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DEMAND DEFROST SYSTEM
DEFROST CONTROL
The demand defrost control measures heat pump outdoor 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 difference between the ambient and the colder coil temperature is the difference or
FAULT DETECTION
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
PIN IDENTIFICATION (See Figure)
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_COMMON to this pin for two (2) seconds to initiate a forced defrost. Remove the short after defrost initiates.)
PIN IDENTIFICATION
FRC_DFT
TEST_COMMON
TST
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, proceed to DEMAND DEFROST CONTROL CHECKOUT in this Service Facts.
▲WARNING: Do NOT connect 24 VAC to T1
PROCEDURE FOR TESTING SENSORS
Measure the temperature the subject sensor is exposed to. If the sensor is mounted on a tube, place the lead on an Annie
Unplug the sensor and measure the resistance with a good quality ohmmeter (Simpson 260 or equiv.). Read the value as quickly as possible to prevent the meter current from changing the resistance reading.
Using the chart on the right, locate (as close as possible) the actual sensor temperature. The measured resistance should be relatively close to the resistance value shown in the chart.
Example: |
|
Sensor temp. | = 19°F. |
Measured Resistance | = 46K ohms |
This sensor is good since the measured value is relatively close to the chart value.
FLASH RATE INDICATIONS:
TEMP °F | RESISTANCE |
86 | 7.85K |
81 | 8.85K |
75 | 10.24K |
70 | 11.59K |
65 | 13.14K |
59 | 15.32K |
55 | 16.93K |
50 | 19.41K |
45 | 22.20K |
41 | 24.76K |
36 | 28.45K |
32 | 31.84K |
25 | 37.50K |
19 | 46.44K |
14 | 53.94K |
10 | 57.64K |
5 | 67.06K |
0 | 78.05K |
1 | — Flash/Second | = Normal operation. |
2 | — Flashes/Second | = Fault A (Low ∆T) i.e.: Inoperative compressor, loss of charge, open ambient sensor, shorted coil sensor. |
3 | — Flashes/Second | = Fault B (20 defrosts terminated on time) |
|
| Fault C (High ∆T) i.e.: SOV stuck in heating, shorted ambient sensor, open coil sensor, closed TXV, |
|
| OD motor failure, OD fan on in defrost, undercharged unit. |
4 | — Flashes/Second | = Fault A & C or A & B |
For additional information consult Pub. No.
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