Bulletin 30-015.003
Copyright © 2017 Unico Inc.
Page 11
SEQUENCE OF OPERATION
The sequence of operation depends on the options
installed and type of control thermostat used. Most
thermostats have a fan AUTO-ON switch. When the
fan switch is set to ON, the “G” circuit is closed and
the blower relay is energized. The indoor blower starts
after about a 45 second delay. The following
paragraphs describe the sequence of operation when
the fan is set to AUTO. If the fan switch is set to ON,
the sequence is the same except the “G” circuit is
always closed and the indoor fan is always operating.
Cooling Cycle (A/C or Heat Pump).
When the
thermostat calls for cooling, the “Y” and the “G”
circuits are closed, and a 24 V signal is sent to the
compressor contactor in the outdoor unit and fan relay
in the indoor unit. After about 45 seconds, the indoor
blower starts. At the same time, the compressor and
outdoor fan also start. Depending on the control
circuitry in the outdoor unit, there may be a time delay
before the outdoor unit starts. If the system was just
turned off, the time delay could be as much as five
minutes. The cooling system is now operating.
For heat pump thermostats setting the switch to
‘cooling’ immediately closes the “O” circuit, which is
used to energize the reversing valve solenoid if
required by the heat pump. Otherwise, the “B” circuit,
which closes when switched in heating, is used to
energize the reversing valve solenoid. (Refer to the
heat pump manufacturer’s instructions to see which
mode the solenoid needs to be energized – whether in
heating or cooling.)
When the thermostat is satisfied, the 24 V signals open
and the outdoor unit stops. The indoor blower
continues to operate for about 40 seconds, then stops.
The system is now off.
Heating Cycle (Heat Pump).
Setting the thermostat
to HEATING will automatically switch the reversing
valve solenoid. This setting closes the “B” circuit
which sends a 24V signal to energize the solenoid if
required by the heat pump. Otherwise the “B” circuit
is not used and the solenoid is not energized during
heating.
When the thermostat calls for heating, the “Y” and “G”
circuits are closed, sending a 24 V signal to the
compressor contactor in the outdoor unit and the fan
relay in the indoor unit. This starts the indoor blower
and the outdoor compressor and blower. There is a
time delay of about 45 seconds for the indoor unit. The
heating system is now operating in stage one.
If the first stage does not satisfy the thermostat, the
second stage thermostat calls for more heat. This
closes the “W2” contacts and energizes the sequencer
for electric heat. When the second stage thermostat is
satisfied, the “W2” circuit is broken and the sequencer
is de-energized. The electric heating system is now off.
When the first stage thermostat is satisfied, the 24 V
signals open and the outdoor unit stops. The indoor
blower continues to operate for about 45 seconds, then
stops. The system is now off.
Heating Cycle (Electric Heat).
When the thermostat
calls for heating, the “W2” and “G” circuits are closed.
The W2 circuit completes the 24V signal to the
sequencer in the electric duct heater, which cycles on
the electric heating elements. The heating system is
now operating.
When the thermostat is satisfied, the 24 V signals open
and the indoor blower stops after about 40 seconds. At
the same time the sequencer cuts the power to the
electric elements. The system is now off.
Heating Cycle (Hydronic Heat).
When the
thermostat calls for heating, the “W1” circuit brings on
the fan at low speed and “W2” at full speed. The HotW
relay is used to energize either a boiler, valve, or pump.
For ‘combo’ systems, where potable water is
circulated through the hot water heating coil, it is
necessary to ensure that the water is never stagnant in
the coil. The switch will activate a timer so that the
HotW relay will energize a pump to circulate water
once per day for 5 minutes regardless.
If an aquastat is utilized between R and aqua, the fan
relay circuit will remain open until the aquastat is
closed. R-Aqua allows the HotW relay to function
without an optional aquastat. If an aquastat is used, this
jumper must be removed.
When the thermostat is satisfied, the 24 V signal to the
heat relay opens the HotW circuit which stops the
pump or closes a valve. The fan circuit opens and de-
energizes the fan relay. After about 45-seconds the
blower stops.
CHECKING AIRFLOW
CAUTION.
DO
NOT
OPERATE
BLOWER WITH FREE DISCHARGE OR
LOW STATIC PRESSURES (BELOW 1
INCH WC (250 Pa)) TO PREVENT
MOTOR FROM OVERLOADING.
After the system is installed and before charging
system, check for proper airflow. Record the plenum
static pressure. With this information, the amount of
airflow can be determined by counting the flashes on
the SCB.
As a recommended further check on airflow, use the
Turbo-Meter (Davis Instruments Catalog No.
DS105I07) to measure the CFM from each outlet. This
hand held vane type velocity meter that fits directly
