If you disable a hydrofan and then enable it again, the dupes won't use it any more. To work around this you have to deconstruct it and construct a new one.
Side note: It would be neat if we could set a target temperature for the hydrofan. I just want to keep the nearby buildings from overheating; the dupes shouldn't have to push the temperature below freezing.
Build a hydrofan. Disable it. Enable it. Now it can't be used again.