AbstractThe paper deals with some of the major aspects of heat‐exchanger design for electric heat pumps. After a discussion of heat‐transfer theory, it describes a method that can be used in the design and sizing of air‐to‐refrigerant heat exchangers and in calculating temperature distributions. As an illustration, economically optimum sizes for exchanger coils are given for heat pumps of output 13.9 kW, 5.6 kW and 5 kW at 5°C outside the ambient temperature. At several stages, manufacturer's experimental data have been used, and the final results are compared with the design of heat exchangers used in commercially available models. Some temperature measurements made on a heat‐pump installation in an experimental house are also reported.At least doubling the size of presently used indoor coils is shown to be economically justifiable, increasing the seasonal coefficient of performance from about 2.4 to 2.8‐3.0. Reassessment of outdoor‐fan size is also shown to be necessary.Throughout the work it is assumed that the heat pump is required for heating only, as would be the case in the