If you have a typical house, your heating system is likely to be based on mains (piped) gas, electricity. A much smaller percentage of homes rely on oil tanks or bottled LPG for heating and these are the only ones that provide any kind of buffering against damage or failure of infrastructure. The vast majority of properties, being based on piped gas or electricity (eg electric underfloor heating or heatpumps) have the unfortunate characteristic of failing promptly should there be a failure in the supply (The same applies to pellet stoves). Whereas we are not advocating that you should rip these out and replace with an oil or bottled LPG system (which would most likely cost significantly more to run), there is actually a lot of sense in implementing a secondary heating system which is immune to this problem. You can even install multiple heat sources, to hedge your bets as to which one might be affected.
Woodstoves are probably the most common alternative/supplementary heating system to be implemented – and they can be installed easily in most houses, either re-using the original brick or clay liner chimney, or by passing a flexible stainless steel chimney flue down inside the existing chimney. Even if your house does not have a chimney, you can pass a double-walled stainless steel liner through a wall and up the outside of the house, ideally terminating at a point higher than the apex of the roof. If the chimney stack is small and in good condition, then you can utilse this directly, in which case the chimney will gradually heat up over several hours, and work like a giant storage radiator, re-radiating heat into the house over a period of several hours after the fire has extinguished. The best place for a chimney stack is in the centre of a house, where all this radiated heat is captured by the house itself. Chimney stacks on end walls of houses are less efficient, and more likely to suffer with sooting, as they cool more readily, being exposed to cold outdoor air.
Heating or Hot Water
You can buy woodstoves that simply heat the room that they are in, but in the event of a heating failure, this heat can usually be dispersed throughout the rest of the house to a greater or lesser extent, by leaving the room doors open. This distribution can be improved by using air circulation fans. Even the mini fans designed to sit on top of stoves, using TEGs (thermo-electric generators) can make a significant difference.
Many stoves may be fitted with an optional back-boiler, usually in the range 9000-11000BTU/hr (2.5 – 3.2kW) – this is not enough to run the domestic radiator system, but it is sufficient to heat a hot water tank and an additional radiator. If you are planning this for dealing with power cuts, bear in mind that you will need to use ‘gravity-circulation’ rather than a pump. To be effective, the plumbing circuit needs to be as big a diameter as possible (eg 28mm), and the pipework needs to be as short as possible, and run gradually upwards for the whole run. If the piperun is short, you can actually get decent flow through 22mm or even 15mm pipe, but flow will obviously not be as fast as if it were a larger diameter.
Some stoves have a full water-jacket. These are designed for providing minimum heating to the room that they are in – instead most of the heat is transferred to the water. These are generally intended for heating a whole house, and are often described as being suitable for ‘x’ number of radiators. This may seem like a good option, however as these systems need to be fully pumped, consideration to the pump energy requirements must be made. For example, even a low-energy 40W pump running 12 hours per day, will require around 0.5kWh of electricity, which may not sound a lot when connected to the grid, however it will be a big drain on the batteries if the batteries cannot be charged with mains power.
Stoves with built-in water heaters are invariably a little less efficient (by around 10%) and a little less clean than stoves intended purely for heating air, but in the event of an extended power failure, the extra benefit of being able to heat hot water vastly exceeds this disadvantage.
Sizing
Sizing the stove is important. Stoves do not run easily or cleanly when they are run below the rated power output. So you do not want a stove that is too big or too small. Most stoves are rated at 6kW or higher. If you have a very small place to heat – such as a canal boat, shepherd’s hut or a micro-home, then a 4.5kW stove such as the morso squirel is ideal. To heat a larger living room (say 28m2), with a back-boiler for heating a large hot water tank plus a small radiator upstairs, then an 8kW stove such as the Hunter Herald might be ideal.
Efficiency
Old, traditional stoves tend to be around 50% efficient (which is still a LOT more than a traditional open fire – up to 15% at best). More modern stoves are between 70-80% efficient, so it is worth buying something that a) is new enough to have an efficiency rating and b)has a decent 75-85% efficiency. Beware of a few manufacturers claiming their stoves achieve almost 90% efficiency, which is unlikely, given the percentage of heat that is needed to send the smoke up the chimney.
Early stoves were very basic affairs, but manufacturers soon learnt that having two air intakes – one low down on the stove (‘primary air’) and one high up (‘secondary air’) provided better efficiency. The primary air is generally only used in the early stages of a burn, to quickly heat the stove up to operating temperature, then shut down completely once the fuel starts to pyrolyse, air from the secondary inlets being mixed in with the fumes to effect combustion.
Manufacturers then started to incorporate ‘tertiary air’ – where air, often preheated by passing through tubes inside the stove and then injected at the back of the stove. This improves the stove efficiency and burns cleaner as well.
Some woodstoves will only run on wood. Others, usually described as ‘Multifuel stoves’ are able to run on coal OR wood. If you are running on wood alone, then it is better to remove the grate, and to set the fire directly upon the ash. When running with fossil fuels such as coal, you will need to burn on top of a grate inside the stove, and you will need to have some primary air passing through the grate to keep the grate from overheating and to keep the fire going.
Lighting the stove
New users are sometimes worried about how easy it is to light a stove. It can be very easy indeed, if prepared properly, or almost impossible if it is prepared incorrectly. The easiest way is to crumple up individual sheets of newspaper and put plenty into the stove. Onto this, add a couple of handfuls of small kindling – this can be either small offcuts of wood, thin twigs or slivers of wood which have been created with an axe, splitting the wood along the grain to produce short lengths with a width of about 10-20mm. On top of this place a couple of small logs, or small blocks of wood. The secret of successfully lighting a fire is to firstly ensure that you are only burning dry wood (damp or fresh wood is bad for plenty of reasons – hard to light and will produce low temperature smoke which reacts with the steam being driven off, to produce tars and soot which will condense inside your chimney impeding the flow of gases out of the appliance and potentially igniting to causing a chimney fire.