CHW error message

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What causes the error below? The CHW input used all eQUEST defaults. The bldg is high-rise residential. The systems used defaults also (4-pipe FCU). Varkie Thomas
pro Maslak sim DOE-2.2-47h2 12/15/2011 15:03:51 BDL RUN 7

REPORT- ATTN Simulation Messages For Review HVAC Program WEATHER FILE- EPW ISTANBUL,-,TUR
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***ERROR***********************************************************************
In EL1 Sys1 (FC) , cooling coil leaving temperature must
be at least 6F higher than entering water temp.

***ERROR***********************************************************************
In EL1 Sys1 (FC) , cooling coil leaving temperature must
be at least 6F higher than entering water temp.

***ERROR***********************************************************************
In EL1 Sys1 (FC) , cooling coil leaving temperature must
be at least 6F higher than entering water temp.

***ERROR***********************************************************************

Varkie Thomas's picture
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Either your supply air temperature is set too low or your chilled water supply temperature is set too high.

Kelsey Van Tassel

Kelsey VanTassel's picture
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Varkie,

Your leaving supply air temperature off the coil must be 6 degress higher
than your chilled water supply temperature. If your unit fan is in the
draw through configuration, the leaving temperature off of your coil will
need to be your design supply air temperature minus the fan motor heat gain.

For example, if you have a leaving air temperature of 55 deg F out of the
unit and a fan motor heat gain of 3 deg F, then in a draw through
configuration, your leaving coil temperature would be 52 deg F which means
a maximum chilled water supply temperature of 46 deg F. If you switched
the fan to blow through, then the maximum chilled water supply temperature
would be 49 deg F.

Does that answer your question?

-Robby

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Kelsey, Robby,
Thanks. The supply air temp was too low. What CHWS temp does eQUEST assume? This should be an input value too, besides temp rise.
Varkie

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Varkie,

On wizard screen 3 of 3 under "CHW Plant Equipment" the setpoint is
defined. The default is 44 deg F.

[image: image.png]

In detailed edit mode this setpoint can be seen on the "Circulation Loop
Properties" screen:
[image: image.png]

Robby Oylear's picture
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Whoops, here's the image I meant to send with the detailed edit screen:
[image: image.png]

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Robby: This project is being done by an architect who used DD. I looked for the error in CHW plant and forgot to look at HVAC supply temps. I was mistaken, the Detailed Edit does allow you to set CHWS temp but not SD and DD. Varkie

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Does anyone know if there is a way to bypass or ignore this error? I've
worked on a few projects recently where we've been able to get close to
a 46 degree LAT from an AHU with 42 degree water. Not a big deal I
guess, but it would be nice to be able to be only 4 degrees above CHW
temp.

James Hansen, P.E., LEED AP

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Varkie,

The first screen shot I sent shows where you can manipulate the CHW
setpoint in the DD wizard. I'm not sure about SD wizard.

Robby Oylear's picture
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Attached is an article on cold air distribution. UW-Madison-Extension used to have a course on this.

Varkie Thomas's picture
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James -

I've had some luck getting around this one by applying reset schedules to the chilled water loop supply temperature so the "Design CHW Temp" defined on the "Basic Specifications" tab meets the 6 degree difference requirement, and then on the "Controls" tab set up a reset schedule that will reset the temperature of the chilled water loop to what you actually want the system running at.

Cheers
Chris

Chris Lowen
Energy Analyst
LEED AP BD+C

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