Question-and-Answer Resource for the Building Energy Modeling Community
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I would say it is almost definitely because of reheat. When you are using a standard VAV reheat system, you are dependent on the all the zones being roughly the same temperature, however, if one of your west zone suddenly experiences a lot of sun then the AHU will provide cooling to all spaces, thereby making all other spaces put their reheats on for comfort. If you vary the cooling temperature in the zones this will vary how much cool air the VAV has to provide to meet the load and so also vary how much reheat is needed in the other zones to keep them comfortable.
I am not sure why you would want to set your heating and cooling setpoints to the same temperature. I feel EnergyPlus might kick up an error if there is no deadband because the heating and cooling would probably be in conflict a lot, ie, both trying to heat and cool. I would put at least a delta T of 1C if not 2C. But it would also depend on your tolerances for your equipment. if the equipment goes on if there is a delta T in the space of 0.2C then you might be able to get away with 0.3C in the thermostat.
