We had the same '04 Monteray for 12 years and, bit by bit, got to know the Hydro-Hot pretty well! (Maybe it was drip by drip!)
Fluid: The old ones used green (Prestone). As Gerald wrote, it is a true waste of money using the red
in the systems designed to use the green.
Low Fluid: There is a float switch. The original in the HydroHots used in Montereys was made from "high quality" plastic. A metal (brass?) replacement was made. If you know it is full, you can "jumper" around the float switch and stay warm until a replacement is found.
What to add: At about year 10 we were having issues with getting consistent hot water. We agreed to a full trouble shooting sequence from John Carrillo at a rally in Bend. (
http://heatmyrv.com/ ). One of the steps was to analyze the fluid. It had too much Prestone and not enough water. John explained that long term, more water is lost to evaporation that fluid. So even if you add a 50/50 mix, it will gradually end up being too strong.
Less than hot water in the shower (1): Most shower valves, including the one in our Monterey have a mechanical setting inside that can stop the hot from being wide open. It is not a "thermal valve". It can be set to stop you from turning the hot water all the way up. Perhaps well intended if you worry that people will be burned, but for me it was a PITA.
Less than hot water in the shower (2): The reason for the John Carrillo full troubleshooting procedure was to find out why we could not depend on consistent hot water in the shower. In short, our version of the HydroHot had a "stir pump". There is a quick way to "test" it. The inlet hose has to be hot. The outlet hose from the pump has to be equally hot. It won't be if there is a complete failure of the little pump. Three technicians prior to John were sure the pump was working because both hoses were hot to the touch. One used a IR thermometer. The pump had a weird magnetic drive from the motor to the impeller. That means it can pump just enough to confuse anyone trying to measure the temperature of the inlet and outlet tubes. With a weakened or tired magnetic drive, it will pump enough to heat the hoses but not stir the coolant. John's approach was that you first "troubleshoot" everything else. Then, if there is still inconsistent water temperature, you change the stir pump. He put in one that was a lot beefier than the first two. Our hot water became hot and consistent!
Burner tubes: They crack. It must get hot in there! Ours was replaced twice.
Service techs: In twelve years, the Hydro-Hot was the most failure prone system and the second most expensive. No point in listing all the missed or wrong trouble shooting events, but the key to having it run safely and efficiently is a technician that knows what they are doing. They are had to find.
