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Heat Controller, Inc. VMH SERIES Service Manual
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Auto fan action in heating mode.
8.4.3.4 Defrost mode：
Conditions of defrost:
When the unit is running, if any one of the following conditions is satisfied, the unit starts defrost:
1 .Defrost is available after 40 minutes of run time if T3 (Condenser coil temperature) is＜37°F/3°C and
there is a 11°F/6°C temperature difference for 3 minutes.
2. Defrost is available after 80 minutes of run time if T3 (Condenser coil temperature) is＜37°F/3°C and
there is a 7°F/4°C temperature difference for 3 minutes.
3. Defrost is available after 120 minutes of run time if T3 Condenser coil temperature) is＜37°F/3°C and
there is a 4°F/2°C temperature difference for 3 minutes.
4. If any of the above conditions are satisfied and the T2 (Evaporator coil temperature) has decreased
9°F/5°F below the normal maximum indoor pipe temperature, unit starts defrost.
Note: T2 is the maximum value of the evaporator temperature during the anti-cold wind function. It’s
recorded by the system automatically.
Terminating defrost:
If any one of following items is satisfied, defrost will stop and unit will return to normal heating mode.
1 .Defrost time has reached 10 minutes
2 .T3 (Condenser coil temperature) >46°F/8°C for 80 seconds.
T1-Ts-ΔT
°F °C
5° (2.5)
4° (2.0)
3° (1.5)
2° (1.0)