They refer to Hydro-Hot as the “Little-Brother” of the Aqua-Hot because they both provide a continuous on-demand supply of hot water, have a 3-zone interior hydronic heating system and engine preheating, although with Hydro-hot it’s an option and not a factory feature. The Hydro-Hot can provide the same heating features as the Aqua-Hot, when there is a demand for continuous domestic hot water, the demand period for the interior heating system is put on standby mode.
BTU / Size for Aqua-Hot vs Hydro-Hot
Both are 50,000 BTU diesel-burner and have 1650 watt 120 volt AC electric heating elements. Both are available in 12 volt DC voltage systems but Aqua-Hot also offers 24 volt.
So basically when it comes to servicing the Hydro Hot, it's the same as the Aqua Hot.