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Cooling Coil has leaving humidity ratio > entering humidity ratio

asked 2018-09-23 23:29:47 -0600

Siv's avatar

Does anybody have an idea as to how this particular warning can be rectified or what causes this issue ?

SizeWaterCoil: Coil="CHW CLG COIL", Cooling Coil has leaving humidity ratio > entering humidity ratio.
   **   ~~~   **     Wair,in =  4.120009E-003
   **   ~~~   **     Wair,out = 8.912150E-003
   **   ~~~   ** ....coil leaving humidity ratio will be reset to:
   **   ~~~   **     Wair,out = 4.120009E-003
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answered 2018-09-25 17:21:29 -0600

TomB's avatar

I have seen this warning, and the following info may help you understand the issue.

  • Obviously, a chilled water cooling coil cannot increase the humidity ratio
  • The erroneous Wair, out shown is the Humidity Ratio you've entered either on the airloop's sizing system, or on the coil object itself
  • The erroneous Wair, out is not calculated with reference to actual coil conditions - it is a design target, not a calculation result
    • The Humidity Ratio off the coil is limited by the air on conditions - which will vary with your design day conditions - you can't add humidity via the cooling coil !
    • EnergyPlus is saying to you "I tried to get the air off humidity ratio you've asked for, but sorry mate, it is not physically possible, so here is one that works"

To fix the issue, you need to understand what you are asking the coil to do, and if it is psychometrically correct. The relevant input settings should be found on OS:Coil:Cooling:Water and/or OS:AirLoopHVAC.OS:Sizing:System My approach is to leave coil settings on Autosize, which will mean that the settings in the OS:Sizing:System will dominate. On the OS:Sizing:System, check Central Cooling Design Supply Air Temperature and Central Cooling Design Supply Air Humidity Ratio. For bonus points, plot these on a psychometric chart vis-a-vis the air on conditions. To see if all the above is helpful to you, try setting the Central Cooling Design Supply Air Humidity Ratio to 0.00412 ('Gee wiz, that's dry'), see if the warning changes or goes away.

To understand the hierarchy in the sizing routines, see Sizing guidance The EnergyPlus documentation in Engineering Reference, has a illuminating section on the physics of cooling coils

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I reduced the supply air temperature and the issue was resolved. Looks like I was doing something which was not psychrometrically rational.

Siv's avatar Siv  ( 2018-09-28 03:59:51 -0600 )edit

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Asked: 2018-09-23 23:29:47 -0600

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Last updated: Sep 25 '18