[EnergyPlus_Support] Re: CTF vs. CondFD results

["Liam" <liambrn@xxxxxxxxx>]

To answer my own question, CondFD results come within a few percent of CTF results when I dropped the timestep frequency to 10/hour.  A colleague suggested this.  It's counter-intuitive to me, however, as I would have expected more timesteps/hour to be better, as tends to be the case in ESP-r, among others.    

A conversation with the same colleague revealed multiple ways of interpreting this sentence in the documentation: "...A minimum value of 20 [timesteps/hour] is suggested for ConductionFiniteDifference...".  My interpretation is that there should be 20 or more timesteps/hour.  However, I think the intent may be 3 minutes/timestep or more, given that I had success with the second interpretation, only.  

Regardless, I am a little apprehensive about the fact that the CondFD model is so sensitive to timestep duration.  I'd hate to perform a sensitivity analysis each time I run a simulation.  

If anyone has experience with selecting an appropriate timestep regime with the CondFD algorithm, I'd love to hear about it. 

Thanks,
Liam O'Brien 

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Liam" <liambrn@...> wrote:
>
> Hi All,
> 
> I'm modeling a building of fairly heavy mass so I want to use the CondFD algorithm to better characterize transient effects.  A comparison between CTF and CondFD yields differences in predicted heating loads of 100% or more.  
> 
> I repeated the comparison for several of the example files, e.g., 4ZoneWithShading_Simple_1 and got similar differences.
> 
> Everything in the models is identical except for HeatBalanceAlgorithm.  I used 20 time-steps/hour for both algorithms.
> 
> Any thoughts as to what might be happening?
> 
> Thanks in advance!
> 
> Liam O'Brien
>




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