A breathing assistance system (1) for delivering a stream of heated, humidified gases to a user, comprising a humidifier unit (5) which holds and heats a volume of water, and which in use receives a flow of gases from a gases source (13) via an inlet port (23), the flow of gases passing through the humidifier and exiting via an exit port (9), the system further having a temperature sensor (15) which measures the temperature of the gases exiting the humidifier unit, an ambient temperature sensor (60) which measures the temperature of gases before they enter the humidifier unit, and a flow sensor (61) which measures the flow rate of the gases stream, the system also having a controller (8a) which receives data from the temperature and flow sensors, and which determines a control output in response, the control output adjusting the power to the humidifier unit to achieve a desired output at the humidifier unit exit port.
