to conditioning element 18 these can be seen along the left hand side of controller 10 a number of these switches close when a burner unit element enters a particular state this relationship is symbolized in fig1 by a dotted line drawn from the element whose state controls to the switch which controlled by the state of the element thus recycle interlock switch 40 closes when recirculating fan 60 reaches a certain speed the lockout interlock switch 54 closes when the combustion fan 61 reaches a certain speed the low and high fire switches 52 and 58 close when the damper and fuel throttle 66 reach their low and high fire positions respectively the following table defines the purpose of each and the reference numbers for the switch which controls the signal and the screw terminal which receives the signal ______________________________________ switch terminalsignal name purpose ref no ref no ______________________________________pilot valve hold selects pilot valve hold 44 45 modemanual open selects manual opening 46 47 for main valvepre ignition senses status of pre 48 49interlocks ignition interlock switch stringstart switch allows manual start 50 51 of operating sequencehigh fire switch closes when damper and 52 53 fuel throttle enter high fire positionlow fire switch closes when damper and 58 59 fuel throttle enter low fire positionstop switch allows manual stop of 42 43demand operating sequence by opening when closed applies demand.