If you mean Aqua-Hot, I have used ours into the single digits at night. It would not be safe to store at those temps but using it keeps everything from freezing on our coach.
The AquaHots in the U320 coaches are bigger capacity than the HydroHot in the Nimbus. Maybe Rudy will tell us the differences and how it will affect cold weather use.
1.Would it be fair to say that tha bays do not stay as hot as the coach 2. plumbing there would be more likely to freeze than the living quarters plumbing?
I hate to have to winterize the coach for 6 weeks of possible serious cold before we head out and go south!
The aqua hot's 1500 watt 120 volt element so keep the tanks and front of the coach and the bath over freezing. Might never shut off the fans. Maybe set really low?