Adding intentionally undersized hydronic supplemental system
The house is 5500 sf 1929 stone colonial with steam heat. Steam boiler is brand new together with all new near boiler piping and works perfectly and silently. Philadelphia, PA locale.
Just don't tell me "if it an't broke........."
From a "what if" hypothetical point of view, lets suppose that I put in maybe half or two-thirds the required radiation, basically whatever size baseboard or radiator conveniently and aesthetically fits the room. (Steam system has recessed convectors with finish plastered fronts-nice stuff from the Warren Webster Company late of Camden, NJ) Then run hydronic with the steam thermostat set well down to come on only if the water can't keep up. Maybe turn the water off mid winter, maybe not. Maybe toss in some hydronic element below the steam convector element inside the Warren-Webster enclosures for a totally hidden effect.
Using an intentionally undersized system would keep the radiation as small as possible so as no to detract from the decor. Plus, I wouldn't run heat to every part of the house. Maybe use pex-al-pex for the water.
My theory: It would avoid running the steam on mild days, saving substantially on fuel bills.
Well, the idea is not to modulate the steam, but to...
totally shut it off during most of the heating season to save the cost of firing up a 400k btu machine during moderate weather. Modulation of steam, when it's actually making steam is not a problem, conserving a little bit of steam when it's running through thermostatic devices is not going to make any $ difference in the slightest, I believe.