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Understanding heat requirements

The design and cost of a biomass installation is dependant upon the heat output it is designed to meet.

In general the peak heat load requirement for building heating will be needed for relatively short periods (during the coldest part of the year).

Most installations can regulate their range of operation down to 30% of peak output, but during a low load (say under 20% capacity) most go into an inefficient on/off 'slumber' mode. Buffer tanks and back up fossil fuel boilers are integrated into systems to manage this process.

Rather than specifying an installation to accommodate the peak load and possibly the low 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 biomass system. A peak load boiler (normally the existing oil/gas boiler, but it can be another biomass boiler) covers the extra capacity.

Calculating your boiler size

You must seek qualified professional advice before specifying a system. The following offers an introduction to the general considerations:

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. There are some rules of thumb used to work out approximate figures:

  • Building size calculation

Multiply the volume of the building by 0.035.

e.g. Volume = length x width x height.

Lets say this equals 5000m3 x 0.035 = 175kW

  • Replace the fossil fuel use

If you know your annual fuel spend it is possible to calculate a rough estimate of the size of boiler you might need in kW.  Work out how much oil or gas is being used in kWhs. Allow for system efficiency losses to work out how much useful heat and hot water is needed. Then convert that by assuming the typical full load hours. For example:

400,000kWhs heat required

Assume 2,500 full load hours

= 160kW rated boiler (400,000/2,500)