Abstract: A system for supplying sanitizing gas to a spa includes a sanitizing gas generator for generating sanitizing gas, a conduit for conducting the sanitizing gas from the generator to the spa, and an isolation device coupled to the conduit between the spa and the generator. An arrangement for creating subatmospheric suction pressure in at least a region of the conduit is employed whereby the isolation device is subjected to said suction pressure and the sanitizing gas is caused to flow from the generator to the spa. The isolation device has a drain opening through which liquid can flow to protect the generator from a backflow of water from the spa and a valve responsive to said suction pressure to seal said outlet port sufficiently to prevent significant loss of the suction pressure and of the sanitizing gas when the subatmospheric suction pressure is present.