A humidification chamber (1A) for a respiratory humidifier (30) in which a level of liquid water is maintained by the use of floats (2, 3) which actuate valves (8, 10 and 9, 11) to control the passage of water through a water inlet (4). Two floats (2, 3) are each provided with actuating mechanisms to separately control the respective valve members (10, 11). The separate valve members (10, 11) are covered by an elastomeric moulding (12, 21, 22) which couples the valve members (10, 11) and provides a seal. The valves (8, 10 and 9, 11) are co-axially aligned and the system provides improved safety by allowing for reliable operation in the case where one of the floats (2, 3) fails. An alternative embodiment of the dual valve construction is also disclosed.