Many authorities propose domestic kitchen extract fans as effective measures against surface condensation. This paper describes field study, and computer modelling, work to improve quantitative information for fan specification. A detailed computer model is described and used to determine air flows, humidity levels and condensation quantities with different fans and humidistat or manual control. Conclusions are that: low rate fans are unable to prevent condensation; condensation is difficult to remove once present even with extended running from humidistat control; moisture migration through open doors cannot be prevented because of thermally induced air movement and full transient analysis is essential in assessing mould risk.