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Modu-Fire
®
Gas-Fired Boiler Maintenance
27
5.5
SEQUENCE OF OPERATION
5.5.1 Standard Modulating
1. When the On/Off (Main power) switch is turned
on, the switch illuminates. Power is provided to
the flame safeguard control, the temperature
control, and through normally closed contacts of
relays 2 and 3 to the damper close terminal.
Power is also applied through the “start logic
string” a series of normally closed switches, in-
cluding water flow, gas pressure, high stack
temp, high limit temperature, and operating
temperature to the flame safeguard programmer.
2. The water flow limit switch is closed when
there is water flow through the boiler.
Note: The closing of this switch does not prove
that flow is adequate. Refer to Technical Data
Sheet # TD110 for proper flow requirements.
3. When adequate gas pressure is available, the
low gas pressure limit switch is closed. Manual
reset is required following conditions resulting
in low gas pressure.
4. When the temperature sensed by the high limit
temperature control is below the set limit, the
switch is closed. Manual reset is required fol-
lowing conditions exceeding high limit tempera-
ture.
5. When heat is required as indicated by the outlet
water temperature, power is applied to Terminal
6 of the programmer, which initiates the burner
ignition and operation sequence.
6. The programmer first energizes Terminal 4
which supplies power to a normally open con-
tact of relay 2, the air damper position limit
switch, and the air flow switch. If the damper
motor is on the low fire side of the damper posi-
tion limit switch, relay 3 is energized through
normally closed contact of relay 2. This supplies
power to the open terminal of the air damper
through the normally open contact of relay 3.
The air damper opens to its start position and
the damper position limit switch applies power
to terminal 6 on the damper motor. This ener-
gizes relay 2 and stops the air damper motor.
The motor contactor is energized through relay
2. The air flow switch initially shows low air
flow with the “LOW AIR” indicator. This indi-
cator will remain on until sufficient air flow is
sensed. If the damper motor is not open enough
for purge, relay 2 is not energized, the combus-
tion air blower will not start and the combustion
sequence does not go any farther.
7. Once sufficient airflow is indicated by the air
flow switch, Terminal 7 of the combustion con-
trol is energized. This starts the Prepurge pe-
riod. The combustion chamber is Prepurged for
30 seconds.
8. A 10-second “trial for ignition” period is initi-
ated with both Terminal 8 and 10 being ener-
gized. Terminal 10 of the combustion control
powers the ignition transformer. The trans-
former output creates a spark at the igniter.
Terminal 8 powers the pilot gas valve.
9. When a pilot flame is detected by the UV scan-
ner a signal is sent to the combustion control.
When this signal is detected, Terminal 10 of the
combustion control is de-energized terminating
the spark.
10. After the 10 second “trial for ignition” period, if
pilot flame has been established, Terminal 9 of
the Combustion Control is energized. This
opens the main gas valve and energizes the air
gas ratio control. The 10 second Main trial for
ignition” period begins. If pilot flame has not
been established, the unit will lock out on pilot
flame failure.
11. After the 10 second “main trial for ignition pe-
riod” if flame has been established, Terminal 8
of the combustion control is de-energized, clos-
ing the pilot valve. Terminal 21 is energized,
energizing Relay-1. If main flame has not been
established, the unit will lock out on main flame
failure.
12. Relay 1 releases the temperature control to
modulate.
13. The unit modulates between the low fire setting
and the high fire setting to maintain the desired
outlet water temp set point.