With the derate and efficiency rating, the output would be about 100,000
btuh. This would be just a bit more than needed for radiant; but not
excessively so to make the boiler short-cycle. However, multi-zoning
opens only SOME of the zones. As they are probably slab floor with
embedded tubing; the temperature difference between the entry to the
slabs and returning remains steady when already near the room
temperature desired.

A common problem with low-mass copper boilers is the short-cycling as
SOME of the zones are closed. Supplying 100,000 btuh when only part of
the house is asking for heat means that the boiler hits the high limit
quickly and short-cycles as indicated. A Weil McLain Ultra and Buderus
boilers have built-in controls to compensate (at more expense.) If the
high limit is raised, the boiler water temperature will rise more, but
it will not solve the problem. There is no mention that the water has to
be hotter because the rooms are too cold.

Additional information describing the purpose of the multi-zoning IN
THAT HOUSE (not in general principles); for instance: the garage is only
heated to 40F, the living rooms have setback during the day and the
bedrooms are heated at night, etc. might indicate that the larger areas
could be heated in common, so the boiler input more closely matches the
demand instead running short-cycles. If only one or two zones are open
at a time, the boiler will short-cycle.

A 100,000 btuh boiler can not run long when supplying only 20,000 btuh
at a time. However, a cooler room can be heated by the slab storage of
another warmer room without running the burner. If the circulator and
burner only runs off the thermostat of the larger room that demands heat
most frequently, the smaller room would be heated by the zone
thermostat opening and closing the zone valve during that demand. This
is the easiest to set up without great cost of sophisticated controls
and modification. It is a master-slave condition, with the most
demanding room as burner master and the smaller rooms as circulator slave.

I hope this understandable in laymans terms.

IF you have any more questions on this subject PLEASE feel free to send me a private E mail as some folks detest the techinical aspects of heating and hate for me to post it here.