I think David has explained it pretty well. The setup on coaches can vary with year and model, but ours has all hydronic system switches together on one panel on the galley cabinet. As to whether there is exactly a tank involved within the unit or just heat exchange tubing, I'm not sure, but Orman and others here can address that.
As mentioned, the electric element is not up to the snuff that the diesel burner is, so use it more for maintaining heat status that has been attained by the diesel side, and to save diesel fuel when plugged into power, especially power you've already paid for with camp fees. The electric side won't keep up with the demands of a shower or perhaps a washing machine. And you should know that since there are two sides to your hydronic unit, heating hot water and heating your living area and bays, one of them must take priority over the other; hot water wins. If you take a shower or run hot water somewhere else, an AquaHot/HydroHot will circulate through that water exchange coil and usually shut down the coach heat in order to maximize and keep up with the hot water demand. Rather than water, the living space heat is provided by circulating boiler fluid/antifreeze through separate exchange coils and pumping it in loops to fan-forced heat exchangers at floor vents. We often notice our floor heat exchanger blowers stopping when we turn on a hot tap. Once that water demand stops, the vent fans kick on again. That may be less common on an Aqua Hot as opposed to our smaller Hydro Hot, but I'd assume it occurs on both.
The third switch on our panel is for circulating hydronically heated antifreeze through the engine to warm it up prior to starting when it's cold. Similarly, your engine coolant heat circulates automatically through your HHot so when you come off the highway to camp each day, the HHot is already warm and needs less diesel to initially run. It also assists the HHot if you use it to warm the rig's interior during travel in lieu of or in addition to the dash heater. Contrary to some confusion among new owners, the engine preheat switch is not involved with nor is it necessary to this automatic engine-assisted AHot feature; the switch allows the HHot or AHot to heat up the engine, not the other way around since the engine coolant circulates through the hydronic unit automatically anyway - no switch needed.
On ours at least, the diesel burner switch on the galley panel is also used to reset the unit if it should go into a fault for some reason, such as a low battery shutdown or other reason. Simply turning the diesel switch off for a couple minutes, then back on, may be all it takes to reset and get things going again. Otherwise you'll need to check the lamps on the HHot control panel in the HHot bay to get a clue as to what may be wrong.
Joel
