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Re: [EnergyPlus_Support] Gas Heating Coil Nominal capacity change with heating setpoints change


Dear Kelly,
You have not provided enough detail to understand your problem fully, although, based on your description, it certainly seems like the heating load should be smaller for a lower heating setpoint.  Nonetheless, there are many factors involved.
The information below is copied from my personal references on E+.  Most of it should be up-to-date, but I'm not completely sure about that.  Hopefully, it provides some ideas about factors which affect your model and result in similar heating capacity.
As always, I recommend creating numerous Output:Variables.  Wwhen you review the hourly values for, say, a DesignDay, you can probably figure out what is going on.
Finally, if you (or someone else on the forum) sees an error, please let me know.
Controls setting impact on AutoSizing
? Zone airflow is sized based on the combined Delta T of the AHU (System Sizing) and the zone (Zone Sizing).  i.e., if the AHU heating design leaving temp = 8C and the zone leaving temp = 35C, 35C will be used for the sizing calc.
? A zone with the same maximum leaving temp as the AHU will get autosized with zero capacity.  (The delta T for sizing = 0, therefore capacity = 0)
? For reheat systems, make sure the Reheat coil design outlet temp is consistent with the value used in Zone Sizing.
? Set Point Manager:Warmest affects the AHU leaving temp, but is not considered in sizing calcs.  Therefore, if the SPMgr:Warmest maximum Setpoint temp is lower than the AHU design discharge temp, you will have inadequate capacity. (i.e., the sizing calcs count on the full delta T, but the SPMgr prevents its full availability.)
? SetPointManager:OutdoorAirReset may not allow an air or water system to achieve the value used by autosizing.  For example:  The model uses 13C as the cooling coil design discharge temperature.  The OutdoorAirReset manager, however, is specified to allow 13C only when the outdoor air temperature reaches 40C.  A problem arises when the outdoor temperature on a DesignDay only reaches 28C, permitting the discharge temp to only reach, say, 16C.  This may cause inadequate cooling.
Autosizing Process in EnergyPlus
(with thanks to Mike Witte, GARD Analytics; the overall process is mostly reported in the eio output)
1. First, an ideal loads simulation is performed to determine design heating and cooling loads for each zone.   .EIO reports these as  Zone Sizing Information â??Calc Des Loadâ??
2. The respective heating/cooling sizing factors are then applied to these zone loads.  .EIO reports these as Zone Sizing Information â??User Des Loadâ??
Note:  This is the only place that the sizing factors are used.
3. Using the supply air temps from Sizing:Zone and the zone setpoint(?)  temp at peak load determine design heating and cooling supply flow rates. EIO reports these as Zone Sizing Information â??Calc Des Air Flow Rateâ?? and â??User Des Air Flow Rateâ??.
Note: User Des Air Flow Rate is based on User Des Load OR uses a hard value from Sizing:System if specified.
4. For each zone, the greater of the heating or cooling supply flow becomes the design supply flow for that zone. EIO reports these as Component Sizing Information, AirTerminal:SingleDuct:Uncontrolled, name, Maximum Air Flow Rate, value
5. For each air handler, the zone air flow rates are summed, or the user can override total system flow in the Sizing:System object
� EIO reports these as System Sizing Information, systemname, 
o Calculated Cooling Design Air Flow Rate [m3/s], value 
o User Cooling Design Air Flow Rate [m3/s], value
o Calculated Heating Design Air Flow Rate [m3/s],  value
o User Heating Design Air Flow Rate [m3/s], value
Note: Heating and cooling design flows are the same if it's a single duct system.
6. The greatest of the two User Design Air Flow Rates for the system will become the system supply flow rate.
? EIO reports these as Component Sizing Information, 
o Branch, branchname, Maximum Flow Rate [m3/s], value
o AirLoopHVAC, airloopname, Design Supply Air Flow Rate [m3/s], value
o Component Sizing Information, Fan:OnOff, componentname, Maximum Flow Rate [m3/s], value
7. At this point, the central coils are sized to meet the supply air temperatures specified in Sizing:System.  Coil sizes will also be impacted by the mixed air temp at time of design load. 
? For DX coils there's the added constraint that a DX coil will not autosize outside the range of cfm/ton that the model accepts (300-450 or something in that range).  So, it will figure out a capacity that meets the supply temp, then check if the cfm/ton is within range.  If not, the DX capacity will be adjusted up or down to fall within that range.


On Fri, Nov 20, 2015 at 1:06 AM, <mailto:hkllzhang@xxxxxxxxx>hkllzhang@xxxxxxxxx [EnergyPlus_Support] <<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
>  
> Hi, all

> 
> 
> Further to my previous question, now Iâ??m doing simulations about heating setpoints changing in DOE Post-1980 Medium Office model. VAV system is used in the model, DX two-speed electric cooling coil and gas cooling coils are installed in the central plant, and electric reheat coils are equipped at the outlet of VAV box. I tried to consider the influence of system autosizing on the energy performance. So for one set of simulations, I just change the heating setpoints, and leave system and component sizing (nominal capacity, maximum flowrate, etc.) as â??autosizeâ??in the idf file. For the other set of simulations, I fixed the sizing values as they were in the file with original setpoint, then changed the setpoints.

> 
> 
> With the heating setpoint goes down, the gas heating coil nominal capacity goes up, and electric reheat coil nominal capacity stays the same, which looks confusing to me. Could anyone help me to explain that?
> 
> 
> 
> Thanks a lot!

> 
> 
> Best,

> Kelly

> 
> 
> 




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