Abstract The performance of highly extended surface heat exchangers has been evaluated in two heat pump applications: as the evaporator in a compression heat pump and as the absorber in an absorption heat transformer. Film heat transfer coefficients based on the equivalent plain tube area (p.t.a.), ranged from 200 W m −2 K −1, for gas-liquid contact, to 3800 Wm −2 K −1, in the evaporation of a liquid stream. Another measure of the heat transfer characteristics of the exchangers were the ‘ UA o’ values, which were significantly better than the theoretical values calculated for each operation. The experimental results ranged from 12 W K −1 (gas) to 214 W K −1 (evaporation), while the theoretical values were 1 W K −1 (gas) and 43 K −1 (evaporation).