27
®
Modulation
During stage 16 (modulation), the unit will position the fuel and air motors as programmed for the
profile selected.
In auto modulation mode, the system will modulate as necessary to either track the input or maintain
the pressure/temperature of the boiler at the entered setpoint (see Option Parameter 5.3 - 5.8). If
boiler sequencing is enabled and active, the unit will start, stop and modulate according to instruc-
tions issued from the unit controlling the lead boiler.
In manual modulation mode, the system will either modulate in response to the keyboard up/down
keys (if the NX550 Display is mounted) or the external up/down signals.
Safety Shutdown
A safety shutdown will occur under the following conditions:
• If any step of the gas proving sequence fails.
• In stages 1-9 (inclusive) if a flame is detected
• In stages 11-16 (inclusive) if a flame is not detected
• In stage 4 if the air pressure switch goes high (air pressure present)
• In stages 7-18 (inclusive) if the air pressure switch goes low (air pressure not present)
• In stages 5, 7 and 8, stages 10-16 (inclusive) and stage 18 if a motor is not in the correct position
• In any stage, if an internal or external fault not previously mentioned occurs which may affect the
safe operation of the burner (refer to Fault and Fault Finding).
PS
NX PRESSURE SENSOR
#1
#2
TO BURNER
