In air-conditioning and refrigeration (HVACR) equipment, microfin tubes are commonly used to enhance the boiling performance of evaporators and heat pumps. Such tubes are mechanically expanded by a mandrel into a fin pack to create an interference fit between the outside surface of the tube and the fin collar to minimize the thermal contact resistance between tube and fin. However, during this expansion process, the internal enhancements also end up getting crushed or deformed to varying degrees, which degrades the in-tube thermal performance. Extensive data on boiling heat transfer coefficients and pressure drop in microfin tubes have been reported in the open literature. However, these data are almost invariably based on unexpanded or pristine tubes, and not on mechanically expanded tubes. Due to lack of knowledge about the influence of fin deformation on the thermal-hydraulic performance of microfin tubes, it is always questionable to use pristine tube performance data for designing HVACR heat exchangers and systems. In this work, we first present an analysis of the changes in the inside surface area of microfin tubes arising from tube expansion. An element-by-element thermal-hydraulic model of a coaxial tube heat exchanger, which takes the surface area change calculations as an input, has also been developed, since boiling heat transfer and pressure drop characteristics of microfin tubes are often measured using such heat exchangers. This model was then exercised to estimate the increase in refrigerant side thermal resistance and the change in refrigerant pressure drop for the flow boiling of R-410A through mechanically expanded microfin tubes at a saturation temperature and mass flux that is representative of conditions commonly encountered in residential and commercial air????conditioning systems. It is demonstrated that the effect of fin deformation on the thermal-hydraulic performance of actual heat exchangers needs careful investigation. The analysis presented in this work leads to a practical method of estimating the effect of fin deformation on the in????tube boiling thermal resistance and pressure drop. A more realistic design of air????conditioning/refrigeration heat exchangers and systems can thus be achieved.