Condensing Boiler with Indirect DHW

If I had to choose

I would go with the boiler whose installer services it in their sleep and comes when you call them. True of any boiler of course!



Your plan and approach is a sound one. Of the names you mentioned, I see Viessmann as the top of the line (although the price has dropped over the past year or so to be competitive), followed by Triangle Tube and Lochinvar Knight in a dead heat. This is not to disparage the other names, just a top-three listing and my opinion only. Others have theirs and with more hands-on experience than I ever will.



(I am on the design side but have installed my own heating systems, just so you know, and have specified all of the brands just named, a number of times each.)



What I like about Viessmann Vitodens is their materials (316 Ti or Titanium Stabilized stainless steel), and overall engineering. Boiler as System.



What I like about the Triangle Tube Prestige Solo: Designed by engineers from the ground-up, blank sheet of paper kind of design. Fire-tube (the others are water-tube), with higher water volume. 439 stainless, excellent quality and built-in ODR controls. 



What I like about Lochinvar are the controls and intuitive interface. The Lochinvar is built around the Giannoni heat exchanger, which to me as a relatively high pressure drop, compared to the Triangle Tube.  But the firing turn-down on the Lochinvar is 5:1. Viessmann and TT are about 3.5 to 4:1.  Turndown and low fire on milder days means a lot to me. All are good, the Lochinvar wins that score.



The Viessmann HX also is similarly helical in design, than the Giannoni, but heavier in material mass. Both make the need for a dedicated boiler pump much more likely.



(In theory, any of these boilers could operate with a single circulator if properly sized, but the realities of variable system flows, control valves opening and closing, etc. means that you really do want a dedicated boiler circulator with any of these boilers. Too little flow and the small water volume flashes to steam you see.)



The TT offering, because of the relatively high volume tank type heat exchanger is most forgiving and voted "least likely to need a dedicated circulator" in its High School year book.  But the way I like to see systems set up, a properly-sized boiler circulator will allow much greater design flexibility on the radiation/emitter side.



Still, it gets down to service and good installation. Who represents the brand you choose locally and with confidence. That would be my choice.



My $0.02



Brad

If not the Boston area

probably not.