In large green field plants such as refineries and petrochemical plants, a certain area is defined as the boiler island and all equipment in that area is required to be supplied by the boilermaker; also, all the TPs would be at the battery limits requiring considerable amounts of piping and pipe rack. All auxiliary plants mentioned earlier would form a part of the boiler.

Boiler island in a power plant has a different connotation: the total power plant is divided into three components, namely the boiler island, turbine island, and balance of plant (BOP). Besides the main boiler, the chemical dosing and dust-collecting packages are usually parts of the boiler island.