Sudden Increase or Decrease in indoor air temperature in energyplus

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Hi everyone. I have a residential building in Energyplus and the setpoint and setback are 22C and 18C for the heating season( I have 2 zones- one master and a slave). This case is in Ottawa(Canada) and because I've set the flows on Autosizing I have to define System: sizing Just from looking at similar cases in a cold climate, I set them like this: Preheat design temp.= 7C, Central heating design supply are temp.= 40 The result of flow rates are: maximum supply fan flow rate: 0.8 m^3/s,m maximum flow rate of the main zone: 0.7 m^3/s The problem is that when I open the variables file in excel I see a significant rise of temp. of the zones (specially the main zone) from set back to set point! for example, I have defined setpoint 18C from 10 pm until 6 am when people are asleep. Then I have defined the setpoint on 22 from 6 am to 9 am and then again on 18 when they are at work until 6 pm. the result is that in 6:30 the temp. is 18 and in the next time step (10 minutes later) the temp. of the main zone is about 19.4!! and in the next 10 minutes, it is 20 and then 20.5 and it does not reach 22 until 9 am! What is the reason for this strange high rising in temp. very immediately, and then it slows the process? What is wrong here?

And the more strange thing is that the temp. drops significantly from 21.5 at 9 am to 19 at 9:10 am when the people leave the house!! (I have defined no cooling for the heating season and cooling coil consumption is zero in results) and then from 19 to 18.6 for the next time step and the difference reduces as the time pasts!!!