Service Manual
SERVICING
109
MBE/AEPF WITH TWO STAGE ASZ & DSZ HEAT PUMP UNITS
3.0 Cooling Operation
When used with the ASZ & DSZ two stage heat pumps, dip
switch #4 must be set to the OFF position on the VSTB inside
the MBE/AEPF. The “Y1” output from the indoor thermostat
must be connected to the purple wire labeled “Ylow/Y1”
inside the wire bundle marked “Thermostat” and the purple
wire labeled “Ylow/Y1” inside the wire bundle marked “Out-
door Unit” must be connected to “Y” at the heat pump. The “Y2”
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/
Y2” at the heat pump. The orange jumper wire from terminal
“Y1” to terminal “O” on the VSTB inside the MBE/AEPF must
be removed.
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 “Y1” and 24Vac is supplied to “G” and “Ylow/Y1” of the
MBE unit. The VSTB inside the MBE will turn on the blower
motor and the motor will ramp up to 60% of the speed
programmed in the motor based on the settings for dip switch
5 and 6. The VSTB will supply 24Vac to “Y” at the heat pump
and the compressor and outdoor fan starts in low speed
operation.
3.2 If first stage cooling cannot satisfy the demand, the room
thermostat will energize “Y2” and supply 24Vac to “Y/Y2”of
the MBE unit. The blower motor will change to the cfm for high
speed operation and the VSTB will supply 24Vac to “Y2” at the
heat pump. The compressor and outdoor fan will change to
high speed operation. When the “Y2” demand is satisfied, the
thermostat will remove the “Y2” demand and the VSTB will
remove the 24Vac from “Y2” at the heat pump. The blower will
drop to 60% of the programmed cfm and the compressor and
outdoor fan will change to low speed operation. On most
digital/electronic thermostats, “Y2” will remain energized
until the first stage cooling demand “Y1” is satisfied and then
the “G”, “Y1” and “Y2” demands will be removed.
3.3 When the first stage cooling demand, “Y1”, is satisfied, the
room thermostat removes the 24Vac from “G” and “Y1”. The
VSTB removes the 24Vac from “Y’ at the heat pump and the
compressor and outdoor fan are turned off. 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 “G” and “Y1” and 24Vac is sup-
plied to “G” and “Ylo/Y1” of the MBE/AEPF. The VSTB will turn
on the blower motor and the motor will ramp up to 60% of 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. The compressor will start on low stage and outdoor
fan will start on low speed on a “Y1” heating demand but the
blower motor will deliver only 60% of the programmed cfm
for high speed heating operation.
4.2 If a thermostat that provides a “Y2” demand in heating is used
and first stage heating cannot satisfy the demand, the room
thermostat will energize “Y2” and supply 24Vac to “Y/Y2” of
the MBE unit. The blower motor will change to the cfm for high
speed heating operation and the VSTB will supply 24Vac to
“Y/Y2” at the heat pump. The outdoor fan will change to high
speed operation and compressor will shift to high stage. If
the “Y2” demand is present and becomes satisfied, the
thermostat will remove the “Y2” demand and the VSTB will
remove the 24Vac from “Y/Y2” at the heat pump. The blower
will drop to 60% of the programmed cfm and the outdoor fan
will change to low speed. On most digital/electronic thermo-
stats, “Y2” will remain energized until the first stage heating
demand “Y1” is satisfied and then the “G”, “Y1” and “Y2”
demands will be removed.
4.3 If the heat pump operation cannot satisfy the demand, the
room thermostat energizes “W2/W3” and 24Vac is supplied
to terminal “E/W1” of the VSTB inside the MBE/AEPF unit. The
VSTB will supply 24Vac to heat sequencer HR1 on the electric
heater assembly.
4.4 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, M3and M4, which will close
and 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 second stage auxiliary
heat demand, the PJ4 jumper on the VSTB inside the MBE/AEPF
must be cut. If the “W2/W3” demand cannot be satisfied by the
heat pump, the temperature indoors will continue to drop. The
room thermostat will then energize “W3/W4” 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/W4”
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/W4”
will remain energized until the first stage demand “Y1” is
satisfied and then the “G”, “Y1”, “Y2” “W2/W3” and “W3/W4”
demands will be removed.