You can't eliminate boiler cycling with a simple thermostat, or even
two of them. They have hysteresis.

Indeed. The idea is to promote slow cycling, perhaps using even a third or
half the heat bank capacity before attempting recovery. The questions is
whether the thermodynamic advantage of running at full pelt outweigh that of
the lower return temperature at the less efficient low burner setting.

We are considering two systems he

1. Radiators, low hysterysis single thermostat, modulating condensing
boiler.
2. Heat bank, v.high hysterysis double thermostat, unmodulating condensing
boiler.

The high hysterysis would lead to a modulating boiler become effectively
non-modulating, as the heat bank will be able to absorb whatever heat is
thrown at it until the off thermostat fires.

Which is better, running:

1. a burner at a low modulation rate, but with a lower return temperature to
promote condensing.
2. a burner at its full rated value, with long cycling (i.e. 10-15 minutes)
but with a higher return temperature.

This is all based on the understanding that modulating boilers are actually
more efficient at full rated value, than at their minimum level.

Christian.