MACH® Gas-Fired Boiler
Page 42
5.4
S
EQUENCE OF
O
PERATION
1. When the Boiler On/Off switch is turned on, power is provided through a circuit breaker to the boiler control and
the combustion blower.
2. If the high gas, low gas or low water level control is open, the boiler control locks out and displays an error.
3. When the water temperature is below the boiler control setpoint minus the hysteresis (On Differential), a heat
request is generated.
4. Provided all limits are made, the boiler will attempt to start.
5. 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.
6. A trial for ignition begins. The sequence of events is illustrated graphically below.
7. After ignition, the fan may be driven to low fire before the boiler is released to modulation.
8. The control modulates the firing rate between low and high fire to maintain the desired outlet water temperature.
9. 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.
10. 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.