The following issues should be considered before installing a Wood Fuelled Boiler. 

• Design issues

With the exception of fuel type, the size of the boiler required is probably the single most important issue when designing a system.

The heating output from all boilers is specified in kW, or sometimes BTUs, and represents the rate at which the system can deliver heat energy. The size of system required is dependent on many factors including levels of insulation and draught proofing of the building, the lowest outside temperature and patterns of use. An initial estimate of the size of central heating boiler required can be made by:

Boiler size (kW) = volume to be heated (m3) / 34 (for a reasonably well insulated house).

As wood boilers burn most cleanly and efficiently when working at their maximum output it is best not to over-specify but to choose a boiler which is sized to meet the average heating requirements, with additional heating sources to provide extra heat on the coldest days. A qualified heating engineer should perform the final sizing of any boiler system. Wood chip boilers are available with power outputs ranging from 15 – 100 kW.

• Planning issues

The following issues should be considered when deciding to install a wood fuelled boiler.

1. Fue - It is important that you have storage space for the fuel, appropriate access to the boiler for loading and a local fuel supplier.
2. Flue - The vent material must be specifically designed for wood fuel appliances and there must be sufficient air movement for proper operation of the stove. Chimneys can be fitted with a lined flue.
3. Regulations - The installation must comply with all safety and building regulations (see Part J of the Building Regulations).
4. Smokeless zone - Wood can only be burnt on exempted appliances, under the Clean Air Act. This mainly applies to domestic appliances.
5. Planning - If the building is listed, or in an area of outstanding natural beauty (AONB), then you will need to check with your Local Authority Planning Department before a flue is fitted.

An accredited installer will be able to provide more detailed advice regarding suitability.

• Feedstock storage issues

When deciding which fuel should be utilized it is important to consider the availability, delivery and storage of the fuels:

1. Wood Chips -

Wood chips may be delivered by road, or produced on site from round wood. In both cases the chips then need to be held in storage, often for several weeks, before being transferred in smaller batches to the boiler. The two key qualities for wood chips are the Moisture Content and the Particle Size.

In principle, wood chips should be as dry as possible, less than 30% moisture content is ideal. However, drying takes time and requires storage. Consequently, the drier the chip the more expensive it is. If sufficient storage is available it is sometime economical to self dry the chips.

A major problem with most equipment is the presence of tramp material which can break, or otherwise damage, the working parts. Most systems have difficult dealing with oversize material which wedges in conveyors, feeders etc. preventing normal operation. Mechanical solutions are expensive and good supervision of the supplying contractor (or chipping process) is essential.

Transport and delivery of fuel chips is a major area for consideration when installing wood heating. Whilst delivery is, in many ways, similar to that for fuel oil wood fuel orders may need to be placed well in advance of need as guaranteed deliveries can take time to arrange.

It is essential that the wood chips supplied are compatible with the boiler fitted.

2. Wood Pellets -

Wood pellets are available from many sources and their distribution is increasing as systems become more popular. Wood pellets are uniform in size and flow easily so can be transported automatically from large silos to the boiler as required. There are various automatic feeding systems available including worm feed, suction probe, rotating worm screw, silo bag and underground pellet tank.

Wood pellets are more dense than wood chips and therefore require a smaller storage volume for the same heat output.

If there is the space and requirement to use  wood chip then store needs consideration. Chip store design is important, with 80% of the cost of wood heating during a typical life span of 25 years resting with the fuel - not installation. The wood chip supplied must be of good quality (produced from a chipper designed for the job) and be clean (no grit, soil, old nails etc.). Consider how the chips will be moved from the store to the boiler. Whenever possible transportation should be minimised.