You can run the Aqua-Hot with no limitations with water system drained.
The bay heater is probably calling for heat, making the circulation pump run.
Please note: You do not get all of the water out of the Aqua-Hot by draining it or blowing air through it. If the water left inside freezes it will damage the water heating loop. If this water heating loop is damaged, it will require the unit either be replaced or remanufactured. Very expensive. The only approved winterizing method is to completely fill the Aqua-Hot (and rest of the water lines in the coach) with RV water line winterizing fluid.
- Roger Berke -