Low Global Warming Potential (GWP) refrigerants have significant environmental benefits, but they are flammable. In the interest of safety, they are best kept in hermetically factory-sealed heat pump packages outside the built environment. With this approach, a secondary fluid is needed to transfer heat between the air within the Air Handler (AH) and the refrigerant outside the home. In this study advances in microchannel technology and additive manufacturing (AM) of plastics is used to develop a low-cost, high-effectiveness Microchannel Plastic HX (MPHX) to be used in place of traditional water coils. For a given thermal load, the MPHX results in a more compact AH or lower AH fan power draw. The use of thin-walled plastics is viable for high effectiveness HX because the dominant resistance to heat transfer in the air handler HX is on the air side. A subscale MPHX with cross duct area of 5.08 cm x 10.16 cm (2in x 4in) and estimated heat load up to 100W is designed and fabricated and its thermal performance is characterized over range of water and air flow rates. The experimental data are used to validate the MPHX themo-fluidic model. The validated model is further used to compare the performance advantages of the MPHX design relative to a FTHX.