Getting your boiler size right will save you money, time and
trouble! You must seek qualified professional advice to size your
system appropriately. It is important not to over specify the size
of the boiler as this will make it run less efficiently.
Although some systems can regulate their range of operation very
well, typically during low-load (<25%) there is excessive air
and a regular 'on-off' cycling. An inaccurate design load will lead
to reduced efficiency, higher emissions and increased fuel use.
The "load" on your boiler depends very much on the type of use.
In general, for heating and hot water systems, the peak load
requirement will be needed for a short time (during the coldest
part of the year) and the summer load can be very low. The Carbon
Trust Practical Guide recommends that a wood fuel boiler should
cover 50-60% of the maximum system load, which translates
to approximately 85% of the heating need being 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. Smaller units commonly 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 required boiler capacity
Calculating required boiler capacity (typically stated in
kilowatts, kW) is not as straightforward as replacing your current
boiler with a biomass boiler of the same size; installers use a
combination of experience and detailed heat load calculations. The
Carbon Trust Practical Guide for Potential Users Part 2 - Technical
Manual: Section 2.4.4 Plant sizing provides general guidance on how
to size your installation. There are also some online tools which
you can use to estimate the size of system you will need. However,
it is essential that you seek professional advice before specifying
a system.