What can be chipped?

Chipping can be performed on small round wood (SRW), or mixed brash.  The resultant bulk density will be around 200-400 kg/loose m3, which is less than that of the solid wood, and even packed logs, but more than loose brash. 

With suitable equipment chips may be more easily handled, transported and stored than branches and brash, however SRW and logs should usually be transported before chipping owing to the higher bulk density if stacked properly.

Timber length

Longer pieces allow a longer machine chipping cycle, but add to the stress of hand loading. In the case of hand loading a compromise on piece size need to be made to reduce operator’s strain. This can lead to decreased quality however as a greater number of pieces yields a greater number of end pieces which typically produce oversize slivers when the short end turns sideways after passing through the rollers. When grapple loading, feeding long delimbed stems with a very small end diameter, butt first, is the best way to optimise output as well as to reduce the proportion of oversized particles.

Processing site

The processing site should allow minimum handling, movement and transportation both to optimise outputs and minimise operator’s strain.

Wood chip stacks


Stacked wood chips do not allow air to flow easily through the stack. Unless the biomass is very dry this tends to lead to composting and the growth of moulds. Composting leads to loss of biomass and can present a fire hazard owing to the high temperatures created in the core of a large stack.

Advice varies between different sources as to the safe maximum size for stacks of wood chips to minimize the risk of fire, with figures typically around 8-10 m high. The high core temperature in a stack of wood chips can, however, be used to assist drying as moisture is driven out from the core to the periphery.

Spores from moulds growing on piles of stored wood chips have been shown to cause health problems from particle irritation, type I and type II allergic responses, potentially toxic volatile organic compounds (VOCs) and mycotoxins, and inflammatory reactions to cell wall components when inhaled. A P3 filter dust mask should be worn when working in close proximity with stores or deliveries of stored chips.

Choosing a chipper


Wood chip boilers have constraints on the parameters of the wood chips to be used. The choice of chipper used is important if the wood chips are to be used with many types of feed mechanisms, especially auger feeds as found on many wood chip heating systems. General purpose chippers designed primarily for volume reduction do not usually produce chips suitable for use with many auger feeds which require chips of closely defined dimensions, with low levels of fines or slivers. Slivers in particular can cause difficulties as they can bridge or jam the auger.

You should give careful consideration to:

  • the size/capacity of the chipper in relation to the expected use in terms of overall volume of timber to be chipped, and expected level of use
  • manually feeding vs. grapple feeding; with manually fed chippers, weight and size of material are limiting factors for both operator and machine, limiting outputs (and presenting ergonomic and long term health implications for the operator). The use of correct lifting technique and aids will limit the physical strain but it will remain an important factor nonetheless

Chipper maintenance

Regular maintenance of the chipper components and settings will guarantee a consistent particle size distribution; the main factors that will influence chips size are:

  • The infeed speed of the chipper.
  • The number of knives for disc and drum chippers.
  • Knives setting and maintenance.
  • Anvil adjustment.
  • Disc or drum speed.
  • Blade maintenance.
  • Use (or not) of sieves / screens.

Health and safety considerations

Material can sometimes ‘kick back’ out of the infeed because of form, length, or aggressive infeed rollers. Operator should not remain in front of the chipper infeed more than necessary.

Due to the ‘megaphone effect’ of the conical shape of infeed chutes, sound pressure is very high at the very position where the operator stands to load the machine. Some chippers designs achieve reduced noise levels, and adjustments to the chipper settings can although help. The operator must wear ear protection relevant to the level and length of exposure.

Comprehensive guidance regarding the safe use of chippers and training requirements are available from the HSE.

More about chippers

There is more information about chippers here and further information to download: