Table of Contents
First it is important to get straight whether you want a wood burning stove because -
- it is a cosy thing to have on a warm winter’s night
- it is a serious form of total heating
- it is a bit of a backup to a central heating system
- you have access to lots of free timber
- it seems like a green alternative
Potentially, wood burning stoves are one of the greenest forms of domestic heating in the UK in terms of energy and materials sustainability. This is because they are almost CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming neutral and there is a very considerable amount of timber goes to waste anyway. Sulphur emissions are almost zero. However there are several possible drawbacks to their use
- buying and storing logs
- stoking, management and temperature control
- As soon as insulation levels are seriously improved in a house the issues with woodburning stoves as room emitters become, first of all, how to burn them low enough efficiently without going out and secondly how to get most of the heat into a central heating system and then a hot water store or masonry store in the case of a kakkelovn. Log burning works best and is cleanest when the stove is burning rapidly but then there may be excess heat into the room and this needs capturing for later.
- smoke emissions, as has been experienced recently in Germany and the US where their widespread popularity has led to public debates on the subject. Legally in the UK the question of smoke emissions depends on whether you are in a smokeless zone. The legislation around the Clean Air Act does allow for dark smoke to be emitted providing ‘that the alleged emission was solely due to the lighting up of a furnace which was cold and that all practicable steps had been taken to prevent or minimise the emission of dark smoke’. The Log Pile Website has good information on emissions and also on the safety aspects of burning wood. Possibly the the most sensible use of wood stoves is in some PassivHausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. designs where they act as as a backup form of heating in case of extremely cold weather. Otherwise the heating is mainly solar, electrical items and occupants.
see the section on burning timber
combustion air supply for stoves in habitable rooms
Combustion stoves all need a supply of air to burn the fuel and it varies depending on the stove output and fuel. The manufacturers state how much air is needed in the form of so many square millimetres of opening area. It is a requirement of the Building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) that this is provided because not doing so could cause the stove to produce deadly carbon monoxide if it is starved of air. The question becomes how the air gets to the stove. With a well sealed draught proof house it would be self defeating to use the old fashioned method of putting vents in an outside wall. Anyway people object to the draught and block up the vents, which is dangerous. The best way is to use a stove which has a built in connection so that you can run a ventilation duct directly from the stove to an external wall.
Choosing the type of stove
The problem with log burning stoves is that they tend to need constant supervision to make sure that the correct amount of fuel is loaded and that the air supply is properly adjusted. If this isn’t done they are either going out, overheating or burning smokily. (compared with wood pellet and woodchip stoves which are easy to control). There are four main reasons you might consider using a wood burning stove and they all lead to different types of stove, especially if the uses are combined. The basic functions are:
- Space heating only – basically a fire box with a door for loading fuel and emptying ash.
- Central heating (including domestic hot water) – ditto but with a water jacket around it
- Cooking – the fire box with hobs and an oven
- Focal point / Charm factor – any of the above but usually with a window to see the flames
- combined heat and power
A lot of stoves in the past were aimed at draughty, poorly insulated houses and were designed to kick out as much heat as possible. With a well insulated house the challenge is rather to be able to control a stove so that it burns cleanly and reliably on very low output. A really well insulated house such as a PassivHauswould overheat with any stove except under the severest weather conditions. Very few wood stoves are designed with eco houses in mind, especially when it comes to combining room heating with water heating or cooking. See below. Probably the first decision is whether you want the stove to be the main source of heat in the house or whether it simply augments an existing central heating system and gets lit on cold winter nights
Single room heating
If this is what you want then you probably need one of the smallest models of free standing stove on the market, usually made of decorated cast iron and with a self cleaning window in the door. The self cleaning bit is achieved by the incoming combustion air ‘washing’ down over the inside of the window. Also it should have secondary and possibly tertiary burn so that little smoke, creosote and soot is produced. The reason it should not be too large is that it will mainly be heating the room it is in (unless you keep all the internal doors in the house open to spread the heat around) and if the room is well insulated it will probably need less than 1 kWkilowatt - a measure of how fast energy is flowing. e.g. electricity might flow through an electric kettle at the rate of 2 kW. to heat it, even in very cold weather. If the room is poorly insulated or draughty then you will need to seek advice on how big a stove to get.
Whole house heating
If you seriously want to have a stove as your primary source of heating (and possibly cooking) then you need to have a heating engineer or experienced plumber do calculations to establish the optimum size. In the case of stoves which combine central heating with being a decorative focal point in a room then the ratio of how much heat comes directly into the room compared with how much goes into heating the water becomes very important. For instance you may find you need to run the stove flat out in order to heat bath water and radiator water in a hurry). Say this took 5 kW (3kW for bath water and 2 for radiators. If the stove was designed to split its heat output 50/50 between the boiler and the room heating then you would also get 5 kW of heat into the room where the boiler was situated. This would make nonsense of energy efficiency because even a large living room in a well insulated house would need less than 1kW of heat during freezing weather conditions so the extra 4kW would have to be wasted out through an open window in order to prevent over heating. What happens in spring, autumn and summer becomes a joke. We have become so used to well regulated gas and oil boilers with sophisticated thermostatic controls that designing a log burning system down to low energy standards is extremely difficult.
The cost of log heating
This kind of falls into two extremes. If you have a mate who gets loads of scrap timber and logs and drops them round for free and you have a chain saw then your heating is virtually free. Great!
If you have to buy logs from a supplier and have them delivered it is very different. The current wholesale cost of air dried logs is around £325/tonne before delivery and VAT. If you get the logs delivered in handy little net bags it can double or triple this cost.
Cooking on stoves
As with water heating (above), cooking on wood burning stoves is almost always out of balance with room heating and water heating. In summer it is in direct conflict, even with models containing baffles and adjustable grates. The reason that so many people drool on about cast iron ranges such as AGA and ESSE is that they remember them like some shrine to heat in an otherwise cold and damp house – the only place that was reliably warm as the frost crept across the stone floor. Possibly mother baking cakes helped too. Nothing wrong with that except these antiquarian behemoths are usually inefficient, extremely slow to react and in terms of cooking in and on them, – well – juggling plates springs to mind. All sensible cooks have a couple of gas rings or induction hobs and a microwave close to hand as well. Nostalgia isn’t what it used to be
Should you decide to go for a combined water heater and ‘visual’ room stove then bear in mind that normal old fashioned water jackets on stoves do not work well with wood burners. They were designed for solid fuels such as coal and coke. Wood burns at a lower temperature when in contact with a relatively cool water jacket. This causes poor combustion and tarry condensates on the cold surfaces including any glass windows in the stove. There is more information on the Cosi website with recommended stoves to deal with this. For added efficiency in these types of installations you may want to use a valve such as the Laddomat which controls the flow of water between the boiler and the storage tank in such a way that the boiler reaches full operating temperature quickly. It also extracts any remaining heat out of the boiler when it is going out.
The Log Pile Website - sources of log and pellet fuel and lots more info.