[Bldg-sim] CAV to VAV issues in EnergyPlus

[Wahid Hamed]

Hi Ryan,

Check VAV minimum airflow set point within the zone which might be a big
reason to increase the reheat load and the cooling load accordingly.

Regards,
Wahid




Wahid H. Mohamed, LEED AP BD+C
Mechanical Project Engineer, Energy Modeler
Saudi Diyar Consultants
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On Fri, Oct 4, 2013 at 8:52 PM, Ryan Del Balso <rdelbalso at gmail.com> wrote:

> Thanks for the quick reply Daniel.
>
> I have checked the Report: Equipment Summary tables. Here all is identical
> between the two models, except for the Heating Coil for the zone I
> modified. For the VAV system, the Heating Coil nominal capacity is 28,369 W
> vs 68,845 W for the CAV system. All the other heating, cooling and fan
> systems are sized the same between the models.
>
> In the Report: HVAC Sizing Summary everything is identical.
>
> For the zone I switched the Time Setpoint Not Met has increased
> significantly for both cooling and heating, and during occupied and
> unoccupied periods.
>
> In the Outdoor Air summary: both mechanical ventilation and infiltration
> are minimally different in all zones (differences are in the hundredths).
>
> As for where the energy differences are:
> Fan Energy: slight decrease in overall fan energy (I have only changed a
> small zone)
> Heating: increase in heating energy
> Cooling: increase in cooling energy
>
> I think this sums it up. From this, maybe the issue is with the heating
> rather than the cooling coils. There is a significant difference in the
> heating coil nominal capacity.
> Ryan
>
>
>
>
> On Fri, Oct 4, 2013 at 12:29 PM, Studer, Daniel <Daniel.Studer at nrel.gov>wrote:
>
>> Ryan,****
>>
>> ** **
>>
>> In terms of debugging your model, it would be constructive to know where
>> the energy use differs: increased heating, cooling, or fan energy? Also,
>> you may want to take a look at the table file and double-check that the
>> coil and fan sizes, and design OA flow rates are identical between the two
>> models. It’s possible that in your conversion you modified something that
>> is causing your system to size differently (presuming you’re not
>> hard-sizing everything). That’s where I’d start.****
>>
>> ** **
>>
>> Thanks,****
>>
>> Danny****
>>
>> ** **
>>
>> Daniel Studer****
>>
>> Engineer | Commercial Buildings Research Group****
>>
>> National Renewable Energy Laboratory (NREL)****
>>
>> 15013 Denver West Parkway | Golden, CO  80401****
>>
>> ** **
>>
>> ** **
>>
>> *From:* bldg-sim-bounces at lists.onebuilding.org [mailto:
>> bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Ryan Del Balso
>> *Sent:* Friday, October 04, 2013 1:16 PM
>> *To:* bldg-sim at lists.onebuilding.org
>> *Subject:* [Bldg-sim] CAV to VAV issues in EnergyPlus****
>>
>> ** **
>>
>> I will start out by saying I am not an EnergyPlus expert. I am trying to
>> convert one of the DOE commercial building E+ prototypes from a CAV system
>> to a VAV system to determine the energy savings. I have modified the
>> baseline CAV model for one of the zones thus far to a VAV system, but the
>> VAV model is using more energy than the CAV model. I also have a lot more
>> unmet hours for both heating and cooling. I cannot figure out what is
>> causing these issues (I assume they are related). My current guess is that
>> I continue to use the same cooling coil (Coil:Cooling:DX:SingleSpeed) in
>> the VAV model, rather than converting the coil to Coil:Cooling:DX:TwoSpeed
>> as I see in most of the other DOE prototype VAV models.****
>>
>> My questions are:
>> 1. Is it ok to use the Coil:Cooling:DX:SingleSpeed in a VAV system, or
>> should I convert that to a Coil:Cooling:DX:TwoSpeed?****
>>
>> 2. If it I convert the VAV model to the Coil:Cooling:DX:TwoSpeed is this
>> analogous to modifying the baseline compressor to a variable speed
>> compressor?****
>>
>> Thanks for any help.
>> Ryan****
>>
>
>
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