SERVICING
40
On heat pump units, when the room thermostat is set to the
cooling mode, 24Vac is supplied to terminal “O” of the VSTB
inside the MBE unit. The VSTB will supply 24Vac to “O” at
the heat pump to energize the reversing valve. As long as the
thermostat is set for cooling, the reversing valve will be in the
energized position for cooling.
3.1
On a demand for cooling, the room thermostat energizes
“G” and “Y” and 24Vac is supplied to terminals “G” and
“Y/Y2” of the MBE unit. The VSTB will turn on the blower
motor and the motor will ramp up to the speed pro-
grammed in the motor based on the settings of dip switch
5 and 6. The VSTB will supply 24Vac to “Y” at the heat
pump.
3.2
The heat pump is turned on in the cooling mode.
3.3
When the cooling demand is satisfied, the room thermo-
stat removes the 24Vac from “G” and “Y/Y2” of the MBE
and the VSTB removes the 24Vac from “Y” at the heat
pump. The heat pump is turned off and the blower motor
will ramp down to a complete stop based on the time and
rate programmed in the motor.
4.0 Heating Operation
On heat pump units, when the room thermostat is set to the
heating mode, the reversing valve is not energized. As long
as the thermostat is set for heating, the reversing valve will
be in the de-energized position for heating except during a
defrost cycle. Some installations may use one or more
outdoor thermostats to restrict the amount of electric heat
that is available above a preset ambient temperature. Use
of optional controls such as these can change the operation
of the electric heaters during the heating mode. This
sequence of operation does not cover those applications.
4.1
On a demand for first stage heat with heat pump units,
the room thermostat energizes “Y” and “G” and 24Vac is
supplied to “G” and “Y/Y2” of the MBE. The VSTB will
turn on the blower motor and the motor will ramp up to the
speed programmed in the motor based on the settings of
dip switch 1 and 2. The VSTB will supply 24Vac to “Y”
at the heat pump and the heat pump is turned on in the
heating mode.
4.2
If the first stage heat demand cannot be satisfied by the
heat pump, the temperature indoors will continue to
drop. The room thermostat will then energize terminal
“W2” for second stage heat and 24Vac will be supplied
to “E/W1” of the MBE. The VSTB will supply 24Vac to
heat sequencer, HR1, on the electric heater assembly.
4.3
HR1 contacts M1 and M2 will close within 10 to 20
seconds and turn on heater element #1. At the same
time, if the heater assembly contains a second heater
element, HR1 will contain a second set of contacts, M3
and M4, which will close to turn on heater element #2.
Note:
If more than two heater elements are on the heater
assembly, it will contain a second heat sequencer, HR2,
which will control the 3
rd
and 4
th
heater elements if available.
For the 3
rd
and 4
th
heater elements to operate on a third
stage heat demand, the PJ4 jumper on the VSTB inside
the MBE and AEP* must be cut.
If the second stage heat
demand, “W2”, cannot be satisfied by the heat pump, the
temperature indoors will continue to drop. The room thermo-
stat will then energize “W3” and 24Vac will be supplied to “W/
W2” of the MBE. The VSTB will supply 24Vac to HR2 on the
electric heater assembly. When the “W3” demand is
satisfied, the room thermostat will remove the 24Vac from
“W/W2” of the MBE and AEP*. The contacts on HR2 will
open between 30 to 70 seconds and heater elements #3 and
#4 will be turned off.
On most digital/electronic thermo-
stats, “W3” will remain energized until the first stage
demand “Y” is satisfied and then the “G”, “Y”, “W2”
and “W3” demands will be removed.
4.4
As the temperature indoors increase, it will reach a point
where the second stage heat demand, “W2”, is satisfied.
When this happens, the room thermostat will remove the
24Vac from “E/W1” of the MBE. The contacts on HR1 will
open between 30 to 70 seconds and turn off both heater
element(s). The heat pump remains on along with the
blower motor because the “Y” demand for first stage heat
will still be present.
4.5
When the first stage heat demand “Y” is satisfied, the
room thermostat will remove the 24Vac from “G” and “Y/
Y2” of the MBE and AEP*. The VSTB removes the 24Vac
from “Y” at the heat pump and the heat pump is turned off.
The blower motor will ramp down to a complete stop
based on the time and rate programmed in the motor
control.
5.0 Defrost Operation
On heat pump units, when the room thermostat is set to the
heating mode, the reversing valve is not energized. As long
as the thermostat is set for heating, the reversing valve will
be in the de-energized position for heating except during a
defrost cycle.
5.1
The heat pump will be on and operating in the heating
mode as described the Heating Operation in section 4.
5.2
The defrost control in the heat pump unit checks to see
if a defrost is needed every 30, 60 or 90 minutes of heat
pump operation depending on the selectable setting by
monitoring the state of the defrost thermostat attached to
the outdoor coil.
5.3
If the temperature of the outdoor coil is low enough to
cause the defrost thermostat to be closed when the
defrost board checks it, the board will initiate a defrost
cycle.
