Describes work carried out by British Gas to establish the magnitude of heat losses from gas-fired boilers arising from natural ventilation through the boiler during its shutdown period. Discusses the general principles of ventilation heat losses to the flue and via the draught diverter and presents data for heat loss decay rates for a range of boilers. Calculates ventilation heat losses for typical boilers. Concludes that for a typical open-flued domestic heating boiler, ventilation heat losses are approx 6% or less of total heat input. Example calculations are given to enable estimation of annual heat loss for a given plant where usage pattern, load factor and other relevant data are known. Considers the effect of flue dampers on these heat losses.