The HHO Gas doesn't freeze, besides it is being used as it is produced thus the name Hydrogen On Demand. In most hydrogen generators the water will freeze and expand. Which can 1 - ruin your generator and 2 - Until your HHO generator thaws out it will not produce HHO gas, which could be awhile depending on how frozen it is.
The generators we carry do not freeze, the Revolution won't freeze until -54 Degrees Fahrenheit. Some companies are telling you to put alcohol and other things into your generator to keep from freezing. This will mess up your electrolysis process, so never do this.