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Excessive Frosting on Air-Source Outdoor Unit (Defrost Failure)

Likely causes

Defrost temperature sensor (coil sensor) failure

Check: frost undetected, defrost cycle not triggered

What to do: Measure coil sensor resistance (NTC 10kΩ: 10kΩ at 25°C) → replace if deviation exceeds 20%

Defrost timer/control logic misconfigured

Check: cycle interval too long or duration too short

What to do: Check defrost initiation conditions (e.g., coil temp below -5°C) and max defrost duration in controller parameters → adjust to manufacturer's recommended values

Outdoor fan motor failure

Check: reduced airflow → accelerated coil icing

What to do: Check outdoor fan current and rotation → replace motor or capacitor if fan is stopped or running slow

At high humidity and 2–5°C outdoor conditions, frost accumulates faster than defrost cycle frequency

Check: Inspect with instruments or visually

What to do: Verify 4-way valve switching during defrost: measure hot gas pipe temperature rise rate → if coil bottom temp is still below -5°C after 10 min of defrost, replace valve

4-way valve incomplete switching during defrost

Check: hot gas reversal does not reach entire coil

What to do: Log outdoor conditions during excessive frosting → if high humidity at 2–5°C, shorten defrost interval from 60 to 45 minutes

Field tip

Quick field check for defrost issues: touch the outdoor coil — if the side exposed to ambient air and the opposite side differ by more than 5°C in temperature, hot gas distribution is uneven → inspect 4-way valve or distributor

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