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