Trane 4TTZ0 Installer's Manual page 16

11.3 Defrost Control (Heat Pump only)
The demand defrost control measures heat
pump outdoor ambient temperature with a sen-
sor located outside the outdoor coil. A second
sensor located on the outdoor coil is used to
measure the coil temperature. The difference
between the ambient and the colder coil tem-
perature is the difference or delta-T measure-
ment. This delta-T measurement is representa-
tive of the operating state and relative capacity
of the heat pump system. By measuring the
change in delta-T, we can determine the need
for defrost. The coil sensor also serves to
sense outdoor coil temperature for termination
of the defrost cycle.
FAULT IDENTIFICATION
A fault condition is indicated by the fault LED
on the control board inside the heat pump
control box.
In normal operation, the status LED will flash
once each second. If the light is flashing more
than once per second or not at all, refer to the
Service Facts for that unit.
PIN IDENTIFICATION
1. TEST_COMMON (Shorting any of the
other pins to this pin causes the function
of the other pin to be executed. Leaving
this pin open results in the normal mode of
operation.)
2. TST = Test (Shorting TEST_COMMON to
this pin speeds up all defrost board tim-
ings.)
3. FRC_DFT = Forced Defrost (Short
TEST_COMMON to this pin for two [2]
seconds to initiate a forced defrost. Re-
move the short after defrost initiates.)
16
OUTDOOR CONTROL
BOARD – PIN
IDENTIFICATION
DEFROST CONTROL CHECKOUT
Normal operation requires:
a. Status LED on board flashing 1 time/sec-
ond.
b. 12 VDC between D & B in communicating
mode.
c. 24 VAC between R & B in 24V mode.
d. Defrost initiation when FRC_DFT pin is
shorted to TEST_COMMON pin.
If a defrost control problem is suspected, refer to
the service information in control box.
18-BC53D2-6