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7
ST7997 OIL BURNER MOUNTED ELECTRIC FAN TIMER
Table 2. ST7997 Operating Sequence.
Mode Action System Response
HEAT Thermostat calls for
heat. (W terminal is
energized.)
a. Thermostat calls for heat (closes oil primary control T-T connections.
b. Ignition system and oil primary control start the furnace. Oil flows as
long as oil primary senses flame.
c. Burner motor is energized, LED turns on, and heat fan on delay
timing begins. When timing is complete the circulating fan is
energized at heat speed and warm air is delivered to the controlled
space.
Thermostat ends call for
heat.(W terminal is
de-energized.)
a. Oil primary control is de-energized, terminating the burner cycle, and
LED turns off.
b. Heat fan-off delay timing begins. When timing is completed, the
circulating fan is de-energized.
c. ST7997 returns to standby mode (oil primary control and circulating
fan are off, unless continuous fan operation is selected at the
thermostat).
Burner fails to light. a. Oil primary control locks out within lockout timing (timing depends on
oil primary control).
b. Burner motor is de-energized and LED turns off (even though the
thermostat is still calling for heat).
c. If heat fan has started, it continues through the selected heat-fan-off
delay period.
Established flame fails. a. Burner motor is de-energized, LED turns off, and oil primary control
goes into recycle mode.
b. If selected heat fan-off delay timing is longer than the recycle delay
timing, the heat fan continues to run through the next trial for
ignition.
COOL Thermostat begins call
for cool. (G and Y
terminals are energized.)
a. Cooling contactor is energized and LED turns on immediately.
b. Circulating fan is energized at cool speed when cool on-delay time is
complete.
Thermostat ends call for
cool. (G and Y terminals
are de-energized.)
a. Cooling contactor is de-energized and LED turns off immediately.
b. Circulating fan turns off when cool fan off-delay time is complete.
FAN Thermostat begins call
for fan. (G terminal is
energized.)
a. Circulating fan is energized immediately at heat speed and LED
turns on immediately.
Thermostat ends call for
fan.(G terminal is
de-energized.)
a. Circulating fan is de-energized and LED turns off immediately.
LIMIT Limit switch string opens. a. LED flashes at 2 Hz rate (1/4 second on/off).
b. Oil primary control shuts off burner.
c. Circulating fan is energized immediately at heat speed.
Limit switch string closes
(with existing call
for heat).
a. Oil primary control is energized, initiating burner lightoff and LED
changes to continuously on.
b. Circulating fan continues to operate at heat speed.
Limit switch string closes
(without existing call
for heat).
a. LED turns off.
b. Circulating fan turns off when heat fan-off delay time is complete.
c. Normal operation resumes; device is in standby awaiting next
thermostat command.