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COP value for a DX coil that results in a specific EER

asked 2017-01-25 19:52:40 -0500

Jim Dirkes's avatar

updated 2017-04-01 11:07:59 -0500

Dear forum, My past practice has been to derive the COP need to produce a specific EER (e.g., the EER specified as minimum for ASHRAE 90.1)

I was "cleaning house" recently and revised the spreadsheet where that COP is derived. Now it doesn't work :( The derivation is shown below.image description

Using Eqn #3c for this example, the derived COP is 3.497 to produce an EER of 10.0, but E+ thinks the EER is 8.7. Something in my derivation must be off, but after reviewing it about 100 times, I can't see it.

If any of you has an equation that works properly, that would be a great help. If you can point out my reasoning flaw, that would be even better.

Thanks in advance. image description

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answered 2017-01-26 12:37:57 -0500

Adam Hilton's avatar

updated 2017-01-26 12:39:16 -0500

EnergyPlus solves the performance curves at rated conditions and uses the multipliers accordingly. Check out page 807 in the Engineering Reference. I don't know if that's the only thing that's causing your numbers to be off, but I suspect that's at least part of it. Note that 'TotCapTempModFac' should be solved at rated conditions, not 'TestB'.

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Thanks for that.
Two thoughts in response: 1) I didn't think the EER calculation used the rating conditions at all. I might be wrong! 2) I've already got the EIR calculation in my spreadsheet, so I'll run those numbers to see what difference it makes

Jim Dirkes's avatar Jim Dirkes  ( 2017-01-26 14:40:15 -0500 )edit

Correct. The curves should be 1 at rating conditions, EnergyPlus doesn't assume that though and sometimes the difference between 1.001 and 1.000 can be significant.

Adam Hilton's avatar Adam Hilton  ( 2017-01-26 15:58:37 -0500 )edit

All is working well! EER, as it happens, does not need to know the rating conditions per se. The nominal capacity and airflow for the coil are the only values needed.

Jim Dirkes's avatar Jim Dirkes  ( 2017-02-01 15:40:40 -0500 )edit

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Asked: 2017-01-25 19:52:40 -0500

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Last updated: Jan 26 '17