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Modulating boilers ALL vary the flame. Vitodens, and perhaps others, also modulate the circulator.
The Munchkin (without Vision 1 that allows outdoor reset) varies the flame to keep delta-t across the HX as close to 20° as possible during a heat call. You set a fixed supply temperature.
When outdoor reset is used, the flame is modulated to achieve the computed "target" temperature. Delta-t will not have an artificial fixed amount.
If indoor feedback is added to outdoor reset, the rate of change in space temperature and/or the difference between desired temp and actual temp will modify the outdoor reset target temperature.
If indoor feedback alone is used, the desire is to use the rate of change and difference between actual/desired to maintain actual=desired as closely as possible. Again, delta-t is not fixed.
Smooth modulation continues as long as the lowest possible firing rate does not exceed the high limit during a heat call (Munchkin without Vision 1), exceed the target tempeature (outdoor reset) or result in actual > desired (indoor reset or feedback). When this happens, the boiler cycles on and off.
Zoning only affects modulation inasmuch as how it affects the load. For example if just one small zone calls, you're extremely unlikely to get modulation until other zone(s) call and increase the load. Proportional control (TRVs) are superior to digital zones (valves or circulators) since nearly all zones will have some load at nearly all times. This allows smooth modulation until the load of the structure as a whole drops below the minimum modulation rate.