SERVICING
46
4.0 Heating Operation
On heat pump units, when the room thermostat 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 “G” and “Y” and 24Vac is
supplied to “Y” at the heat pump unit and the “G” terminal
on the EBTDR board. The heat pump is turned on in the
heating mode and the blower motor starts after a 7 second
on delay.
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 heat
sequencer HR1 on the heater assembly.
4.3
HR1 contacts M1 and M2 will close 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 con-
tacts, M3 and M4, which will close and turn on heater
element #2. The blower motor is already on as a result
of terminal “G” on the EBTDR board being energized for
the first stage heat demand.
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.
If the second stage heat demand, “W2” cannot be satisfied by
the heat pump, the temperature indoors will continue to drop.
The room thermostat will then energize “W3” and 24Vac will
be supplied to HR2 on the heater assembly. When the “W3”
demand is satisfied, the room thermostat 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.
On most digital/electronic thermostats, “W3” will
remain energized until the first stage heat 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 satisfied.
When this happens, the room thermostat will remove the
24Vac from the coil of HR1. 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”.
The heat pump is turned off and the blower motor turns off
after a 65 second off delay.
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 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-41 AEP* & MBE WITH SINGLE STAGE CON-
DENSERS
AEP* & MBE ELECTRONIC BLOWER TIME DELAY RELAY
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 for specific wiring
connections, dip switch settings and system configuration.
