I had a love-hate relationship with my HydroHot! It broke a lot and consumed a lot of part$. Three years out of the 12 we had it, it didn't work very well. Water temperature was inconsistent. Showers were OK with a lot of valve twiddling. In hind sight, it was usually a stir pump problem. When it was working, it was as good as in a stick house, which is the love part.
The first thing to check if hot water is not doing what you expect is the outside shower. If you close the valve on the shower head and leave the knobs on, it mixes hot and cold everywhere else.
If that is not it, based on my experience, inconsistent water temperature is probably a failed or
partially failed stir pump.
(I think stir pumps are unique to the HydroHot. I don't think the bigger AquaHots had them.)
Stir pumps are hard to trouble shoot. I had three of them. All it does is keep the coolant circulating. Due respect to Gerald, but you can't check it by seeing if the outlet hose is hot. As he said, the pump has a magnetic drive. It is a crappy magnetic drive. If it is functioning at even a 10% flow rate, the hose gets hot, you think it is working, but nothing is getting done. I had two different "professional" AquaHot techs declare my stir pump was fine. One used an IR meter. Then one day, John Carrillo performed a full, end to end, troubleshooting of the entire system and showed me it the stir pump was not working. (
https://heatmyrv.com/).
I replaced the mixing valve once too. On the HydroHot, you can't reach it with any normal arm and have the leverage to exercise it. Even if you can exercise it, you can't see to put it back where it was. You have to crawl under and take the back panel off. When I changed it, it didn't fix the inconsistent lukewarm problem.
Another issue can be the coolant. John Carillo explained that, over time, water evaporates faster than the coolant it is mixed with. When we add the 50/50 mix, the effect is that it gets a too high level of coolant to water. If you have to add a little, it is better to use distilled water than a mix!
Related is the coolant we use. The coolant used in the HydroHot around 2000 used the green poison, not the "safe" stuff used in later AquaHots. Add enough of the safe stuff and the mix will be wrong resulting in lowered ability to heat water or air.
Regarding the either water or heat option, the HydroHot is big enough to do both unless maybe the water is icy cold. To "fix" it, it is a simple and short jumper on the board. (No, I can't remember which two pins!)
Maybe more important than anything else, is that the burner tubes crack. I bought two when the cracks were still small. I never found out what happens if the cracks get big.
Other repairs included needing a new (extra large) "radiator" cap twice, a bearing deep inside the burner, the electric element, the electric element relay, the high temperature limit switch and cracked plastic plumbing in the back.
When we sold it, I swear it was working perfectly. The buyer, Rick Lalande, immediately had trouble with it.
