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Work out your heat needs and boiler or stove size

Getting your boiler or stove size right will save you money. Over specifying your boiler will lead to higher fuel costs and carbon emissions.

Large boiler tanks

How much energy your boiler needs to generate (the 'load') also depends very much on how the boiler is used. For example, in general the peak load requirement for building heating will be needed for a relatively short time (during the coldest part of the year), whereas in industrial processes the load is likely to be much more consistent throughout the year.

Although some systems can regulate their range of operation very well, during a low load (under 25% capacity) most go into an inefficient on/off 'slumber' mode.

Don't make the mistake of simply specifying your wood fuel boiler to be the same capacity as your current boiler. Typically a wood fuel boiler needs only two thirds the capacity of a fossil fuel boiler; however, it is important to get professional advice on this.

Rather than buying a boiler to accommodate the peak load, it is often more energy and cost efficient to have a top-up system in place. The Carbon Trust recommends that a wood fuel boiler should cover 50-60% of the maximum system load; this translates to approximately 85% of the heat supplied by the wood fuel boiler. A peak load boiler (normally the existing oil/gas boiler, or a renewable source such as solar water heater) covers the extra capacity.

Systems can also use an accumulator tank to manage for variation by storing hot water. The volume of storage in litres should be about 40-60 times the boiler heat output in kW.

Calculating your boiler size

You must seek qualified professional advice before specifying a system. However, at this stage, there are a number of online guides and tools to help you get a good enough estimate of boiler size.

Calculating your stove size

As a rough guide, most stove manufacturers suggest using the following equation based on the room size you want to heat:

depth x height x width (in metres) divided by 14 = heat load requirement in kW

This is based on a rectangular room, average insulation and an outside temperature of -1 degree C. You should always discuss your requirements in detail with the supplier or manufacturer, particularly for unusual shaped rooms or well insulated homes.

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