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Limitations/Advantages of 'Batch Heaters' like Versa Hydro
Given that my chimney guy is undergoing foot surgery that will seriously delay our proposed atmospheric boiler install, I'm having to again look at the Versa Hydro. I have just recently grasped that with the Versa Hydro you can keep the tank temperature at 140 for bacterial safety but then the variable speed DT circ will modulate down and sip the heat out of the tank to achieve the 85, 90, 100 degree supply temperatures desired. I've been told that the circulator modulation, representing the Versa's thermal efficiency, is the comfort part of the equation. However, in this scenario, the temperature at the combustion chamber will be at the higher temperature,--out of condensing range--which is what drives the combustion efficiency i.e. fuel savings.
So even if I'm not truly condensing--as I would be with a regular mod con and indirect, but I'm getting the advantages of the thermal efficiency and built-in 55 gallon buffer tank--reduced short cycling--, is the whole package still substantially superior in overall efficiency to an 82-85% atmospheric (ECM circ, zone valves, etc.) with either a standard indirect or turbomax reverse indirect? (Reverse indirect also presents the issue of constant maintenance of high water temperature in the tank.)
Of course I have to factor in additional installation and maintenance costs for the Versa.