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SERVICING
56
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. The contacts of the LVDR relay on the
defrost board closes and supplies 24Vac to “O” and
“W2”. The reversing valve is energized and the contacts
on HR1 close and turns on the electric heater(s). The
unit will continue to run in this mode until the defrost
cycle is completed.
5.5 When the temperature of the outdoor coil rises high
enough to causes the defrost thermostat to open, the
defrost cycle will be terminated. If at the end of the
programmed 10 minute override time the defrost thermo-
stat is still closed, the defrost board will automatically
terminate the defrost cycle.
5.6 When the defrost cycle is terminated, the contacts of the
HVDR relay on the defrost board will close to start the
outdoor fan and the contacts of the LVDR relay will open
and turn off the reversing valve and electric heater(s). The
unit will now be back in a normal heating mode with a
heat pump demand for heating as described in the
Heating Operation in section 4.
SEQUENCE OF OPERATION
This document covers the basic sequence of operation for a
typical application with a mercury bulb thermostat. When a
digital/electronic thermostat is used, the on/off staging of the
outdoor unit and auxiliary heat will vary. Refer to the
installation instructions and wiring diagrams provided with
the MBE for specific wiring connections, dip switch settings
and system configuration.
MBE/AEPF WITH TWO STAGE
ASX & DSX CONDENSERS
1.0 COOLING OPERATION
When used with the ASX & DSX two stage condens-
ers, 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 “Outdoor Unit” must
be connected to “Ylow/Y1” at the condenser. 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 condenser. The orange
jumper wire from terminal “Y1” to terminal “O” on
the VSTB inside the MBE/AEPF must remain con-
nected.
1.1 On a demand for cooling, the room thermostat energizes
“G” and “Y1” and 24Vac is supplied to “G” and “Ylow/Y1”
of the MBE/AEPF unit. The VSTB inside the MBE/AEPF
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 “Ylow/Y1” at the condenser and the compres-
sor and condenser fan starts in low speed operation.
1.2 If first stage cooling cannot satisfy the demand, the room
thermostat will energize “Y2” and supply 24Vac to the
MBE/AEPF unit. The blower motor will change to the
cfm for high speed operation and the VSTB will supply
24Vac to “Y/Y2” at the condenser and the compressor
and condenser 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 “Y/Y2” at the condenser. The blower will drop
to 60% of the programmed cfm and the compressor and
condenser fan will change to low speed. On most
digital/electronic thermostats, “Y2” will remain en-
ergized until the first stage cooling demand “Y1” is
satisfied and then the “G”, “Y1” and “Y2” demands
will be removed.
1.3 When the first stage cooling demand, “Y1”, is satisfied,
the room thermostat removes the 24Vac from “G” and
“Y1”. The MBE/AEPF removes the 24Vac from “Ylow/
Y1’ at the condenser and the compressor and condenser
fan are turned off. The blower motor will ramp down to a
complete stop based on the time and rate programmed
in the motor.
2.0 Heating Operation
2.1 On a demand for heat, the room thermostat energizes
“W1” and 24Vac is supplied to terminal “E/W1” of the
VSTB inside 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 for dip
switch 1 and 2. The VSTB will supply 24Vac to heat
sequencer HR1 on the electric heater assembly.
2.2 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 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.