A humidifier for humidifying a flow of air to be delivered to a patient includes a base unit having at least one wall defining a receiving space. The base unit also includes a variable volume reservoir configured to hold a body of water and receive the flow of air to humidify the flow of air for delivery to the patient. The receiving space is configured to receive the variable volume reservoir and the variable volume reservoir is conformable to a shape of the receiving space. The humidifier further includes a heater for heating the body of water.
