Heavy frost on a cooler is a result, not a cause. Before changing the unit, establish which of the two conditions below applies, because they have different fixes.
| Condition | What it looks like | Usual cause |
|---|---|---|
| Frost builds, defrost clears it, then it returns quickly | Even layer across the coil, cleared by each cycle | Defrost termination set too early, or too long an interval |
| Ice remains in the coil after defrost | Hard ice at the bottom of the coil, growing cycle by cycle | Defrost capacity too low, drain blocked, or fan running during defrost |
| Frost forms only at one end | Localised ice at the header or the first rows | Uneven refrigerant distribution or a partially blocked distributor |
| Frost forms on the outside of the casing | Ice on the housing, not the coil | Cold bridging through the panel, not a cooler fault |
Defrost problems and insulation problems look similar from the room. Measuring the coil after defrost separates them.
Each check takes minutes and eliminates one cause. Do them in this order.
| Check | How | What it tells you |
|---|---|---|
| Coil after defrost | Open the unit immediately after the cycle ends | Residual ice means the defrost is short of capacity, not short of time |
| Heater current | Clamp meter on each heater branch | A single failed element looks like a defrost problem but is a heater problem |
| Drain | Pour water into the drain pan | Slow drainage means ice refreezes in the pan and blocks the next cycle |
| Defrost termination sensor | Compare its reading with the coil | A displaced or failed sensor ends defrost before the ice has cleared |
A heater that has failed on one branch is the most common single cause, and the cheapest to fix.
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