Service Manual

SERVICING
106
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 seeif 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 contactson HR1 close and turns on the
electric heater(s). The unit will continue to run in this mode
until the defrost cycle is completed.
a.For models with defrost control PCBDM133 or PCBDM160, a
30 second compressor delay at defrost initiation/termina-
tion is optional. As shipped from the factory, the control is
set for the delay (“DLY”), which will turn the compressor off
for 30 seconds while the reversing valve shifts to/from the
cooling mode position. To bypass the delay, which typically
reduces sound levels during defrost mode, change the pin
settings from “DLY” to “NORM”.
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 thermostat 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 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. See section 5.4a.
MBE/AEPF WITH GSX, SSX, ASX, DSX, VSX
MBE ELECTRONIC BLOWER TIME DELAY RELAY
AEPF AIR HANDLER
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
auxiliary heat will vary. Refer to the installation instructions and
wiring diagrams provided with the MBE/AEPF for specific wiring
connections, dip switch settings and system configuration.
MBE/AEPF WITH SINGLE STAGE GSX, ASX, SSX, and VSX CONDENS-
ERS
When used with a single stage GSX, SSX, ASX, and VSX condens-
ers, dip switch #4 must be set to the on position on the VSTB inside
the MBE/AEPF. 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 condenser. The orange jumper wire from terminal
“Y1” to terminal“O” on the VSTB inside the MBE/AEPF must
remain connected.
1.0 Cooling Operation
1.1 On a demand for cooling, the room thermostat energizes “G”
and “Y” and 24Vac is supplied to “G” and “Y/Y2” of the MBE/AEPF
unit. The VSTB inside the MBE/AEPF will turnon the blower motor
and the motor will ramp up to 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 condenser and the compressor and condenser
are turned on.
1.2 When the cooling demand is satisfied, the room thermostat
removes the 24Vac from “G” and “Y”. The MBE/AEPF removes
the 24Vac from “Y’ 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 pro-
grammed 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 con-
tain 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. For the
3
rd
and 4
th
heater elements to operate on a second stage heat
demand, the PJ4 jumper on the VSTB inside the MBE/AEPF must
be cut. With the PJ4 jumper cut, the VSTB will run the blower
motor on low speed on a “W1” only demand. If the first stage
heat demand, “W1” cannot be satisfied by the heat pump, the
temperature indoors will continue to drop. The room thermostat
will then energize “W2” and 24Vac will be supplied to HR2 on the
heater assembly and the blower motor will change to high speed.
When the “W2” demand is satisfied, the room thermostat will
remove the 24Vac from “W2” and the VSTB will 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 remain 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 thermo-
stat will remove the 24Vac from “E/W1” and the VSTB re-
moves the 24Vac from HR1. The contacts on HR1 will open