A method for exchanging heat between an effluent fluid and a blood fluid and a heat exchanger for heat exchange. The heat exchanger comprises a first and a second fluid circuit extending through the heat exchanger. The heat exchanger further comprises a stack of fluid plates and a membrane arranged between each of the fluid plates where one interspace is formed between each fluid plate and membrane. The first and the second fluid circuit is each constituted by a passage extending through the fluid plates and membranes and along the fluid plates and membranes in at least two interspaces According to the method for heat exchanging an effluent fluid is passed through the first fluid circuit and a blood fluid is passed through the second fluid circuit such that the effluent fluid is passed along one side of a membrane and simultaneously the blood fluid is passed along the other side of the membrane. Heat is thus exchanged between the effluent fluid and the blood fluid over the membrane.
