Specifications

035-17377-000 REV A (0201)
Unitary Products Group 49
FLAME SENSE
Flame sensing occurs at all times. If “W1” is not present and
a flame is sensed for 2 seconds, the draft motor is energized
and the main valve is kept off. The ICB halts any operation
until a flame is not detected. Once the flame detection is lost,
the ICB performs a post-purge. Normal operation is allowed
concurrently with the purge (i.e. this purge can be considered
the purge associated with a call for “W1”).
If “W1” is present, a flame is sensed, but the main valve is not
energized, the draft motor is energized until the flame detec-
tion is lost. Normal operation is now allowed.
The flame detection circuitry continually tests itself. If the ICB
finds the flame detection circuitry to be faulty, the ICB will not
permit an ignition sequence. Also, the draft motor is ener-
gized. If this failure should occur during an ignition cycle the
failure is counted as a recycle.
GAS VALVE
The UCB and ICB continuously monitor the GV.
If the ICB senses voltage at the GV when not requested, the
ICB will energize the draft motor. The ICB will not operate the
furnace until voltage is no longer sensed at the GV. The draft
motor is stopped when voltage is not sensed at the GV.
Any time the UCB senses voltage at the GV without a call for
heat for a continuous five-minute period, the UCB will lock-on
the indoor blower and a flash code is initiated (Table 60).
When voltage is no longer sensed at the GV, the UCB will de-
energize the indoor blower following the elapse of the fan off
delay for heating.
If voltage has been sensed at the GV for at least 15 seconds
during the fan on delay for heating and GV voltage or “W1” is
lost, the indoor blower is forced on for the length of the fan off
delay for heating.
During a call for heat, if the UCB does not sense voltage at
the GV for a continuous five-minute period the UCB will ini-
tiate a flash code (Table 60). The indoor blower motor will not
be locked-on while there is no GV voltage.
However, if during a normal ignition sequence, the NPC has
been tested to be open, IDM energized, NPC closes and then
the first stage main valve (MV1) is detected as being on, the
ignition sequence stops and the IDM is de-energized. (Since
the NPC is wired serially with the main valve relays, de-ener-
gizing the IDM will open the NPC). This failure is counted as
a recycle and will result in an eventual recycle lockout. If the
main valve continues to be energized after the IDM is de-
energized the IDM is re-energized. The second stage (MV2)
main valve is not monitored for closure after energization.
PRESSURE SWITCH
Once the draft motor has been proven during a normal igni-
tion sequence, if proving is lost for 2 seconds, the GV will be
de-energized, the ignition cycle is aborted, and the ICB
flashes the appropriate code. The draft motor is energized
until again proven or “W1” is lost.
ROLLOUT SWITCH
The rollout switch is wired in series with the draft motor prov-
ing switch. As such, the ICB cannot distinguish the rollout
switch operation from that of the draft motor proving switch.
Consequently, the control will only respond as in the same
manner as outlined above (under Pressure Switch). An
open rollout will inhibit the gas valve from actuating.
INTERNAL MICROPROCESSOR FAILURE
If the ICB detects an internal failure, it will turn cease all out-
puts, ignore inputs, and display the proper flash code for con-
trol replacement. The ICB remains in this condition until
replaced.
LOW AMBIENT COOLING
To determine when to operate in low ambient mode, the UCB
has a pair of terminals connected to a temperature-activated
switch. When the low ambient switch is closed and the ther-
mostat is calling for cooling, the UCB will operate in the low
ambient mode.
Low ambient mode operates the compressors in this manner:
10 minutes on, 5 minutes off. The indoor blower is operated
throughout the cycle. The 5-minute off period is necessary to
defrost the indoor coil.
Low ambient mode always begins with compressor opera-
tion. Compressor minimum run time may extend the minutes
of compressor operation. The defrost cycle will begin immedi-
ately following the elapse of the minimum run time.
When operating in low ambient mode, the UCB will not lock-
out the compressors due to a freezestat trip. However, a
freezestat trip will de-energize the associated compressor. If
the call for cooling is still present at the end of the ASCD and
the freezestat has closed, the unit will resume operation.