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24.4. Defrost Control
For defrost calculations:
•
T3 = Outdoor coil temperature sensor.
•
T30 = Minimum value of T3 during 10 to 15 minutes of run-time period.
Defrost mode begins when any one of the following conditions are met:
•
After 29 minutes of run-time - T3 < 19.4°F (-7°C) and T3 + 4.5°F (2.5°C) ≤ T30.
•
After 35 minutes of run-time - T3 < 23°F (-5°C) and T3 + 5.4°F (3°C) ≤ T30.
•
After 40 minutes of run-time - T3 < -11.2°F (-24°C) for three minutes.
•
After 120 minutes of run-time - T3 < 5°F (-15°C).
Run-time count starts
0
This formula depends on humidy. The higher the humidity, the more frequent the defrost cycles. With no humidity, there
will be less frost build-up. Defrost is a function of humidity.
24.5. Defrost Theory
Tdef = The unit must run for a minimum of 11 minutes or longer in order for it to go into defrost mode. At that time it will look
at one of the following five conditions to determine when to go into defrost. Those conditions are:
• Condition 1: After 29 minutes of accumulated run-time: Coil temperature ≤ 19.4ºF and coil temperature ≤ Tdef - 5.4ºF .
• Condition 2: After 35 minutes of accumulated run-time: Coil temperature ≤ 23ºF and coil temperature ≤ Tdef - 2.7ºF .
• Condition 3: After 40 minutes of accumulated run-time: Coil temperature ≤ 10ºF for three min utes.
• Condition 4: After 120 minutes of accumulated run-time: Coil temperature ≤ 5ºF .
• Condition 5: If T3 is less than 37ºF after 120 minutes of continuous run-time and T3 is less than 27ºF for three minutes,
it will start defrost.
NOTE:
T3 = Outdoor Coil Sensor
24.6. Defrost Termination
If any of the following conditions are met, the unit will exit defrost mode and return to normal heating mode.
• Condition 1: Coil temperature is higher than 70ºF.
• Condition 2: Coil temperature is higher than 55ºF for continuous 80 seconds.
• Condition 3: Defrost run-time of 10 minutes.
This also includes any manual defrost update by using a remote control.
After 29
minutes of
run-time T3 <
19.4°F and
T3 time
T3+5.4°F is still
mea-
less than or
sured
equal to T30.
and T3
temp
deter-
mined
6 min
14 minutes
15
29 < 30
10
After first 15 minutes of
operation, if T3 <
19.4°F then defrost,
otherwise monitor T3
for next 14 minutes.
Figure 40. Defrost Calculations
After 40 minutes of run-time
T3 < 11.2°F.
5 min
35
40
After 36 minutes of
run-time T3 < 23°F and
T3+5.4°F is still less than
or equal to T30.
98
2 Hours
2 hours have passed
and T3 < 5°F.
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