Identical buildings have different HVAC load profiles

asked 2017-01-04 19:27:31 -0500

juneau
31 ●1

I created two copies of a midrise multifamily residential building in OpenStudio 1.13 and applied split AC with gas furnace to one and LG VRF measures from BCL to the other. Looking at the HVAC Load Profile tables in the OpenStudio output report shows that the VRF building has significantly higher monthly cooling and heating loads compared to the split AC building.

I checked the weather files between the two models (same), I compared the sensible heat gain components (same values, other than VRF shows air cooling and heating occurring with the zone equipment and the split systems show air cooling and heating in the terminal unit columns). From reading these boards, I understand that there is no way to dis-aggregate the load components in Open Studio, but I tried running the zone component loads report in EnergyPlus and did not see any differences there either. As far as I can tell, neither system is doing mechanical ventilation and the infiltration rates are the same. In July, the VRF building has a cooling load of 23.11 MBtu compared to 12.96 MBtu for the split AC building.

Any suggestions for diagnosing this issue would be greatly appreciated.

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answered 2017-01-05 14:37:22 -0500

mdahlhausen

9314 ●17 ●11 http://openstudio.net/

updated 2017-11-18 20:16:33 -0500

I'm assuming you mean there is an annual heating and cooling energy use difference between the VRF system and the Split AC system. Load implies a unit energy per unit time.

The differences are likely due to (1) differences in equipment COP and (2) part load curves, particularly on the VRF system.

I've noticed the default VRF curves in the OpenStudio object give high energy use values, so I altered the VRF measure to replace the default curves. The EnergyPlus example files have VRF system you can draw from.

Also make sure to model heat recovery if your VRF system has it.

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Thanks for your response. Yes, there is an annual energy use difference between the two systems and it's good to learn that the default VRF curves in the OpenStudio are not ideal. However, I am noticing a difference in what I believe to be the total heat gains and losses in the two buildings. I'm basing that assumption on the "Monthly Load Profiles" table in the results summary report for OpenStudio where the monthly cooling and heating load totals are reported in MBtu. The two buildings should be the same so I'm not sure why one building would have nearly twice the cooling load of the other.

juneau ( 2017-01-05 18:22:51 -0500 )edit

I tried to add the OS VRF measure to my run for the base building and I get the following error: "Weather Simulation requested, but no weather file attached" I confirmed that I have a weather file selected in the Site tab and the model runs fine when the OS VRF measure is not selected in the Measure tab.

I don't understand why adding a measure would affect the weather file.

juneau ( 2017-01-11 16:42:15 -0500 )edit

Someone else had a similar issue with the weather file on Unmet Hours. Deleting the project folder and moving the .osm file to a different location before opening seemed to clear the problem.

I can now run the OS VRF measure but I am unclear on how to replace it with the altered the VRF measure. Do I just replace the files in the scripts folder and re-run?

Thanks.

juneau ( 2017-01-11 18:48:23 -0500 )edit

you should be able to copy the measure to the "My Measures" folder. It shouldn't be changing the weather file. hmmm. You could try saving a copy of the model, and using the "apply measure now" option in the drop-down at the top to apply it.

mdahlhausen ( 2017-01-11 19:46:12 -0500 )edit

Thanks. I was able to load both measures (OS VRF & yours) and run them separately using apply measure now. I took out the DOAS from the OS version and also changed the supply fan operating mode schedule to onoff to cycle with the demand. Your curves do result in less cooling/heating/fan energy use compared to the default VRF measure, but it still uses twice the cooling energy of the system composed of individual split systems. I wonder if the issue is with the split system definition? It's from the DOE prototype building (14 SEER, AC gas heat). I've not found another one on the BCL.

juneau ( 2017-01-12 16:31:52 -0500 )edit

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