Honeywell A4021 Series Product Handbook page 7

Line voltage
Heat demand
Leak test
Valve V1
POST
Valve V2
CONFIG
Vent valve
(normally open)
GPS
Leak test
Valve V1
PRE
Valve V2
CONFIG
Vent valve
(normally open)
GPS
Lock--out
(Ignition controller)
Reset ign. control.
power--up start
heat--demand
Fig. 6. System sequence of A4021A in post and pre--configuration.
end start
heat--demand heat--demand
lock--out reset burner controller
start whithout valve proving !