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Re: [EnergyPlus_Support] Fan Cycles On For No Reason

The occasional timesteps which are showing the max flow rate are a 
bug that has been squashed in version 1.0.1 (CR4427).  In your run, 
it's occuring in 24 timesteps out of an annual simulation, so the 
impact on results is quite small.  There is no way around this in 
v1.0.0, so you either ignore it or move to the v1.0.1 beta.

Regarding the short cycle operation, this can be modeled with an 
appropriate part-load curve on the DX coil and the fan.  EnergyPlus 
models each HVAC timestep as a quasi-steady state, so flows and 
temperatures are averaged over the timestep.  If the required run-
time is 2 minutes, for example, then a 10-minute time step would show 
an average supply air flow rate of 2/10 of full flow.  Cycling losses 
are accounted for using the plr curves.  These curves define the 
actual run time required to achieve a given part load cooling output. 
 So, it should be possible to model your system fairly well by paying 
close attention to the plr curves.

If you were to use this system type for ventilation purposes, then 
you would probably set the DX unit operation to ContFanCycComp so 
that the fan would run at full flow (as scheduled) to provide the 
desired OA, and the compressor would cycle as needed to meet the 
cooling load.


On 11 Apr 2002, at 17:05, Edward Lyon wrote:

> The attached file is a further modification of a project I am
> working on. I have a single zone with a cycling fan system for
> heating and cooling. I have been looking at the temperature and
> mass flow of the output node of the primary air loop.  The flow
> volume is usually very low because I have an oversized AC system
> that operates on very short cycles.  However I also observed that
> the flow volume is maxed at 24 times through the year with no
> apparent reason for the system to run at all and with neither
> heating or cooling apparent in the flow temperature.  The first
> occurance of this problem is approximately 01/03 2am.  

> 1. I think there may be a bug lurking here
> 2. I don't think EnergyPlus can properly model this system because
> of the short cycles.  At some times the fan does not really run
> long enough for the dx to charge and start cooling.  I was
> considering linking some ventilation air to the fan operation, but
> with the short cycles, I doubt the outside air volume would be
> correct. 

EnergyPlus Testing and Support      

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