The effects of starting the outdoor, coil far~prior to the termination of the defrost on refrigerant pressure transients in a heat pump were investigated experimentally. Experiments ,were conducted ona 3-ton spfit-!y,_p,e heat purnp in psychrometric chambers at a unive~’sity S Energy Systems Laboratory. At the termination of a typicai defrost period, the high-pressure’refrigerant in the outdoor coil of a heatpump is exposed to the low-pressure.suction line of the compressor, and the low-pressure refrigerant in the indoor coil is exposed to the high~pressure d~charge’ line of the compressor. A sudden rise and fall of pressure is experienced in the compressor suction line, and a Sudden fall and rise of pressure is experienced in the compressor discharge line. Such pressure transients subject the compressor and the refrigerant fines to a mechanical shock. Shocks are detrimental to the refiabifity and longevity of the heat pump. By starting the outdoor coil fan prior to termination of the defrost cycle, the outdoor coil was cooled and pressure transients at the defrost termination were reduced. Experiments were conducted for fan prestart timings of 20, 40, and 60 seconds. The investigation showed the 20-second fan pre-start to be ideal for reducing the pressure transients at the defrost termination.