Have you used the Hydro-Hot Engine Pre-Heat???

[David T. Richelderfer]

Yes - when I turn on the Diesel Burner switch (its associated green light comes on) I can hear the burner first turn on and a few seconds later I can hear the ignition and initial combustion followed in a few more seconds by a short burst of black smoke coming out its exhaust pipe.

Yes -  when I turn on the Engine Preheat switch (its associated green light comes on) I can hear the engine preheat pump start humming.  Well before starting the Engine Preheat the AquaHot should have been heating by either the electric water heating element and/or the diesel burner.  Both the electric element and Diesel Burner can be on at the same time.

On our AquaHot, the AHE-100-02S model, there is no control panel remoted from the Aquahot unit.  Its brains are all located on the front wall within the unit's stainless steel box beside several auto-fuses and four thermostats.  If the unit faults, then we simply turn off the diesel burner switch and turn it back on to reset it.  If it doesn't reset, then I start looking at the fuses.

[Joel Ashley]

Russ, consider also Pat Long's discovery of a kinked hose in his system.  Though his was preventing hot fluid getting to registers, age-softened, poorly-routed rubber hoses seem to confound coach systems regularly.  My experience has been with both aged fuel fill/vent hoses and even coolant ones on a brand new coach.  Easy to conceive a similar condition could be blocking your water pump line.

Joel

[Russ Mann]

Russ, consider also Pat Long's discovery of a kinked hose in his system.  Though his was preventing hot fluid getting to registers, age-softened, poorly-routed rubber hoses seem to confound coach systems regularly.  My experience has been with both aged fuel fill/vent hoses and even coolant ones on a brand new coach.  Easy to conceive a similar condition could be blocking your water pump line.

Joel

I'll carefully check that. I've had to replace a fuel filler hose and the coolant hose from the overflow tank to the radiator. The overflow hose was weeping through the wall of the hose just enough to cause a very slow leak with nothing on the ground. We're full timers so we never really noticed any deposits on the ground because we don't stay in one place very long.

[Orman Claxton]

Some Aqua-Hot will not have an Engine Preheat. 
On the road heat supply as below
While traveling on the road, leave the diesel switch off. Set the inside temperature to the desired point. Heat will be supplied  by the engine heat ( circulated by the AH pump)
I hope this helps