284806-YIM-A-0107
Unitary Products Group
41
OPERATION
SEQUENCE OF OPERATIONS OVERVIEW
For these units, the thermostat makes a circuit
between "R" and "Y1" for the first stage of cooling.
The call is passed to the unit control board (UCB),
which then determines whether the requested opera-
tion is available and, if so, which components to ener-
gize.
For gas heating, the UCB monitors the "W1" call but
does not handle the operation of the gas furnace. An
ignition control board controls the gas heater operation.
For electric heat units, the UCB passes the call to the
electric heater.
In both cases, when the "W1" call is sensed, the indoor
air blower is energized following a specified heating
delay.
If at any time a call for both heating and cooling are
present, the heating operation will be performed. If
operating, the cooling system is halted as with a com-
pletion of a call for cooling. Heating always takes prior-
ity.
COOLING SEQUENCE OF OPERATION
CONTINUOUS BLOWER
By setting the room thermostat fan switch to "ON," the
supply air blower will operate continuously.
INTERMITTENT BLOWER
With the room thermostat fan switch set to "AUTO" and
the system switch set to either the "AUTO" or "HEAT"
settings, the blower is energized whenever a cooling or
heating operation is requested. The blower is ener-
gized after any specified delay associated with the
operation.
When energized, the indoor blower has a minimum run
time of 30 seconds. Additionally, the indoor blower has
a delay of 10 seconds between operations.
VAV BLOWER OPERATION
For units with VFD, the speed of the indoor blower
motor is controlled by duct static pressure. The duct
static set point is the pressure that the drive will main-
tain when operating the unit in VAV mode. If the duct
static pressure reaches or exceeds the high-limit set-
point, then the supply fan motor will be shutdown.
NO OUTDOOR AIR OPTIONS
When the thermostat calls for the first stage of cooling,
the low-voltage control circuit from "R" to "Y1" and "G"
is completed. For first stage cooling, compressor #1,
condenser fan motor #1, and condenser fan motor #2
for 15 and 20T only (if the ambient temperature is
above 60ºF), are energized. After completing the spec-
ified fan on delay for cooling, the UCB will energize the
blower motor.
When the thermostat calls for the second stage of cool-
ing, the low-voltage control circuit from "R" to "Y2" is
completed. Compressor #2, and condenser fan motor
#2, 25T only (if the ambient temperature is above
60ºF), is energized, provided it has not been locked-
out.
If there is an initial call for both stages of cooling, the
UCB will delay energizing compressor #2 by 30 sec-
onds in order to avoid a power in rush.
Once the thermostat has been satisfied, it will de-ener-
gize Y1 and Y2. If the compressors have satisfied their
minimum run times, the compressors and condenser
fans are de-energized. Otherwise, the unit operates
each cooling system until the minimum run times for
the compressors have been completed. Upon the final
compressor de-energizing, the blower is stopped fol-
lowing the elapse of the fan off delay for cooling.
To be available, a compressor must not be locked-out
due to a high or low-pressure switch or freezestat trip
and the anti-short cycle delay (ASCD) must have
elapsed.
ECONOMIZER WITH SINGLE ENTHALPY SENSOR -
When the room thermostat calls for "first-stage" cool-
ing, the low voltage control circuit from "R" to "G" and
"Y1" is completed. The UCB energizes the blower
motor (if the fan switch on the room thermostat is set in
the "AUTO" position) and drives the economizer damp-
ers from fully closed to their minimum position. If the
If the unit is operated with the manual bypass
switch in the LINE (BYPASS) position and
there are VAV boxes present in the duct sys-
tem, then boxes must be driven to the full-open
position using a customer-supplied power
source to prevent over-pressurizing and possi-
ble damage to the ductwork.
