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Re: [EnergyPlus_Support] Re: Residential heater sizing and cycling question
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
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Fax: (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu
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