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Chiller is ON but Cooling Tower is OFF

asked 2023-01-30 09:37:00 -0500

Keigo's avatar

updated 2023-12-14 02:38:33 -0500

I got stuck over the weird behavior of my model. My model is a small office building with the following chilled water loop and condenser water loops.

image description

I bullet-point the situation.

  • The problem is that cooling towers and condenser pumps are off even when there is a cooling demand on the chilled water loop and the chiller is operating. The annual end use of Pumps and Heat Rejection is extremely small.
  • The problem occurred after I changed Pump Control Type from Continuous to Intermittent.
  • When there is a cooling demand on the chilled water loop, a cooling demand on the condenser water loop is 0.
  • There is a lot of Chiller Condenser Heat Transfer even though the condenser pumps and cooling towers are off, which makes no sense.
  • Chillers cool the chilled water even though the condenser loop is off.
  • Chiller Condenser Inlet Temperature is constant throughout the year (25.74°C for Chiller1 and 25.84°C for Chiller2), which is the weirdest thing in my model. Chillers cool chilled water without running cooling towers probably due to this constant condenser inlet temperature, but I don't know why the temperature at this node is constant.
  • Unmet hours are not detected even though the condenser loop is off.
  • Free convection in CoolingTower:TwoSpeed is not modelld.
  • Plant Loop Volume is autosized. I hardsized it to 0m3 as a trial, but the problem was not solved.

The charts below are one-week extracts of the simulation results in summer.

Cooling towers and condenser pumps are off.

image description

Chiller Condenser Inlet Temperature and Cooling Tower Outlet Temperature do not match at all.

image description

I have put the idf files here. Their version is V9-6-0. The model with continuous pumps is just for comparision. I closed the link.

I have modelled similar systems several times before, but this is the first time I have this problem. Any suggestion for solving this problem would be appreciated.

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answered 2023-10-06 04:41:41 -0500

Keigo's avatar

updated 2023-12-14 02:32:05 -0500

I think this is a bug. I updated my idf file to V23-1-0, but the bug still exists.

I have another mode which has the same bug i.e., there is a cooling demand and the chiller is on, but the cooling tower is off due to the weird system node temperature in the condenser loop. However, I could not reproduce the bug with simple ExampleFiles. I'm not sure, but some building size might be necessary to trigger the bug.

In the another model, the bug seems to have to do with the condenser loop temperature setpoint. The bug occurs when the Offcet Temperature Difference (i.e., the cooliing tower approach) in SetpointManager:FollowOutdoorAirTemperature is 5.6ºC, but the bug disappears when it is set to 0ºC, but the approach of 0ºC is impossible in reality.

I'd like to report the bug and leave it to the developers to resolve.


I uploaded my idf files showing the same error here. There are 3 different models in total. The access is restricted for now. Although the project names are hidden and the locations are changed for confidentiality, I can only pass the files to EnergyPlus developers to fix the bug. Please let me know your email address to grant access.

This is the bug I most want fixed now.

The bug affects all of my ASHRAE90.1 Baseline cases.

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Keigo's avatar Keigo  ( 2023-10-06 04:48:52 -0500 )edit

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Asked: 2023-01-30 09:37:00 -0500

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Last updated: Dec 14 '23