Still trying to stay on the green side
Another scenario I am considering. Assume I forego zoning which was suggested to allieviate the upstairs from going cold when I build a fire in the living room (2-4/mnth). The upstairs and downstairs convecting loops otherwise seem to be well balanced. On average, the upstairs is about 2*F cooler then what the prog. thermostat is reporting. (This could probably be corrected using the existing balancing valves on the system although I'm afraid to touch them as they are original installs). With my system size, I think should be able to to run in condensing mode for a fair pecentage of the heating season. This could alleviate the need for a buffer and the possible IDHW. I would also relocate the thermostat to a more central location although closer to the front door in the centrl hallway.
The IDHW would be replaced with a high-eff. gas-fired direct-vent unit that could be located closer to where the demand is and save a run of @40ft of 3/4" insulated pipe run through unconditioned space. It currently takes about a minute to even start seeing a sensable temp rise in the master bathroom shower or faucets. Other bathrooms, laundry, dishwasher would also be served better. Would the benefits of this more centralized location for HW supply outweigh the efficiencies lost in going away from IDHW. Any recommendations on stand alone hi-eff HW heaters. Is there away to compare the EF value to the efficiencies of an IDHW?