Yes, as stated in the original post I am using the WinterDesignDay and SummerDesignDay with different temperature setpoints (75F in winter and 70F in summer). However I am still receiving the error 'effective heating set-point higher than effective cooling set-point' even though my regular day schedules are the opposite (70F for heating and 75F for cooling). The error happens during sizing.

For each type of design day, the cooling setpoint needs to be higher than the heating setpoint when using DualSetPointWithDeadband thermostat control method. You can have a heating setpoint of 75F and a cooling setpoint of 80F during WinterDesignDay, and a heating setpoint of 65F and a cooling setpoint of 70F during SummerDesignDay -- all while still running at heating 70 and cooling 75 during the regular simulation.

Is there a throttling range for the DualSetPointWithDeadband control method, something that equates to 'effective' setpoint? Is this hard coded or a user input? Haven't been able to find anything concrete in the E+ references. I have some zones where the operational setpoints are 70F heating and 72F cooling, I think this is actually causing my error and not the design days.