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SERVICING
58
The outdoor fan will change to high speed operation. 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 thermostats, “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 sup-
plied 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 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.
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 thermo-
stat 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.
4.5 As the temperature indoors increase, it will reach a point
where the “W2/W3” demand 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 the 1
st
and
2
nd
heater elements. If the “Y2” demand is present and
becomes satisfied the room thermostat will remove the
24Vac from “Y/Y2” of the MBE and the blower motor will
change to 60% of the programmed cfm. The VSTB will
remove the 24Vac from “Y/Y2” at the heat pump and the
outdoor fan will change to low speed operation. The heat
pump remains on along with the blower motor because
the “Y1” demand for first stage heat will still be present.
4.6 When the first stage heat demand “Y1” is satisfied, the
room thermostat will remove the 24Vac from “G” and “Ylo/
Y1” of the MBE/AEPF. The VSTB removes the 24Vac
from “Ylo/Y1” 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 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. 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.
S-60 ELECTRIC HEATER (OPTIONAL ITEM)
Optional electric heaters may be added, in the quantities
shown in the specifications section, to provide electric
resistance heating. Under no condition shall more heaters
than the quantity shown be installed.
The low voltage circuit in the air handler is factory wired and
terminates at the location provided for the electric heater(s).
A minimum of field wiring is required to complete the instal-
lation.
Other components such as a Heating/Cooling Thermostat
and Outdoor Thermostats are available to complete the
installation.
The system CFM can be determined by measuring the static
pressure external to the unit. The installation manual
supplied with the blower coil, or the blower performance table
in the service manual, shows the CFM for the static mea-
sured.