If your home was perfectly insulated, it would need no heating – its temperature would rise to the body temperature of the people inside. Your home only needs heating because it loses heat to the outside world, so that the function of a heating appliance is to replace the lost heat.
The unit of the rate of energy conversion is the Watt, named after the Scottish steam engine pioneer James Watt, and defined as one Joule per second. Heating engineers customarily express heat emission in Kilowatts (kW, a thousand Watts) per hour, commonly just referred to as kW. (Old-fashioned one-bar electric fires were usually 1kW output.) Other units you may encounter are the British Thermal Unit (3412 btu's = 1kW) and the Kilocalorie (860 Kilocalorie = 1kW).
It is very important to ensure the heat output of the stove (measured in Kilo Watts (kW)) is equal to the heat requirements of your home. Solid fuel appliances work more efficiently when they are working hard rather than in “slumber / idling or tick over” modes. An oversized appliance will produce excess heat, more than is required to adequately heat your home. It is inevitable you will turn the appliance right down to compensate for this which in turn can have a detrimental effect on your appliance and chimney liners. Your chimney needs to stay warm to work; if your appliance is burning cooler then the gases cannot rise through the chimney quick enough to be expelled into the atmosphere. The cooler gases then turn into a condensate which can cause a corrosive build up in the chimney system. Prolonged periods of slow burning of ALL solid fuels including wood combined with inadequate cleaning will result in chimney blockages and or complete failure of your lining system.
To produce a comfortable room temperature of around 21º/22º Celsius (70º Fahrenheit), when the outside temperature is 0º, you will need about 1kW of heat for every 14 cubic metres of averagely insulated space. 1kW is the equivalent of approximately 1 bar of an electric fire.
The following formula and example will give you a reasonable guide to the heat output you are likely to need. To establish the cubic capacity of the room, measure (in meters) the length (L), the width (W) and the height (H) of your room, L x W x H= m3.
Room Size, Length 6m – Width 4.5m – Height 2.5m
6m x 4.5m x 2.5m = 67.5 (m3)
67.5 m3 / 14 = 4.82 kW required
A 5kw appliance would be adequate for this example.
These calculations are based on heating a room which has been averagely insulated.
To calculate the kW requirement for a poorly insulated room divide m3 \ 10.
To calculate the kW requirement for a very well insulated room divide m3 \ 24.
Larger stoves need to have permanent non closable ventilation installed as wood and solid fuel burning stoves draw in air from the room.
Building Regulations specify that the following fixed ventilation must be provided when installing wood and solid fuel burning stoves.
Appliance rated under 5kW: No ventilations required.
Appliances rated over 5kW: For each kW of rated output above 5kW, 550 sq.mm (0.85 sq. ins.) per kW is required.
A standard ventilation brick will generally provide 1650 sq.mm (2.56 sq.ins).
The information above is guidance only, for accurate calculations we strongly recommend you have a site survey to assess your exact requirements before you make your final decision. Other factors will need to be taken in to consideration such as building type, construction, age, level of insulation, central heating system, and condition of chimney etc, site surveys are often free of charge and can be arranged directly with the retailer or installer.