Schedules are not used for Heating design calculations which are based on a steady state analysis that does not account for timing. In cases where for example a maximum mechanical ventilation rate is defined and modified by a schedule for simulation purposes, for the steady-state Heating design calculations the same maximum mechanical ventilation rate is used continuously.
Note that this comment does not apply to the sizing calculations carried out before Compact and Detailed HVAC simulations because these are driven by the same IDF data as the simulation which includes schedules. Therefore for these HVAC sizing simulations the WinterDesignDay and SummerDesignDay day types define the values for the heating zone sizing simulation.
Cooling design calculations are of course dynamic and so schedules are required for these. It is possible to select the day type to be used for Cooling design calculations on the Calculation options dialog. By default the Compact schedule day type is SummerDesignDay for Cooling design calculations. For cooling sizing calculations carried out before Compact and Detailed HVAC simulations the SummerDesignDay day type is always used.