouse gas; wood is a carbon-neutral fuel in that burning releases CO2 (the same amount as if the trees died and rotted) but new trees absorb it. Growing trees absorb more CO2 than mature trees, so as we harvest mature trees, from a climate change perspective, it is essential that we replace them with new ones. Wood is a renewable resource that provides a habitat for wildlife. It also has a good energy balance, i.e. can be locally produced, requiring very little processing or transport energy (logs more so than pellets, although pellets release less of the pollutants that cause acid rain). Emissions are better than coal, oil or gas as regards NOx and SOx (acid rain) and carbon monoxide but worse for particulates. For space heating, emissions and energy losses from power stations make electricity the worst option environmentally; and wind and photovoltaics are as yet too expensive for heating.

Modern ‘clean burn’ stoves can be used in smokeless zones; they use secondary combustion, baffles or catalyts to maximise combustion of gases and particulates, reducing emissions and increasing efficiency.

You will need to cut kindling from waste wood

You will need to cut kindling from waste wood, twigs, unwanted furniture, doors etc to help light the fire; it needs to be very dry, and can be kept in a basket near the stove to dry it further.

What can I do?

Stoves

basic stoves start at around £400, (c. £1000 for clean burn) new, or cheaper second-hand. If you have access to sheet steel, and cutting and welding equipment, you could even build your own, which would work out cheaper still. Your supplier should be able to recommend an installer, or you can install yourself: stand the stove on a slate or concrete slab (not too close to wooden fire surrounds), make a metal plate for the chimney (can be a thin sheet of stainless) with a hole for the flue pipe (‘Copex’ is useful because it’s flexible), seal flue and plate with fire cement, put fire string round door (both from plumbers’ merchants or online – see ‘resources’. Old chimneys will need re-lining, as tar could be deposited, causing a fire hazard, plus gases could seep through into living areas. Chimneys must be swept every year to remove creosote and tar and avoid chimney fires.

Firewood

5 tonnes of wood per year is more than enough for a basic stove in your main living area – but that’s assuming it’s in use most of the time from October to April; unseasoned wood is heavier because of the water content. Logs are the cheapest way of heating your space – around 1/5 the price of electricity per kWh. It will probably get relatively cheaper too, as fossil fuel prices rise, and carbon-neutral fuels get tax breaks.

stacking the split logs under cover for at least one summer will reduce the moisture content to around 25%

Stacking the split logs under cover for at least one summer will reduce the moisture content to around 25%.

You can buy logs split, or cords (lengths over 1m) and cut and split them yourself, in which case you’ll need a chainsaw and a splitting axe from a farm / garden equipment supplier (ask about protective clothing and health & safety). Trees are best felled in the winter when moisture content is lower, then cut and split to dry better. Store under cover for at least one summer, which should reduce moisture content to around 25%. Ash is probably the best firewood, as it has the lowest moisture content when green (c. 35%), but beech, cherry and hawthorn are also good. Don’t use second-hand construction timber, unless you’re absolutely sure it’s untreated, as it will release toxic fumes; and certainly not chipboard, plywood or mdf – even for kindling – as it contains formaldehyde. Bring some split logs in and stack them next to the stove (not touching) – this will dry them more. Leave a layer of ash at the bottom of the fire, add paper, dry kindling (you don’t need firelighters) followed by a few small, dry logs. There will be an air inlet to adjust air flow – have this open at first, and slowly shut it down as the fire becomes more established. Wood ash contains potash, and can be used as a fertilizer (best applied at the end of the growing season, and not on alkaline soil).

See here for more detailed information on getting and using a wood stove.

 

Thanks to Sune Nightingale of Stoves Online for information.


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