Leah and Larry-
On our coach the water tank is in the same bay as the HydroHot, so I assume that ambient heat in there keeps that tank safe, though as Bill points out, only as long as the hydronic system is on and working. The only bay that has an obvious heat exchanger is the waste tank bay. I've found that many new owners haven't discovered that bay, mainly because it doesn't have a regular hand latch, and they didn't receive an adequate orientation from the seller.
The waste tank bay on our rig is directly in front of the passenger side rear wheels, and is held closed by two nearly hidden swing-clasp latches underneath, along the front edge. The bay door looks like a body panel, but it actually is hinged and swings up when unlatched underneath. Inside are the gray and black water tanks, the solar control module, an array of Aladdin modules, and a myriad of associated tubes and wires. Extending into the bay on the forward wall, from the adjoining storage bay, is a white plastic box containing the water pump and associated fittings; the pump is only accessible, however, by removing a carpeted access panel in that adjoining bay. The white box extends into the waste tank bay so that its contents can be kept from freezing by the heat exchanger, which is readily visible mounted to the waste tank bay's ceiling.
A thermosensor rod is mounted to a plywood frame, center front in the bay, and it monitors the bay temp., turning the bay exchanger on at I believe a factory preset temp. Also protected in the plywood frame is the Aladdin temperature and compass module. Since the bay is unlit, I velcroed a small Maglight flashlight on the frame so I could see in there at night. Because the bay door doesn't require a key to open, I also mounted a fire extinguisher in there; I don't want to be looking for keys if there's a fire.
When the temp. outside starts hugging the 32 mark, I go out to the coach, plug it in to our home's 110v 30 amp RV outlet (the rig's batteries normally stay charged by solar only), turn its master switches back on, flip on the HydroHot electric switch, and set the living room thermostat at 45-50. Sometimes I kick on the diesel burner temporarily to get a faster heat-up. Our coach has an ice-maker in its Dometic refrigerator/freezer, and that's the only place I know of that has heat tape, to protect its water supply apparatus on the back of the refer. Like Bill, I regularly monitor the coach during such cold spells, but it's easier for me since the rig is parked next to our home. If the HydroHot failed, I could configure small electric heaters, light bulbs, or some other temporary emergency heat source.
Because I blow out my lines to winterize things, rather than use antifreeze in the lines, the steps I mention above are only as extra insurance, and mostly to protect the HydroHot tubes themselves; drained tanks, blown out water lines and icemaker valves, and antifreeze in the washing machine and drain traps - that would be enough in a motor home with a water heater and furnace. But it's easy to turn on the HydroHot electric side to protect the HydroHot internal loops themselves, and doing so is added insurance for the rest of the rig as well, albeit overkill in some regards. It's cheap insurance against very expensive hydronics repair. Granted, the best option would be to pump RV anitfreeze through everything, but in 26 years I've not gone that route. Yet.
-Joel
