Sequence of Operations
5
On a “call for cooling”, the thermostat “makes” circuits R-O, R-G,
and R-Y.
Circuit R-G energizes the indoor blower motor relay. Circuit R-O
energizes the reversing valve. Circuit R-Y energizes the contac
-
tor starting the compressor and outdoor fan.
When the thermostat is satisfied, the contacts open, de-energiz
-
ing the indoor blower relay, contactor, and reversing valve. The
indoor and outdoor fans and compressor will stop.
On a “call for heating”, the room heat pump thermostat “makes”
circuits R-G and R-Y. Circuit R-G energizes the indoor blower
motor relay. Circuit R-Y energizes the contactor starting the
compressor and outdoor fan. The reversing valve will not be
energized and the system will be in the heating mode. When
the room heat pump thermostat is satisfied, the contacts open,
de-energizing the indoor blower relay and contactor. The indoor
and outdoor fans and compressor will stop. When the indoor
thermostat is calling for heat (24VAC signal at the Y terminal),
the board begins monitoring the DFT terminal to determine if
the defrost sensor is calling for a defrost cycle. Once the board
sees a 24VAC signal at the defrost terminal, it recognizes that
the coil is cold enough to form ice and begins accumulating the
30, 50, or 90 minute compressor run time (factory set at 50 min
-
utes adjusting of this may be necessary to provide complete de
-
frost for you climate zone). This time is only accumulated while
the compressor is running and the defrost sensor is closed. If
the selected time is reached and the defrost sensor is still call-
ing while the compressor is running, the control will enter a de
-
frost cycle. During a defrost cycle, the board will shut down
the condenser fan by opening the OF1 and OF2 terminals and
energize the reversing valve via the O terminal to enter a hot
gas defrost. It will stay in the defrost cycle for a maximum of 10
minutes, but will exit the defrost cycle if the defrost thermostat
is satisfied anytime before that 10 minutes expires. Upon exit
-
ing the defrost cycle, the board will turn off the reversing valve
and close the OF1 and OF2 contacts to allow the fan to resume
operation.
If the outdoor temperature is cold enough to produce icing on
the outdoor coil during the heating mode (approximately 34°),
the condensate heater will be energized through the heater
thermostat located in the control box. This heater stays ener
-
gized during cold weather to keep any defrosted water from re-
freezing in the pan. The condensate pan under the coil must
be piped per item number 6 under the “Before Installing Unit”
section of this manual.
Periodically clean the inside of the unit to keep the weep holes in
the base pan and in the fan scrolls open to assure proper drain-
age of water from the unit.
Step 7 - Maintenance
To verify proper operation of the defrost system, turn off the
power to the unit. Remove the rear access panel. Disconnect
the black wire (coming from the common terminal of the Com
-
pressor) from the T2 screw terminal on Contactor 2A and tem
-
porarily insulate it. To enter test mode, you must short across
the test terminals. Turn the power back on and then call for Y,
HPS and LPS switches closed, call for DFT and the unit will en
-
ter a defrost cycle. Note the unit will enter a 5-second anti-short
cycle delay if Y is interrupted. When the test pins are shorted,
the board reduces all timing functions by a factor of 256. That
means the 10 minute defrost cycle is reduced to 2.34 seconds
and the 30, 50, 90 minute times are reduced to 7.03, 11.72,
and 21.09 seconds respectively. All other functions of the board
remain the same. If you wish to test it, you should short the test
pins, make sure it’s calling for heat, and if the defrost sensor
is not closed, you can put a jumper from R to DFT to simulate
a defrost call. With this test you should see the board rapidly
cycle in and out of defrost mode verifying that it works properly
and is responding to all inputs accurately. To avoid over-heating
the outdoor blower motor, because the access cover has been
removed, limit the test to three cycles.
To return the heat pump to normal operation, turn off power to
the unit and reset the room thermostat to the normal heat set-
ting. Remove any temporary jumpers from the defrost control.
Reconnect the black wire from the common terminal of the com-
pressor to screw terminal T2 on contactor 2A. Install the rear
access cover. Turn on power to the unit.
Operation of Defrost Control 6TT-3
