Honeywell 7800 Series Manual page 22

RM7888A
OPERATION
WARNING
The user of the RM7888A is responsible for pro-
viding an effective purge that meets the approval
of the authority having jurisdiction for a particular
installation. The RM7888A does not directly moni-
tor the presence or absence of airflow and cannot
guarantee that the air valve or fan is working. Fail-
ure to provide an effective purge can result in a fire
or explosion that can cause damage, serious injury
or death.
Standby-Hold
The RM7888A has a request to sequence out of standby
(special function terminal 1 is energized), but a condition
such as a de-energized Low Fire input or Limits Complete
input signal keeps the sequence in Standby.
When Special Function 1 is energized, the sequence will
hold for eight minutes while waiting for the Low fire signal
to become energized, with an alarm after four minutes of
hold. The sequence will hold for 120 seconds while waiting
for the Limits Complete signal to become energized. If either
time limit expires, a lockout will occur.
When Special Function 1 is de-energized, one of four
Standby states are possible. The Standby state in effect de-
pends on the status of the special function inputs. The four
Standby states are:
1. Off: the relay module is commanding the Low Fire
motor position and the Air Valve off.
2. Purge: The relay module is commanding the High
Fire motor position and the Air Valve on.
3. Control Cool: The relay module is commanding the
modulated motor position and the Air Valve on.
4. Low Combustion Air Only: The relay module is com-
manding the Low Fire motor position and the Air Valve on.
If Special Function inputs 2, 3 and 4 are all de-ener-
gized, the Standby-Off state is in effect: the Air Valve is
commanded off and the Low Fire state is ordered.
TABLE 6—SPECIAL FUNCTION INPUTS/OUTPUT STATES DURING STANDBY.
Special Function 2
(Terminal 17)
De-energized
De-energized
De-energized
Energized
De-energized
Energized
Energized
Energized
65-0175
Special Function 3 Special Function 4
(Terminal 19) (Terminal 20)
De-energized
De-energized
Energized
Energized
Energized
De-energized
De-energized
Energized
During Standby/Standby-Purge states, when special func-
tion 1 is de-energized, special function terminals 2, 3 and 4
provide a signal that requests that the RM7888A command
a particular motor position and Air Valve status. If any one
or more of the special function inputs (terminals 17, 19, or
20) is energized, a Standby-Purge state is in effect with the
Air Valve commanded on and either Low Fire, High Fire,
or the modulated position commanded (see Table 6).
Table 6 defines the combinations of special function
inputs and corresponding output state (Air Valve and Mo-
tor Output) during Standby.
PILOT CONFIGURATION IGNITION TRIALS
(PV RETURN AND MV LO FIRE SEQUENCES)
Pilot Ignition State
Pilot Ignition is entered only from the Standby state with
the Limits Complete and Low Fire input signals energized,
Special Function input 1 (terminal 6) energized and the Air
Valve either on or off, depending on which Standby state
preceded the Pilot Ignition state.
At the start of the Pilot Ignition state, the ignition and
pilot outputs are commanded on, Low Fire is commanded,
and the Air Valve is commanded off. The RM7888A will
lockout if the Low Fire input becomes de-energized after
the first two seconds of the Pilot Ignition state. A one- to
two-second transition state precedes the Pilot Ignition state
so the Low Fire switch has at least three seconds to stabi-
lize. This reduces the possibility of a contact bounce caus-
ing a lockout. The RM7888A will lockout if the Limits
Complete input becomes de-energized.
For the first ten seconds, the Pilot Ignition state attempts
to establish a pilot flame. If, at the end of ten seconds, a pilot
flame has not been established, a lockout will occur. Other-
wise, with a flame established, the igniter is commanded
off, the Flame Proven output is commanded on and the pilot
flame continues. This pilot state, with only the pilot output
on and a pilot flame proven, will continue for at least five
seconds, but may continue indefinitely depending on the
status of the special function input terminals.
If a flameout occurs any time after this point in the burner
sequence, a lockout will occur.
Air Valve
Output
De-energized De-energized
Energized Energized
Energized Energized
Energized Energized
De-energized Energized
De-energized Energized
Energized Energized
De-energized Energized
22
Motor Position
Commanded
Low Fire
High Fire
Modulate
Low Fire
Modulate
Low Fire
Low Fire
Low Fire