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RE: [EnergyPlus_Support] Re: Help with VAV to Constant Air Volume
VAV is a luxuary system with individual zone thermostat control, and is not for beginners.
EPlus CBVAV was introduced a few yers ago to address the constant fan and variable speed operation.
Without the damper control, there is no variable air volume. All the components required for VAV operation should be removed. This also mean no endividual temperature control of the zones served by one AHU. One of the zone thermostat is used for AHU air temperature control, and all the other zones are slaved with a fixed proportion of the total duct air.
The reference node and conroller mixedAir are also more complecated. Many advanced users are still having problem with it. There are five setpoints and three maximum capacity values, and internal load variation to be matched. The match for internal load variation is to be done for each zone separately.
I am not sure how you set up the reference node temperature for the AHU. Normally, it should be at the output node of the fan.
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I have been look at the simulation of VAV system since 2007. The solution is quite simple, but no body has actualy doing it. The EPlus CBVAV model has solved that for the constant fan case.
If you wanted to know how good is your system performance, compare it with the result from the IdealLoadsAirSYstem simulation. No HVAC system can be more efficient than this model. If the IdealLoadsAirSystem result looked abnormal, there may some problem with the bulding geometry or construction entries.
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VAV system can run without the reheat, if you set the AHU temperature closer to the required zone temperature, and allow the variable damper to shut down to a lower minimum value.
I have not look at your constant.IDF yet. You can identify from the result that cooling and heating are on at the same time.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: energyplus.1984@xxxxxxxxx
Date: Fri, 18 Nov 2011 17:16:48 +0000
Subject: [EnergyPlus_Support] Re: Help with VAV to Constant Air Volume
I still have reheat. The File CAV.idf does not have VAV, just that i kept the node names the same as VAV.
Thanks,
J
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "energyplus.1984" <energyplus.1984@...> wrote:
>
> Hi Dr. Li,
>
> Thank you very much. Your comments are very helpful.
>
> Since i am new to Energy Plus, could you help by elaborating your comments. How do i change the reference node temperature? Also how do i make the zone air my zone load.
>
> I have posted a file called CAV.idf which has the Constant Air Volume system.
>
> Many thanks,
> J
>
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@> wrote:
> >
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> >
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> > To do the VAV conversion i converted Zone HVAC Air Loop Terminal Units to Constant Volume, With VAV, the reheat does not come on until the damper is at the minimum. Do you still have reheat? Fans to constant volume, If you do not change the Ref.Node temperature and still running it below the zone termperature requirement, the zone reheat may be on all the time. removed the damper. When the damper is removed, it is replaced with a fixed one, which divide the AHU air according to your setting. The zone air volume is no longer zone load dependent. When the zone load is increased, the reheat is reduced, if the reheat is running. My file is running without errors.
> >
> > This is not a coding problem. It is a design problem. The file you have posted is still in VAV mode, I think. VAV usually does not reduce overall enerage usage, because the power used for the control equipment (VSD, sensors and drivers) are not included. The main reason for the heavy energy usage is the low supply temperature and the use of reheat. The AHU is probably running all the time in Summer, because the ref node temperature is set below the OD temperature. Dr. Li
> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > From: energyplus.1984@
> > Date: Fri, 18 Nov 2011 15:15:02 +0000
> > Subject: [EnergyPlus_Support] Help with VAV to Constant Air Volume
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> > Hi,
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> > I converted my Variable Air Volume file to Constant Air Volume and my heating consumption has increased by almost 6 times. The weather file used is Chicago. What are the possible reasons that would lead to such a change?
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> > To do the VAV conversion i converted Zone HVAC Air Loop Terminal Units to Constant Volume, Fans to constant volume, removed the damper. My file is running without errors.
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> > Is my method of making it Constant Volume ok? Any help is appreciated.
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> > Thanks,
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> > J
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