We had the HHE-500 in our '04 Monterey for 12 years. Although it was a grand adventure with lots of good trips and rallies, the Hydro-Hot was my most disliked system. When it worked, it was nice. When it didn't..... "Hate", "anger" and "frustration" come to mind.
My guess is that the stir pump is in need of replacement. Now for the rest of the story.....
The HHE-500 is a relatively small version of the various models. It uses a smaller coolant tank than the more expensive models. In order to get the smaller tank to function it has to have a working and dedicated stir pump. It is very small and the original has a weird magnetic drive between the motor and delicate impeller. In our twelve years the stir pump was expensively replaced twice.
We had symptoms similar to the OPs at about year three. The first replacement was done by Beaver Coach Sales after they guessed failures of two other component$, including the Mix Valve. With some destructive dis-assembly of the first pump it was clear that the impeller had broken in to small pieces. It was "old and brittle". Because of the magnetic drive, the motor ran fine and confused the diagnosis.
In two out of the last three years we owned it, the hot water reliability was very strange. For a shower, we would have to turn on the diesel, run and run water long enough so the system was at full output. Then, if you stayed alert, you could adjust the shower as necessary to keep from burning or freezing. Taking an electric shower was out of the question. I had forgotten the repair from 7 years earlier. If I had, I might have replace the stir pump myself.
During the two or so years of frustration, three different techs tried to trouble shoot it. With intended humor and a smile, Orman was one of them. We were at Quartzsite. The assumption was that if the hose coming out of the stir pump was hot, it was pumping properly. Orman used his new (at the time) IR gun and measured the temperature of the outlet hose. I used my finger. We agreed it was very hot. Roger Burke's trouble shooting guide suggests the same thing by saying if it is hot to the touch, it is probably working. (Note that Orman did not ask for any payment since nothing appeared to be in need of repair.)
At 7 Feathers 1 or "BAC(k) to the Future" a well known tech did a full service. I'm momentarily forgetting his name, but he was there as the available AquaHot service tech because Orman could not attend. He did a complete and routine annual service. Nothing could be identified as broken. Hot water reliability still sucked.
At the Ale Trail rally, John Carrillo was in town a few days before the FMCA rally. John used to work at the factory. He now has his own service company. At Jerry Carr's request, he came to our rally a couple of days and serviced several Beavers. When it was our turn, I explained our frustration. John explained that a complete and through trouble shooting procedure would be needed. Everything would have to be measured and tested. He included the idea that it was not possible to test the stir pump and if EVERYTHING else tested OK we would have to agree to GUESS that the stir pump needed replacing. What the heck! It was only money! I won't list all that John did because it would require even more typing. It took about 90 minutes to adjust, adjust, measure and clean everything. Everything testable passed the test. John ended up putting in a new, improved version of the stir pump.
I all but hugged John! The 12 year old Hydro-Hot functioned like it was brand new and continued to do so until we sold it. It was the stir pump again!
After John left, I hooked up the "broken" stir pump and it would pump water out of a bucket! I had assumed the failure to be the same as the first one and expected the impeller to be in pieces. I took it apart and the impeller was fine. The cause of the two years of crappy showers and luke warm water in the kitchen was the magnet in the drive was "tired". It was not so tired as to prevent a little coolant to pass and heat the hose for Orman to measure as hot, but it was too tired to pump enough to sufficiently circulate the coolant in the tank.
My happiness in the RV experience improved dramatically when we kissed the Hydro-Hot goodbye! Based on a quick price check, I think I can buy two new water heaters for the motorcabin for what I had to pay John to fix the stir pump in the motorhome!
