This paper investigates frosting and refrigerant distribution in the outdoor coil of a 10 kW CO₂ (R-744) split system in heating mode. Experimental results are presented from a well-instrumented prototype, operating under frosting conditions. Photographs record frost buildup over time, and reveal nonuniform patterns of surface temperature distribution at carefully selected operating conditions (air inlet temperatures at 4.5°C; air inlet humidity 55%; 0.10 and 0.30 inlet vapor qualities). Parallel heat exchanger modules exhibited different frosting patterns, so they were tested individually. Results indicated that such frosting tests can be an inexpensive and efficient way to identify severe refrigerant maldistribution problems and to suggest a possible cause.

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