A breathing assistance system for delivering a stream of heated, humidified gases to a user, comprising a humidifier unit which holds and heats a volume of water, and which in use receives a flow of gases from a gases source via an inlet port, the flow of gases passing through the humidifier and exiting via an exit port, the system further having a temperature sensor which measures the temperature of the gases exiting the humidifier unit, an ambient temperature sensor which measures the temperature of gases before they enter the humidifier unit, and a flow sensor which measures the flow rate of the gases stream, the system also having a controller 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.