Service Manual
SERVICING
107
between 30 to 70 seconds and turn off the heater element(s)
and the blower motor ramps down to a complete stop.
MBE/AEPF WITH SINGLE STAGE
GSZ, SSZ, ASZ, and VSZ HEAT PUMPS
When used with a single stage GSZ, SSZ, ASZ, or VSZ heat pumps,
dip switch #4 must be set to the ON position on the VSTB inside
the MBE. The “Y” output from the indoor thermostat must be
connected to the yellow wire labeled “Y/Y2” inside the wire
bundle marked “Thermostat” and the yellow wire labeled “Y/Y2”
inside the wire bundle marked “Outdoor Unit” must be connected
to “Y” at the heat pump. The orange jumper wire from terminal
“Y1” to terminal “O” on the VSTB inside the MBE/AEPF must be
removed.
3.0 COOLING OPERATION
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/AEPF 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/AEPF unit. 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 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 thermostat
removes the 24Vac from “G” and “Y/Y2” of the MBE/AEPF 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 se-
quence 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/AEPF. 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/
AEPF. 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 con-
tain 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/AEPF must
be cut. If the second stage heat demand, “W2”, cannot be
satisfied by the heat pump, the temperature indoors will con-
tinue to drop. The room thermostat will then energize “W3” and
24Vac will be supplied to “W/W2” of the MBE/AEPF. 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/AEPF. 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 thermostats, “W3”
will remain energized until the first stage demand “Y” is satisfied
and then the “G”, “Y”, “W2” and “W3” demands will be re-
moved.
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/AEPF. 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/AEPF. 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.
5.4 When a defrost cycle is initiated, the contacts of the HVDR
relay on the defrost board open and turns off the outdoor fan.