The vertical header is a usual feature of outdoor heat exchanger of the residential air-conditioning system, typically a multipass microchannel heat exchanger. When operating in heat pump mode, it functions as an evaporator. In such a system, refrigerant maldistribution in the header can deteriorate the performance of the heat exchanger. The objective of study in this paper is the adiabatic upward flow of the refrigerant in the second pass vertical header of microchannel heat exchanger and its effect on distribution. R410A is circulated into the header through the microchannel tubes (5 or 10 tubes) in the bottom pass and exits through tubes (5 or 10 tubes) in the top pass. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The best distribution is found at high flow rate and low quality. The distribution is affected by the flow patterns in the header as well as axial momentum. A distribution correlation, obtained based on the flow rate measurement in each tube, was then incorporated with a microchannel heat exchanger model. The simulation results quantified the capacity reduction of the 2-pass microchannel condenser due to the refrigerant maldistribution.