Service Manual
9
Sequence of operation
Table 1A
Sequence of operation
OPERATION
DISPLAY
1. Upon a call for heat, the control turns on the water heater pump.
WHR: Standby
OUT: 123.8F(129)
2. The control connects 120 VAC to the blower. The blower does not
run at this time.
• If the unit is equipped with a flow switch, it must close before the
control powers up the blower.
• The manual reset high limit must be closed before the control
powers up the blower.
• If the unit is equipped with a solenoid valve and/or a vent
valve, they will be energized along with the blower.
• If the unit is equipped with a gas pressure switch, it must close
at this time.
• If there is an auxiliary device connected to the unit, the unit
will then provide 24 VAC to its enable relay. If the auxiliary
device has a proving switch, it must close before the
sequence continues.
WHR: Standby
OUT: 123.8F(129)
3. The control then starts a 10 second prepurge cycle.
WHR: PREPURGE
OUT: 123.9F(129)
4. Once the prepurge cycle is complete, and the blocked drain
switch is closed, the control starts the 5 second trial for ignition by
sending spark voltage to the spark electrode and opening the gas
valve.
WHR: IGNITION
OUT: 123.9F(129)
5. If the control does not detect flame by the end of the trial for
ignition, the control performs a 10 second postpurge, another
prepurge and tries to light the burner again.
If the burner does not light after 4 trials on Models 150 - 399, the
control locks out for 1 hour and then tries another set of 4 trials.
If the burner does not light after 1 retry on Models 500 - 800 the
control will lock out.
WHR: POSTPURGE, PREPURGE
OUT: 123.9F(129)
6. If the control detects a flame before the trial for ignition ends, it
begins to modulate the burner in order to maintain the set point.
WHR: WHR 20% RATE
OUT: 124.8F(129)
7. Once the call for heat is satisfied, the control will turn off the burner.
The blower will remain on for the 10 second postpurge cycle. The
water heater pump will continue to run for its pump delay time,
then turn off.
WHR: POSTPURGE
OUT: 127.4F(129)
8.
Water heater pump off.
WHR: Standby
OUT: 124.7F(129)
1
Service
(continued)
