Re: [EnergyPlus_Support] Residential heater sizing and cycling question

["Michael J. Witte" <mjwitte@xxxxxxxx>]

As Don and Rich have explained, the HVAC system timestep does vary, and you can 
report results at the timestep level, but this is not relevant to what you are 
trying to model.  Each HVAC timestep in EnergyPlus is a quasi-steady-state 
simulation.  For the gas heating coil cycling loss is modeled by looking at the 
quasi-steady part load ratio and making an adjustment to the efficiency through 
the "Part load fraction correlation (function of part load ratio)" curve in 
COIL:Gas:Heating.  

For example, if the load on the furnace from 9:00am to 9:30 am is 50% of 
capacity, it doesn't matter if EnergyPlus simulates two 15-minute HVAC 
timesteps, or 30 1-minute HVAC timesteps.  Each timestep will evaluate the 
efficiency at 50% part-load.  There may be minor differences in results when 
the timestep shifts down, but you should not need to be concerned with the 
length of timestep.  But, you do need to make sure the Part load fraction 
correlation curve accurately represents the type of equipment you are studying.

Mike




On 4 Apr 2007 at 10:30, taharvey2001 wrote:

> I'd like to be able to model a standard constant volume residential
> forced air heater in EnergyPlus and see the impact of Sizing on
> efficiency and heater cycling. 
> 
> I haven't been able to figure out how to model the heater
> appropriately in E+ or and make sure the heater has sufficiently small
> timestep resolution so the heater can cycle, and the difference in
> efficiency can be seen. 
> 
> As I understand, the HVAC system timestep is decoupled from the 15
> minute building timestep, allowing it to free-run with an adaptive
> timestep (down to seconds if need be) - is this true, and is there
> anything that need to be done to enable this behavior? Is there anyway
> to see the high-res HVAC transients in the output?
> 
> What is the best way to get a const. volume, const BTU heater to model
> residential forced air systems?
> 
> thanks,
> 
> troy
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
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> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the
> Documentation link and press the "search" button.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 


========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx





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