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Re: [EnergyPlus_Support] Re: Residential heater sizing and cycling question

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Re: Residential heater sizing and cycling question
From: "Michael J. Witte" <mjwitte@xxxxxxxx>
Date: Thu, 12 Apr 2007 13:21:26 -0500
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To elaborate further, EnergyPlus will not compute the Part Load Fraction curve 
for you.  It does not model the equipment at that level of detail.  You either 
need part-load curves from manufacturer's data (probably not available), or 
some outside model (could even be a spreadsheet) to generate the relationships 
between run-time fraciton and part-load ratio.  If I unsderstand your goal 
correctly, to use EnergyPlus, you would need 2 different Part Load Fraction 
curves, one for the single-stage furnace, and one for the two-stage furnace.  
Then, EnergyPlus will be responsive to oversizing impacts as explained by Rich.

To set up your HVAC system, I would recommend usining COMPACT HVAC:ZONE:UNITARY 
and COMPACT HVAC:SYSTEM:UNITARY.  These use a canned default Part Load Fraction 
curve, so run it once, take the expidf output file, rename it, and edit the 
Part Load Fraciton curve fit object for your two system types.  See the Intput 
Output Reference description of Compact HVAC for more details.

Mike


On 12 Apr 2007 at 8:15, Richard Raustad wrote:

> I get the feeling that you think you will see the heater turn on and off 
> as it does in a residence. You will not see the heater turn off except 
> when there is no load. What you will see is a reduction in the fraction 
> Heater Rate/Heater Capacity or part-load ratio (PLR). This is the value 
> sent to the Part Load Fraction as a function of Part Load Ratio curve. 
> The output of the Part Load Fraction curve is then used to calculate 
> runtime fraction as RTF = PLR/PLF. The report variable RTF is the 
> cycling rate.
> 
> 
> Troy Harvey wrote:
> 
> > Mike thanks for the reply.
> >
> > I'm not real familiar with the HVAC options, I normally use purchased air. But
> > I've look through the docs, and I'm looking for clarification.
> >
> > What I'm hoping to be able to simulate is the effect of over-sizing
> > residential forced air heaters, and the effect of variable BTU/air volume vs.
> > Const. BTU/air volume, and their impact on heater efficiency (due to heater
> > cycling effect on combustion efficiency, burner/flue reheat, etc). That is I'm
> > hoping that, if I oversize, I should see more cycling, and less efficiency. Is
> > this feasible in E+?
> >
> > 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
> >
> >
> > [Non-text portions of this message have been removed]
> >
> >  
> 
> 
> -- 
> Richard A. Raustad
> Senior Research Engineer
> Florida Solar Energy Center
> University of Central Florida
> 1679 Clearlake Road
> Cocoa, FL  32922-5703
> Phone:   (321) 638-1454
> Fax:     (321) 638-1439 or 1010
> Visit our web site at: http://www.fsec.ucf.edu
> 
> UCF - From Promise to Prominence: Celebrating 40 Years
> 
> 
> 
> [Non-text portions of this message have been removed]
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
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> 
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> 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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References:
- [EnergyPlus_Support] Re: Residential heater sizing and cycling question
  - From: Troy Harvey
- Re: [EnergyPlus_Support] Re: Residential heater sizing and cycling question
  - From: Richard Raustad

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