Mastering Fujitsu Defrost Logic: The "Invisible" Service Call

Have you ever walked up to a heat pump that’s encased in ice, but the board says everything is "Normal"? To troubleshoot a defrost issue, you have to understand the integrating logic. Fujitsu doesn't just look at temperature; it looks at time + temperature delta.

The Variable Map

To speak the language of the control board, you need to know these four values:

• Tn: Outdoor heat exchanger (coil) temperature.

• Ta: Ambient outdoor temperature.

• Tn10: Coil temperature exactly 10 minutes after the compressor started.

• Tnb: Coil temperature 5 minutes prior to the current reading.

1. Triggering the First Defrost

The system is protective during its first cycle. It will not operate defrost if the compressor has been running for less than 17 minutes.

2. The "Marksman" Precision: 2nd Cycle and Beyond

After the first cycle, the board gets more aggressive. It looks for a "drop-off" in performance. If any of these are met after 40 minutes of run time, the "valves shift":

1. The 10-Minute Drop: Tn - Tn10 < -9.0°F (Coil is 9 degrees colder than it was at the 10-minute mark).

2. The 5-Minute Slide: Tn - Tnb < -3.6°F (Coil temperature is dropping too fast).

3. The Floor: Tn less than or equal to -4.0°F (If it's that cold, it's time to melt).

3. Integrating Defrost (The Long Game)

If the unit runs for a very long time without hitting the "sliding" triggers above, the system has a "safety" defrost:

• Continuous Run: After 215–240 minutes, if the coil is less than or equal to 23°F, it triggers.

• Short Cycling: If the compressor cycles off 40 times (each run less than 10 mins) without a defrost, it will force a cycle to clear any accumulated frost.

4. Release Conditions (The Exit Strategy)

The system exits defrost as soon as ONE of these happens:

• Coil Temp: Tn reaches 55.4°F (must be at least 1 minute into the cycle).

• Time Limit: The compressor has been in defrost for 15 minutes.