Low air-side heat transfer coefficient (HTC) limits the performance of many heat exchangers. A new method is developed to obtain local air-side HTC of real heat exchangers without the use of complex experimental tools. The method relies on measuring mass transfer and applying the analogy between heat and mass transfer to determine heat transfer. A thin film coating is applied to the heat transfer surface which is exposed to a suitable trace gas, in this case, a 50 ppm ammonia-air mixture. The coating absorbs the ammonia and changes color. A detailed description of the new measurement technique is given, including the application of the fundamental heat and mass transfer correlations, evaluation of suitable coating formulations, determination of surface morphologies, and discussion of the optical principle to quantify color change. For laminar flow across flat plates, the new method results in experimental HTC that are within 17% of the Blasius solution.