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Discrepancy in values in running both Conventional and VRF systems in 2 different locations.

asked 2020-03-09 12:26:25 -0500

bmjohn gravatar image

updated 2020-03-11 08:04:37 -0500

I have run a model using VRF and conventional systems in 2 different locations from CBECC and OpenStudio. The Design Size Gross Rated Total Capacity happens to be the same for both VRF and Conventional but is different for another location. I am not sure why that difference occurs. Does anyone have any pointers? I will appreciate it.

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answered 2020-03-11 07:33:31 -0500

updated 2020-03-12 19:45:42 -0500

When autosizing components the simulation uses the weather data to determine the conditions used for sizing. This includes the zone load and coil inlet condition. I would expect the coil size to change with location. The amount of outdoor air will also impact coil sizing where zone and air loop equipment size OA quantities differently. You can review the eio file to see the zone loads and the html file shows the coil and outdoor air sizing details.

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Thank you, Richard, for sharing that information. I have had the opportunity to read some of your papers for FSEC.

If the above is true, I would assume that the "Design Size Gross Rated Total Capacity" should be different for all locations? In my case I found one location having the same "Design Size Gross Rated Total Capacity" for both VRF and Conventional systems, but when the location changed there was a drastic difference between VRF and Conventional system?

bmjohn gravatar image bmjohn  ( 2020-03-11 11:49:07 -0500 )edit
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Another difference is the performance curves used for each coil model. The CapFT curve is used during sizing to adjust coil size after the size is selected. For example, if the CapFT result at the cooling coil inlet WB and outdoor temperature used for sizing = 1.1, then the coil size is reduced 10%. I assume you are not using the same coil performance curves and therefore the coil size for a specific location could be different.

rraustad gravatar image rraustad  ( 2020-03-11 11:56:39 -0500 )edit

That does make sense, But in my case, I have used the same performance curves (FSEC) and still found the total capacity value to be different.

bmjohn gravatar image bmjohn  ( 2020-03-11 14:14:23 -0500 )edit

What other system are you using. I may need the inputs files to decipher this further.

rraustad gravatar image rraustad  ( 2020-03-11 14:35:35 -0500 )edit

I misunderstood your statement about the performance curves. Yes, you are right, the model uses different performance curves for VRF and Conventional systems. However, I am not yet sure why one of the locations the VRF and conventional models gave the same values for "Design Size Gross Rated Total Capacity" whereas the other locations shown a drastic difference in the values. I could share the CBECC-Com project files for the locations. Is there an email to which I can forward it to? Thanks!

bmjohn gravatar image bmjohn  ( 2020-03-12 09:24:29 -0500 )edit

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Asked: 2020-03-09 12:26:25 -0500

Seen: 75 times

Last updated: Mar 12 '20