SEQUENCE
OF
OPERATIONS
Normal operation (atmo-
spheric or fan assisted)
During the waiting or purge
time, Tw or Tp, the circui-
try verifies that there is no
parasitic flame signal pre-
sent. If the unit is used in a
fan assisted application,
the air pressure switch is
verified to be in the N.C.
(normally closed) or “no
flow” position. Upon powe-
ring the fan, the unit will
not begin the operating
sequence until the N.O.
(normally open) or “flow”
position is activated on the
air pressure switch. After
the scheduled waiting time
Tw, or purge time Tp, the
built-in ignitor and the gas
valve are energized. This
commences the safety
time, Ts. The ignition spark
will ignite the gas and the
flame will be sensed by the
HV electrode (single probe
model) or the detection electrode (dual probe model). Upon sen-
sing the flame, the HV spark will be suppressed and the gas valve
will remain energized. On units with a flame relay, the flame detec-
ting connection will be powered when the flame is established. When
the thermostat is satisfied, the valve and fan are de-energized and
the control returns to the standby mode.
Behaviour under abnormal conditions
- if the air proving switch is in the N.O. (normally open) or “flow”
position at startup, upon a request for heat, the unit will remain in
a waiting mode with the fan de-energized
- if air flow is not proven by the airflow switch after energizing
the fan, the unit control will remain in the waiting mode with
the fan energized until the air flow is sensed
- upon flame failure during normal operation, the unit will maintain
the power to the fan and start again the ignition sequence with
a waiting time (or purge time) followed by a safety time
- if flame is not established and sensed during the beginning of Ts,
the spark will be suppressed shortly before the programmed end
of the safety time to permit the sensing of extremely weak
flames. This time is referred to as Tc (check time). If no flame is
sensed during any part of the safety time, the unit will lockout
Resetting the unit
To reset the unit the external reset switch is depressed. If a first
reset is not successful, wait at least 10 seconds before the next
attempt. Before starting, check if control is locked-out. If locked-
out, depress the reset button; After a manual reset an extended
pre-purge waiting time will occur.
To Next Page
Loading...