A counterflow heat transfer system comprises a heat exchanger and a flow controller arranged to convey a first fluid through the heat exchanger in a first flow direction and a second fluid through the heat exchanger in a second counterflow direction. The heat exchanger comprises at least one first thermally conductive tube conveying the first fluid and at least one second thermally conductive tube conveying the second fluid. The first and second tubes are wound around one another and in contact with one another in an entwined tubular arrangement.