A mathematical model is presented for simulating the thermal behaviour of a cross-flow air exchanger with layers of phase change material (PCM) sandwiched between the hot and cold air streams to prevent frosting. The exchanger operates under winter conditions and uses electric power for storing heat in the PCM, The model is validated with experimental data, and the effects of the dimensionless electric load, Q(e), and of the PCM layer Blot number, Bi, are examined. Results indicate that for a commonly used exchanger with Bi = 0.6 (3 mm PCM layer) and Q(e) greater than or equal to 0.234 (300 W/m(2)), frosting is avoided during both the charge and discharge periods. For Bi = 1.2 (6 mm PCM layer), Q(e) may be set to zero (no storage) and frosting is still avoided, but the increased wall thermal resistance affects the exchanger performance by reducing the dimensionless outlet temperature of the cold air stream.