Wood Pellet Boilers are commonly used in homes and businesses as an alternative to oil or gas fired boilers. Wood pellets for these units are normally housed in a large sealed hopper/tank that is either fitted with screw feeder (auger) connected to the boiler, or the hopper/tank is mounted over the boiler for gravity feeding.
Due to the enclosed nature of these hoppers/tanks the atmosphere inside can become oxygen depleted and a toxic atmosphere can accumulate. Such enclosed spaces would, by virtue of these hazards, be a confined space and subject to the Confined Spaces Regulations, where they are in premises subject to the Health and Safety at Work Act 1974.
Follow the link below for important information from the HSE regarding wood pellet storage.