Apparatus is provided for the flow control of the flue gas to combustion air ratio in a reversible regenerative heating system of the type comprising a pair of regenerators, a first line 7 for supplying air to the regenerators, a second line 8 for removing flue gas from the regenerators, and valves 9,10 respectively for controlling the flow rate of air in the first line 7 and the flow rate of the flue gas in the second line 8. The apparatus comprises a third line 21 adapted to provide a slave flow of fluid proportional to the flow rate of the flue gas, thermistor type anemometers 17 and 18 for sensing the flow rates of combustion air and fluid and a ratio controller 24 for adjusting the flue gas damper valve 10 in accordance with the flow rates so sensed to maintain the ratio of the flue gas to combustion air at a predetermined ratio.