MODU-FIRE® Forced Draft Gas-Fired Boiler
36
5.5 S
EQUENCE OF
O
PERATION
1. In order for the boiler to start, the following limits must be satisfied:
a) Water flow switch
b) High & low gas pressure switches
c) High temperature limit
d) High back pressure switch
e) High stack temperature switch
2. When the Boiler On/Off switch is turned on, power is provided through a circuit breaker to the boiler control
and the combustion blower.
3. If the high gas, low gas or low water level control is open, the boiler control locks out and displays an error.
4. When the water temperature is below the boiler control setpoint minus the hysteresis (On Differential), a heat
request is generated.
5. Provided all limits are made, the boiler will attempt to start.
6. The controller checks that the air pressure switch is open indicating no airflow. The blower is driven towards
the prestart fan speed. When the air pressure switch closes, the 25 second pre-purge time is started. After
the pre-purge, the blower is driven to the ignition speed.
7. A trial for ignition begins. The sequence of events is illustrated graphically below.
8. After ignition, the fan may be driven to low fire before the boiler is released to modulation.
9. The control modulates the firing rate between low and high fire to maintain the desired outlet water
temperature.
10. The burner will continue firing until the outlet water temperature reaches set point plus hysteresis (Off
Differential). At this temperature the fuel supply is shut off and the combustion air fan continues to run for a
30 second post-purge.
11. When the water temperature is reduced by the load on the system, a heat request is generated. The
operating sequence will recycle to step 4.
0
1
2
3
4
5
6
7
Spark
Gas Valve
Flame Detection
Seconds
Fan
*
Flame must be detected at T = 6 seconds
*
Ignition Sequence Timing
0
1
2
3
4
5
6
7
Spark
Gas Valve
Flame Detection
Seconds
Fan
*
Flame must be detected at T = 6 seconds
*
Ignition Sequence Timing
NOTICE! Once the boiler begins the ignition sequence, the firing sequence will continue until main
flame is reached regardless of heat request. The sequence can be interrupted by turning the power
switch off.
