A capillary flow valve for use in a two phase heat transfer system such as a loop heat pipe, including an inlet port for receiving working fluid in a vapor-phase, an outlet port for outputting working fluid in a vapor-phase, and a porous wick material extending across the interior of the valve. Heating the wick evaporates liquid-phase working fluid from the wick and allows the vapor-phase working fluid to pass through the wick to the outlet port. Removing the heat allows liquid to condense in the wick, preventing flow of the vapor-phase working fluid through the wick to the outlet port.