algorithm is provided for adjusting the operating temperature span of an exhaust hood 34 similar to the previous techniques described the control system 72 operates in step 210 the fan speed according to a current temperature span and varies the speed according to the continuously monitored exhaust temperature next in step 212 the control system 72 determines if the exhaust temperature is at least 20 more than the minimum temperature of the current temperature span if not then monitoring continues if however the exhaust temperature is 20 into the temperature span then the control system 72 in step 214 determines if either a the fan has been operating for at least an hour or b it has been an hour since this exemplary algorithm has increased the fan speed if either of the conditions in step 214 are satisfied then the control system 72 analyzes the exhaust temperature in step 216 to determine if it is stable one exemplary test for stability is to determine if the temperature has not varied by u2213 3 u00b0 f over a one minute time period if the exhaust temperature is stable then the control system 72 in step 218 increases the fan speed by 20 or to 100 if already operating above 80 for a predetermined time period such as for example one minute during and after the time period.