A breathing assistance system for delivering a stream of heated, humidified gases to a user for therapeutic purposes, comprising: a humidifier unit that has an inlet port and an exit port, said humidifier unit adapted to in use receive a flow of gases from a gases source via said inlet port, said humidifier unit further adapted to hold and heat a volume of water in use, in use said flow of gases passing through said humidifier unit and becoming heated and humidified, said heated humidified gases exiting said humidifier unit via said humidifier unit exit port, wherein said humidifier unit is a humidifier chamber having a base, and said breathing assistance system further has a heater plate adapted to heat the contents of said humidifier chamber by providing heat energy to said base, an exit port temperature sensor adapted to measure the temperature of gases exiting said humidifier unit, a second temperature sensor located in the flow of gases downstream of the gases source and upstream of the humidifier chamber, a flow sensor adapted to measure the actual flow rate of said gases stream through said system, a controller configured to receive data from said second temperature sensor relating to the measured temperature, data from said exit port temperature sensor relating to the measured temperature, and data from said flow sensor relating to said actual flow rate, said controller determining a control output in response, said control output adjusting the power to said humidifier unit to achieve a desired output at said humidifier unit exit port, and said breathing assistance system further having a heater plate temperature sensor adapted to measure the temperature of said heater plate and provide this temperature measurement to said controller, said controller determining said control output by assessing all of said measured temperatures and said measured flow rate. a humidifier unit that has an inlet port and an exit port, said humidifier unit adapted to in use receive a flow of gases from a gases source via said inlet port, said humidifier unit further adapted to hold and heat a volume of water in use, in use said flow of gases passing through said humidifier unit and becoming heated and humidified, said heated humidified gases exiting said humidifier unit via said humidifier unit exit port, wherein said humidifier unit is a humidifier chamber having a base, and said breathing assistance system further has a heater plate adapted to heat the contents of said humidifier chamber by providing heat energy to said base, an exit port temperature sensor adapted to measure the temperature of gases exiting said humidifier unit, a second temperature sensor located in the flow of gases downstream of the gases source and upstream of the humidifier chamber, a flow sensor adapted to measure the actual flow rate of said gases stream through said system, a controller configured to receive data from said second temperature sensor relating to the measured temperature, data from said exit port temperature sensor relating to the measured temperature, and data from said flow sensor relating to said actual flow rate, said controller determining a control output in response, said control output adjusting the power to said humidifier unit to achieve a desired output at said humidifier unit exit port, and said breathing assistance system further having a heater plate temperature sensor adapted to measure the temperature of said heater plate and provide this temperature measurement to said controller, said controller determining said control output by assessing all of said measured temperatures and said measured flow rate.