Service manual
SERVICING
36
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 and AEP*. 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.
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
installation.
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.
Alternately, the system CFM can be determined by operat-
ing the electric heaters and indoor blower WITHOUT having
the compressor in operation. Measure the temperature rise
as close to the blower inlet and outlet as possible.
If other than a 240V power supply is used, refer to the BTUH
CAPACITY CORRECTION FACTOR chart below.
BTUH CAPACITY CORRECTION FACTOR
SUPPLY VOLTAGE 250 230 220 208
MULTIPLICATION FACTOR 1.08 .92 .84 .75
EXAMPLE: Five (5) heaters provide 24.0 KW at the rated
240V. Our actual measured voltage is 220V, and our
measured temperature rise is 42°F. Find the actual CFM:
Answer: 24.0KW, 42°F Rise, 240 V = 1800 CFM from the
TEMPERATURE RISE CHART, Table 5.
Heating output at 220 V = 24.0KW x 3.413 x .84 = 68.8
MBH.
Actual CFM = 1800 x .84 Corr. Factor = 1400 CFM.
NOTE: The temperature rise table is for sea level installa-
tions. The temperature rise at a particular KW and CFM will
be greater at high altitudes, while the external static pressure
at a particular CFM will be less.