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SERVICING
55
remove the 24Vac from HR2. The contacts on HR2 will open
between 30 to 70 seconds and heater elements #3 and #4 will
be turned off and the blower motor will change to low speed.
On most digital/electronic thermostats, “W2” will re-
main energized until the first stage demand “W1” is
satisfied and then the “W1” and “W2” demands will be
removed.
2.3 When the “W1” heat demand is satisfied, the room
thermostat will remove the 24Vac from “E/W1” and the
VSTB removes the 24Vac from HR1. The contacts on
HR1 will open between 30 to 70 seconds and turn off the
heater element(s) and the blower motor ramps down to
a complete stop.
S-41A MBE/AEPF WITH SINGLE STAGE
GSZ, SSZ, & ASZ HEAT PUMPS
When used with a single stage GSZ, SSZ or ASZ 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 re-
moved.
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 thermo-
stat 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 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/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 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/AEPF 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/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 removed.
4.4 As the temperature indoors increase, it will reach a point
where the second stage heat demand, “W2”, is satis-
fied. 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.