[Bldg-sim] CAV to VAV issues in EnergyPlus

[Ryan Del Balso]

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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