A vapor transfer unit for humidifying a breathing gas, the vapor transfer unit comprising: a housing having: a first end portion, at which is located a gas inlet, a liquid inlet, and a liquid outlet, a second end portion opposite the first end portion, a gas outlet located at the second end portion, wherein the gas inlet and the gas outlet are not coaxial, a gas passage fluidically coupling the gas inlet to the gas outlet, a liquid passage fluidically coupled to the liquid inlet and the liquid outlet, the liquid passage including a plurality of hollow fiber membranes (914a-d) configured to allow liquid (911, 913) to flow from the liquid inlet to the liquid outlet in the interiors of the hollow fiber membranes (914a-d), and wherein the hollow fiber membranes (914a-d) are formed of a material that permits vapor to pass from the liquid passage to the gas passage; a gas input tube having a first end for connection to a source of pressurized breathing gas and a second end opposite the first end, the second end being connected to the gas inlet, wherein the gas input tube has a first length; and a gas output tube having a first end connected to the gas outlet, and having a second end opposite the first end, the second end having a patient interface for delivering humidified gas to a patient, wherein the gas output tube has a second length; wherein the first length is greater than the second length; and each hollow fiber membrane (914a-d) having a fold, about its midpoint, configured to change a flow direction of the liquid flowing around the fold. a housing having: a first end portion, at which is located a gas inlet, a liquid inlet, and a liquid outlet, a second end portion opposite the first end portion, a gas outlet located at the second end portion, wherein the gas inlet and the gas outlet are not coaxial, a gas passage fluidically coupling the gas inlet to the gas outlet, a liquid passage fluidically coupled to the liquid inlet and the liquid outlet, the liquid passage including a plurality of hollow fiber membranes (914a-d) configured to allow liquid (911, 913) to flow from the liquid inlet to the liquid a first end portion, at which is located a gas inlet, a liquid inlet, and a liquid outlet, a second end portion opposite the first end portion, a gas outlet located at the second end portion, wherein the gas inlet and the gas outlet are not coaxial, a gas passage fluidically coupling the gas inlet to the gas outlet, a liquid passage fluidically coupled to the liquid inlet and the liquid outlet, the liquid passage including a plurality of hollow fiber membranes (914a-d) configured to allow liquid (911, 913) to flow from the liquid inlet to the liquid outlet in the interiors of the hollow fiber membranes (914a-d), and wherein the hollow fiber membranes (914a-d) are formed of a material that permits vapor to pass from the liquid passage to the gas passage; a gas input tube having a first end for connection to a source of pressurized breathing gas and a second end opposite the first end, the second end being connected to the gas inlet, wherein the gas input tube has a first length; and a gas output tube having a first end connected to the gas outlet, and having a second end opposite the first end, the second end having a patient interface for delivering humidified gas to a patient, wherein the gas output tube has a second length; wherein the first length is greater than the second length; and each hollow fiber membrane (914a-d) having a fold, about its midpoint, configured to change a flow direction of the liquid flowing around the fold.