Difference in handling heating load and component sizing, EnergyPlus v7.2 vs. v8.2?
2015 03 25 - Added some new information at the bottom
I am currently using the 1.4 versions of the DOE Commercial Reference Buildings (2004 construction) for research purposes. They are provided as EnergyPlus v7.2 IDF files via the web. I recently converted them to v8.2 using the IDF Updater, changing nothing else. There seems to be a difference in how VAV reheat coils are auto-sized and handled. I first noticed this in the Hospital Commercial Reference Building. (Simulation was run in Baltimore climate and, again, 2004 construction).
Both the 7.2 and 8.2 versions of the file generate error messages like this when run:
** Warning ** Calculated design heating load for zone=CORRIDOR_NW_FLR_3 is zero.
** ~~~ ** Check Sizing:Zone and ZoneControl:Thermostat inputs.
However, only the 8.2 version generates the following error message:
** Warning ** The design coil load is zero for Coil:Heating:Water CORRIDOR_NW_FLR_3 VAV BOX REHEAT COIL
** ~~~ ** The autosize value for maximum water flow rate is zero
** ~~~ ** To change this, input a value for UA, change the heating design day, or lower
** ~~~ ** the system heating design supply air temperature
Looking at the html table outputs, the difference manifests in the coil sizing summary. The 7.2 entry:
Maximum Air Flow Rate [m3/s] Maximum Reheat Water Flow Rate [m3/s] Reheat Coil Sizing Air Volume Flow Rate [m3/s]
CORRIDOR_NW_FLR_3 VAV BOX COMPONENT 1.34 0.000367 0.403147
versus an 8.2 entry:
Design Size Maximum Air Flow Rate [m3/s] Design Size Maximum Reheat Water Flow Rate [m3/s] Design Size Reheat Coil Sizing Air Volume Flow Rate [m3/s]
CORRIDOR_NW_FLR_3 VAV BOX COMPONENT 1.34 0.000000 0.403147
As one can see, the difference is in the maximum reheat water flow rate.
More importantly, this manifests itself in the inability of these zones to be heated, which leads to an unrealistically low heating energy use, and unmet heating hours. I've gone through the IDF files in their respective editors, and can't seem to find a difference in the input.
So the primary question is:
1) How do I get v8.2 to treat the file the same as v7.2 and provide a maximum VAV reheat coil flow rate that is not zero, even though the calculated load is zero? (I'd prefer to find a way that isn't manually inputting explicit values, if possible)
With the related questions:
2) How and why does this happen?
3) Would this affect other things that I need to be concerned about?
---- New Information 2015 03 25
The difference also manifests itself in the number of occupied heating hours where setpoint is not met. For example, in the zone mentioned above, the 7.2 run had 0 hours, while the 8.2 run had ~5000. This leads me to not have confidence in the "new" evaluation of the zone's needs.
Some possibly relevant items from the "Known Issues 8.2" document are:
3580 The sizing algorithms for reheat coils in ...
