In recent years furnace manufacturers have been developing new, higher efficiency Category I furnaces that do not have draught hoods or draught diverters but instead use fan-assisted combustion systems. These furnaces still rely on the natural draught created by the buoyancy of the hot combustion products to vent the flue gases. Presents the criteria used to generate venting guidelines and tables for Category I appliances with fan-assisted combustion systems. These venting guidelines and sizing tables are based on limiting the condensing and pressurisation of a vent system attached to a Category I fan-assisted appliance. The guidelines cover either the venting of a single Category I fan-assisted appliance or the common venting of any combination of Category I fan-assisted and draught-hood-equipped appliances on a single floor.

KEYWORDS: developing, vents, volume, boilers, gas fired boilers, fans, combustion, natural draughts, flue gases, regulations, sizing, condensation, pressurisation, exhaust hoods.