Reviews procedures in the design and sizing of flues for condensing boilers serving heating systems operating at both conventional water temperatures and low water temperatures. Explains factors governing the design of a chimney with respect to dispersion of flue products, generation of draught and efflux velocity. Considers chimney area and system resistance. Treats flue construction concerning leak tightness, materials for construction, flue layout, suitable materials for the condensate drain and flue termination. Provides a list for further reading. Supplies worked examples for the design of a chimney system in an annexe.