5
06-237172-001 Rev AA
June 2011
SEQUENCE OF OPERATION / FLAME
RECOVERY / SAFETY LOCKOUT
START UP - HEAT MODE
When a call for heat is received from the thermostat supplying
24 volts to THS the following sequence occurs in the order
listed:
1.
The control resets and then performs a self check routine,
flashing the diagnostic LED once in the first two seconds.
2.
The control begins a pre-purge delay.
3.
Following the pre-purge period, the control energizes the
pilot gas valve. The main gas valve remains de-energized
until the pilot flame is detected.
4.
The control enables the high voltage spark output for the
trial for pilot ignition period.
5.
When the control detects the presence of pilot flame dur-
ing the trial for ignition, the control terminates the sparking
process and energizes the main gas valve.
6.
The thermostat and pilot burner flame are constantly mon-
itored to assure the system continues to operate properly.
7.
When the thermostat is satisfied and the demand for heat
ends, both the pilot and main gas valves are de-energized
immediately.
FAILURE TO LIGHT - LOCKOUT
SINGLE TRIAL MODEL
Should the pilot burner fail to light, or if the pilot flame is not
detected during the trial for ignition period, the control performs
the following actions:
1.
The control enters ignition lockout.
2.
Both the pilot and main gas valves are de-energized
immediately.
3.
The LED indicates the fault code for ignition lockout.
MULTI TRIAL MODEL
Should the pilot burner fail to light, or if the pilot flame is not
detected during the first trial for ignition period, the control per-
forms the following actions:
1.
The pilot and main gas valves are de-energized.
2.
The control then goes through an interpurge delay before
another ignition attempt.
3.
The control attempts two additional ignition trials before
going into lockout and the pilot and main gas valve relays
are de-energized immediately.
4.
The LED indicates the fault code for ignition lockout.
LOCKOUT RESET
Recovery from lockout requires a manual reset by either recy-
cling the thermostat or removing 24 volts for a period of 5 sec-
onds. On models with automatic reset, if the thermostat is still
calling for heat after 5 minutes the control will automatically
reset and attempt to ignite the burner.
FLAME FAILURE RESPONSE
RECYCLE MODE (STANDARD)
With "recycle after loss of flame", upon loss of pilot flame, both
the pilot and main gas valves are de-energized immediately.
After the flame recycle delay, the control attempts to relight the
pilot flame. Multi-try models allow three tries for ignition includ-
ing inter-purges. If the pilot burner relights, normal operation
resumes. If the pilot burner does not relight, the control will go
into lockout as described in “Failure to Light - Lockout” on
page 5.
RE-IGNITION MODE (OPTIONAL)
If the established pilot flame signal is lost while the burner is
operating, the control will respond within 0.8 seconds by de-
energizing the main gas valve. The control then energizes the
HV spark for the programmed TFI period in an attempt to relight
the pilot burner. If the pilot burner does not light within the TFI,
the pilot gas valve is de-energized immediately and on multi-try
models a new TFI sequence begins.
If the pilot burner does not relight, the control will lockout as pre-
viously described in “Failure to Light - Lockout” on page 5.
Multi-try models will make 2 more attempts to light the burner. If
the pilot flame is re-established, normal operation resumes.
