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What is the difference between the Design Coil Load (Btu/hr) and the Nominal Total Capacity (Btu/hr) reported from EnergyPlus.

asked 2014-09-16 13:06:52 -0500

Celia KS's avatar

updated 2020-03-10 10:50:33 -0500

I am having difficulty formulating a response to USGBC about the difference between the Design Coil Load (Btu/hr) and the Nominal Total Capacity (Btu/hr). They are questioning why the difference between the two isn't 15% as required by ASHRAE 90.1 Appendix G. I have over sized all zones by the 1.15 at the zone level like EnergyPlus allows however on this report the difference between the Design Coil Load (Btu/hr) and the Nominal Total Capacity (Btu/hr) is 20%. I found online that the design coil load is indeed the sensible load capacity so this could be the reason that there is a greater percentage than 15% between the two. However, there is not much information on this. I would like to know a bit more about how each of these parameters are calculated in the software. I cannot find much online about it.

Does anyone have some insight on the difference between the Design Coil Load (Btu/hr) and the Nominal Total Capacity (Btu/hr) reported in the Cooling Coils table on the Equipment Summary report? Or direct me to a reference where I can find such information?

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answered 2014-09-16 16:34:04 -0500

updated 2014-10-21 11:43:19 -0500

Your design coil load already includes the 15% since it comes from the zone level. This is what should matter to USGBC.

The design coil load is what happens at the sizing peak load, with the actual inlet temperatures.

The Nominal Total Capacity, or rated total capacity, is what would happen at standard (rated) conditions. It's a way to do an apple-to-apple comparison, similar to what you'll find in manufacturers' catalog.

See page 1949/1987 of the I/O reference manual (v8.2):

Equipment Summary

[...] - Cooling Coils includes the nominal total, sensible and latent capacities, the nominal sensible heat ratio, the nominal efficiency, nominal UA value, and nominal surface area for each cooling coil. These values are calculated by calling the cooling coil simulation routine with the rated inlet conditions: inlet air dry bulb temperature = 26.67C, inlet air wet bulb temperature = 19.44C, inlet chilled water temperature = 6.67C.

For more information about the simulation routine, refer to the coils sections in the engineering reference manual especially pages starting at 402/1304 and at 584/1304

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Just a general comment that page number references are helpful, but they can change, so please include the corresponding software version.

MJWitte's avatar MJWitte  ( 2014-10-21 09:51:22 -0500 )edit

Also, if you just want to link to a major section you can use named destinations. But you can't for example link directly to the "Equipment Summary" heading within the "Output:Table"SummaryReports" section.

David Goldwasser's avatar David Goldwasser  ( 2014-10-21 12:58:54 -0500 )edit

Or try BigLadder's online I/O Reference...

MatthewSteen's avatar MatthewSteen  ( 2014-10-21 18:46:59 -0500 )edit

answered 2014-09-16 16:49:56 -0500

Archmage's avatar

The "Design" load is based on the coil entering conditions occurring during the entire Sizing process with all the design day information etc. The "Nominal" uses the coil entering conditions that are taken from standard rating conditions. The same size coil will have a different apparent capacity when subjected to different coil entering conditions.

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Asked: 2014-09-16 13:06:52 -0500

Seen: 5,227 times

Last updated: Oct 21 '14