You can view the effect of plant margins on pre-heat times within the Heat Loss Calculation
To view the various calculated preheat and plant margin time select the 'yes' option for the question "Show Intermittent Heating and pre-heat times"
A note on the calculation method (from the Heat Loss manual)
The operation of the plant under intermittent conditions is considered, using the equations in CIBSE Guide A.5. The building response factor (fr) is computed from the building U values, admittances and infiltration, this is used to compute the intermittent plant factor (F3):
F3 = 1 + 1.2 (24 - n) (fr - 1) / (24 + n (fr-1))
n = occupancy hours + preheat hours.
A table of plant preheat periods at various plant ratios is displayed, this enables preheating regimes to be examined. It will be found that the longer the preheat period, the lower the installed plant capacity. A plant capacity can be selected from the table to give the required balance between plant cost (low plant capacity) and fuel efficiency (short preheat periods).