Aquahot Issue

I've got an Aquahot 400D in my coach - but am experiencing some weirdness.    Everything seems to behave normally - i.e, the heat exchangers in each zone start to blow when the coach temperature falls below the temp set on the thermostat - and will begin to blow warm air and the temperature readings at the exchangers quickly rise to roughly 90 degrees (tested with a digital infrared thermometer).   However, after a very brief time (and before the temperature in the zone rises to the "set" temperature for the zone) - the exchangers start blowing cold air.   Once they start blowing "cold air" - they continue to blow cold air until they are shut off manually (i.e., turning the thermostat controller to off).   

This happens consistently - and in conditions that I would NOT consider extreme conditions (i.e., outdoor temperatures in the 50 - 55 degree range, thermostat set to maintain 69 degrees inside the coach)

I tried to post this question on the RV Hydronic Heater Repair Website.  Unfortunately,  my account appears to be stuck in "no man's land".  They appear to have my email address on file - so I can't create a new registration.  However, when I follow the instructions to recover the username/password of what must be an "existing" account - I get a message telling me that my username/password reset info has been sent to my email address - yet nothing arrives in my inbox (or in my junk mail files).     
Any input on my issue would be greatly appreciated.  Heck, I'd even be happy with an email address to try to reach the folks at RV Hydronic Heater Repairs to get my account access issue resolved so that I can try posting there!  

I see the RV Hydronic Heater Repairs website has a "contact me" button.  I'd send a message and tell them about the email problem and ask about the Aquahot problem.

ArdraF

How long has it been since the unit was serviced and the fuel filter changed?  As a full timer who has to depend on the AH for heat and hot water, ours is done every year.

It sounds like you have the electric element on. The electric unit can't keep up with the demand, but will give you the performance you describe. I would suspect that your diesel burner is not firing at all.