An electrically conductive plastic (ECP) material can be used to heat water in a reservoir of a respiratory humidifier to encourage heating and/or humidification of gases passing through the respiratory humidifier. The electrically conductive plastic material can at least in part overmould the base and/or walls of the chamber and/or the reservoir of the respiratory humidifier. The reservoir can also partially or fully be formed from the electrically conductive plastic material. Furthermore, the humidification system can be configured to create substantially equal or differential heating of water in the reservoir.