Performance Curves - independent variable boundaries

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When a curve output exceeds the limits (min/max output, OUTPUT-MIN and OUTPUT-MAX keywords), eQUEST throws a CAUTION message in the SIM file, something like this:


In curve: Variable Speed Drive FPLR the dependent value

is exceeding the limits.

Value/Min/Max/First time: 1.004 0.100 1.000 7/21/ 8

I'm curious, what happens when the input type for a performance curve is "Raw Data Points" and during the course of the simulation one of the independent variables is outside the range of the raw data points? For example, here's a curve from the library (BDLLIB.DAT) that's defined via data points:
$LIBRARY-ENTRY SmFricW3VSD-EIR-fPLR&dT CURVE-FIT Sml Fric 3App -044d2 35
$ 70/140 ton, 3F approach evap & cond; R134a extended 460V 10F superheat -044d2 36
$ EIR = 0.182, 0.64 kW/ton, COP 5.5 -044d2 37
IN-RATIO = ( 1.00, 0.75, 0.50, 0.25, $ PLR -044d2 39
1.00, 0.75, 0.50, 0.25, -044d2 40
1.00, 0.75, 0.50, 0.25, -044d2 41
1.00, 0.75, 0.50, 0.28 ) -044d2 42
IN-DT = ( 41.0, 41.0, 41.0, 41.0, $ ECT-CHW DT -044d2 43
31.0, 31.0, 31.0, 31.0, -044d2 44
26.0, 26.0, 26.0, 26.0, -044d2 45
46.0, 46.0, 46.0, 46.0 ) -044d2 46
OUTPUT = ( 1.00, 0.70, 0.46, 0.27, $ EIR ratio -044d2 47
1.00, 0.66, 0.39, 0.22, -044d2 48
1.00, 0.65, 0.38, 0.19, -044d2 49
1.00, 0.72, 0.50, 0.32 ) .. -044d2 50

In the data points, the minimum PLR boundary for the curve is 0.25 - what happens if during the simulation the PLR is 0.20? Does eQUEST use the curve output regardless (and is a CAUTION generated), or is the output at the minimum independent variable used?


Phil Jordan

Phil Jordan's picture
Joined: 2019-05-02
Reputation: 0

Hi Phil,
Good question! When a performance curve uses Raw Data Points (INPUT-TYPE = DATA), the coefficients for the curve are generated based on the data points and can be seen in the eQUEST Basic Specifications tab for the Performance Curve Properties. (You can generate the coefficients yourself mathematically, by using the LINEST Excel function for example.) Outputs for the curve will be determined even if the inputs (independent variable values) are outside the range of the raw data points. For example, a PLR of 0.2 and dT of 25F gives an EIR multiplier of 0.14 in the example curve below. The outputs could even be nonsensical if the simulation used garbage inputs. (An output of -0.2 using inputs of -1,-1.) I don't think there is a keyword that restricts the input variable values, but the OUTPUT-MIN and OUTPUT-MAX fix the output at those values rather than letting it go outside of those boundaries. Additionally, chillers have the MIN-RATIO keyword which defines the load for which the chiller will cycle on and off.


William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

[Pathfinder-EA-logo-2]T: (585) 698-1956 F: (585) 325-6005
bbishop at
134 South Fitzhugh Street
Rochester, NY 14608 [cid:image004.png at 01D50CA4.977450D0] Act on the climate crisis - the Energy Innovation and Carbon Dividend Act

Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

Follow-up questions:
If you specify the OUTPUT-MIN and OUTPUT-MAX will eQuest still through the CAUTION?

If you specify DIAGNOSTIC WARNINGS .. will eQuest still through the CAUTION?

Also where do you specify the DIAGNOSTIC keyword in the eQuest interface? I sometimes find it useful to specify DIAGNOSTIC DEFAULTS to see what hidden defaults might be leading to the results I am seeing. I know I can go through the BDLKEY.OUT and BDLDFT.DAT files to check the defaults but I don't always find what DIAGNOSTIC DEFAULTS prints in the .bdl file.

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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Jones, Christopher2's picture
Joined: 2017-10-12
Reputation: 0