is also shown as directly powering control system 29 further it may be convenient for line voltage to be supplied directly to the various elements of the burner system such as blower 27 ignition 22 and fuel valve 15 with low voltage control signals supplied on the respective paths 30 23 and 17 which switch the electrical power to these controlled elements at the appropriate times to cause them to function thus for example a room thermostat might switch only low voltage which is then used to actuate a relay whose contacts function as demand switch 31 as mentioned above it is important for safe and efficient operation that the combustion chamber 11 receive the design rate of air flow to assure operation of blower 27 the flow of air through duct 24 is sensed in one variation by a switch 25 shown as having a small sail or paddle 19 whose broad surface is perpendicular to air flow in duct 24 and against which this air impinges as mentioned above some airflow switch designs operate on pressure differential generated by the moving air stream the design of switch 25 and sail 19 is such that when airflow velocity reaches a predetermined level an electrical connection is made within switch 25 by sensing the conductivity between paths 26a and 26b control system 29 can determine when the flow of air within duct 24 reaches this predetermined level briefly the normal.