18-AC104D1-1G-EN 15
Section 16. Defrost Control
The demand defrost control measures heat pump out-
door ambient temperature with a sensor located outside
the outdoor coil. A second sensor located on the outdoor
coil is used to measure the coil temperature. The dif-
ference between the ambient and the colder coil tem-
perature is the difference or delta-T measurement. This
delta-T measurement is representative 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.
Termination Temperature
Defrost controls have a selectable termination tem-
perature. As shipped, defrost will terminate at 47°F. For
a higher termination temperature, cut Jumper J2 to
achieve 70°F when at or below 30°F ambient.
Fault Identication
A fault condition is indicated by the flashing light on the
defrost control inside the heat pump control box.
In normal operation, the defrost control light will flash
once each second. If the light is flashing more than once
per second or not at all, refer to the Defrost Checkout
sheet found with the service information in the control
box.
Pin Identication
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 timings.)
3. FRC_DFT = Forced Defrost (Short TEST_COM-
MON to this pin for two [2] seconds to initiate a
forced defrost. Remove the short after defrost initi-
ates.)
Defrost Control Checkout
Normal operation requires:
a. LED on board flashing 1 time/second.
b. 24V AC between R & B.
c. 24V AC between Y & B with unit operating.
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.
DEFROST TERMINATION TEMPERATURE
J1
U1
J2
J3
FRC_DFT
TST
TEST_COMMON
JUMPER 2
TEST PINS
Defrost controls have a selectable termination temper-
ature. Cutting jumper J2 (shown below) will achieve a
termination temperature of 70° when the ambient tem-
perature is below 30° (see table at left).
Defrost Board
Detail
Defrost Termination Temperatures
Outdoor
Temp.
Termination
Temperature
As
Shipped
>22°F 47°F
10°F–22°F ODT + 25°F
6°F–10°F 35°F
Cut
Jumper 2
>30°F 47°F
6°F–30°F 70°F
All
< 6°F
12 min. or 35°F
every 3 hrs.
≤ 0°F Outdoor unit will be turned OFF
≥ 5°F Resume outdoor unit operation
Defrost Termination Temperatures
Outdoor
Temperature
Termination
Temperature
As
Shipped
>22°F 47°F
10°F–22°F ODT + 25°F
6°F–10°F 35°F
Cut
Jumper 2
>30°F 47°F
6°F–30°F 70°F
All < 6°F
12 min. or 35°F
every 3 hrs.
For 024N, 048N & 060N Models
For 036N Models only
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