Log burning stoves

Clearview stove

Potentially, log 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 CO2 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

stoking, management and temperature control
buying and storing logs
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 PassivHaus 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.

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. (Wood pellet and woodchip stoves 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 drafty, 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 PassivHaus would 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 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 kW. to heat it, even in very cold weather. If the room is poorly insulated or drafty then you will need to seek advice on how big a stove to get.

Whole houseIf 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. Probably the the most efficient way to address the problem of peaks and troughs of temperature output from a stove is to go for a very large hot water storage tank (accumulator) and a boiler which is not a visual feature (i.e. is purely a water heater) This way, surplus heat can be stored if the boiler itself is running too hot. It can also be made to work well in conjunction with solar water heating. There is an interesting thread about this on the Navitron forum

Below is the schematic of an installation in a house where they wanted to heat the water from a wood burning cooker with a water jacket (a Windhager) and also from a solar collector. They were keen to have good pressure for the shower without using a pump so the rising main runs through a coil in the main heat store after it has been preheated by the solar collector. (header tanks are not shown)

Below is the stove and hot water store during fitting

Kakkelovn

An interesting and traditional type of stove which mainly overcomes the problem of temperature regulation is the Kakkelovn which has been so popular in Scandinavia and parts of Northern Europe for a couple of centuries. The mass of the masonry absorbs heat from the fire and releases it slowly. Here is an example of a model by the Swedish company Nibe

Combined heat and powerIt is possible to generate electricity from a wood stove at the same time as getting heat. The Austrian SPM company have integrated a stirling engine into a wood pellet stove. It produces up to a kilowatt of electricity.

Cooking on stoves

photo by jrsnchzhrs

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

Hearths and flues

The building regulations (part J) stipulate what type of hearth is required for a stove and other issues such as combustibility of the surrounding areas. Chimneys are also covered by the regs and it should be noted that if a stainless steel twin wall flue is used then (depending on the manufacturer) it probably has a different specification from that of a coal stove or fire. It is important that wood stove chimneys do not run too cool as there is the danger of wood tar condensing out on the inner surface and creating a fire hazard in the flue or even blocking it. Many stove suppliers and installers will give advice on this. The speed with which wood stoves light and how well they run depends crucially on how dry the wood is. It is worth giving consideration to the storage of the timber. Ideally it should be allowed to dry under a cover or roof with good air circulation for several months, preferably over a summer. Kindling will dry in a week or two if there is a storage place near the stove.

The Log Pile Website - sources of log and pellet fuel and lots more info.

Pellet and chip

You may also want to consider wood pellet or wood chip because there are now boilers available with automatic feeds which only need the ash emptying every few weeks. 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.

A nifty little gadget is the Ecofan which sits on top of a stove and uses the stove’s heat to power itself in order to circulate the warm air coming off the stove.

selfbuild-central.co.uk