Is hydro air more efficient than gas/oil?
I live in Pennsylvania and almost every large building would have what you are calling a "hydro-ai system. They utilize a hot water or steam boiler for the heating side and an optional chilled water system for the A/C system. I never worked on the chilled water systems but have 40+ years on the boilers. These systems work great as long as the system is designed correctly. I have only seen these systems installed in very large houses of the super rich since the installed cost is extremely high. They are not for normal size houses due to cost. I would recommend that you hire a professional engineer that specializes in these type systems specifically an engineer that does large buildings and ask for an overall assessment of the system including operational efficiencies and projected costs. My neighbor has a geothermal heat pump that provides heating and A/C and also partial domestic hot water heating tied to a duct system (forced air) at a fraction of my cost to heat and cool the same size house. My own preference for a 2500 approx. house would be a forced air furnace with A/C. (geothermal, if you can afford it). Your house may more than 1 system due to it's size.