After the heater has been finally adjusted as specified in Appendix C to the British Standard Specification of 1974 it shall be exposed for 5 minutes to a steady frontal horizontal draught at a velocity not exceeding 8 metres per second. During this 5 minute period not more than 10 flashes of flame shall appear outside the heater, none of which shall last as long as one second or exceed 50 millimetres in length.