127915-YIM-A-0405
56
Unitary Products Group
low return air temperature, (opens at 26 ± 5 °F and
resets at 38 ± 5°F).
2.
A high-pressure switch to protect against exces-
sive discharge pressures due to a blocked con-
denser coil or a condenser motor failure, (opens at
405 ± 7 psig and resets at 265 ± 20 psig).
3.
A low-pressure switch to protect against loss of
refrigerant charge, (opens at 22 ± 5 psig and resets
at 45 ± 5 psig).
The above pressure switches are hard-soldered to the
unit. The refrigeration systems are independently mon-
itored and controlled. On any fault, only the associated
system will be affected by any safety/preventive action.
The other refrigerant system will continue in operation
unless it is affected by the fault as well.
The unit control board monitors the temperature limit
switch of electric heat units and the temperature limit
switch and the gas valve of gas furnace units.
COMPRESSOR PROTECTION
The compressors also have inherent (internal) protec-
tion. If there is an abnormal temperature rise in a com-
pressor, the protector will open to shut down the
compressor. The UCB incorporates features to mini-
mize compressor wear and damage. An anti-short
cycle delay (ASCD) is utilized to prevent operation of a
compressor too soon after its previous run. Addition-
ally, a minimum run time is imposed any time a com-
pressor is energized.
The ASCD is initiated on unit start-up and on any com-
pressor reset or lock-out.
FLASH CODES
The UCB will initiate a flash code associated with
errors within the system. Refer to UNIT CONTROL
BOARD FLASH CODES Table 33.
RESET
Remove the call for cooling, by raising thermostat set-
ting higher than the conditioned space temperature.
This resets any pressure or freezestat flash codes.
REHEAT MODE SEQUENCE OF OPERATION
The reheat control board allows the user to select two
different modes of operation via a jumper connection
on the board. (See Figure 19.) Each mode is described
below. Refer to Figures 19 - 21 when reading this sec-
tion.
“NORMAL” MODE
When the reheat control board (RCB) detects a need
for dehumidification (24VAC) at "HUM" via the field
supplied dehumidistat connected to RHTB-1 and
RHTB-2 and there is not a call for cooling, it energizes
the hot gas relay (HGR), which energizes the 3-way
valve (SOL 3), the condenser coil valve (SOL 2), and
de-energizes the reheat coil bleed valve (SOL 1). The
Y1 signal is passed to the unit control board (UCB),
which engages circuit # 1, resulting in circuit #1 reheat
mode operation.
When the room thermostat calls for first stage cooling,
with or without a call for dehumidification, the RCB
senses a signal through "Y1", de-energizing the HGR,
which de-energizes SOL 3 and SOL 2, and energizes
SOL 1, engaging circuit # 1, resulting in circuit #1 cool-
ing mode operation.
When the room thermostat calls for second stage cool-
ing, the RCB senses a signal through "Y1" & "Y2" and
engages circuit #1 and circuit #2 in the cooling mode.
Indoor blower operation is initiated upon a call for first
stage cooling, second stage cooling or dehumidification.
Anytime there is a call for 2 stages of cooling, the unit
will not operate in the reheat mode, even if there is a
call for dehumidification at "HUM".
The unit will not operate in the reheat mode if there is
any call for heating.
On units with economizers, the unit will not operate in
the reheat mode if there is a call for cooling and the
economizer is operating as first stage of cooling.
All safety devices function as previously described.
"ALTERNATE” MODE
When the RCB detects a need for dehumidification
(24VAC) at "HUM" via the field supplied dehumidistat
connected to RHTB-1 and RHTB-2, and there is not a
call for cooling, it energizes the HGR, which energizes
the SOL 3, SOL 2, and de-energizes SOL 1. The unit
then operates with circuit #1 in reheat mode and circuit
#2 in cooling mode.