Series Chiller - Continuous Pump Operation

asked 2018-08-01 05:48:21 -0500

jt1234's avatar

I'm working with OpenStudio v2.6 / EPlus V8.9. I'm setting up a primary only, variable flow chilled system with two water-cooled chillers configured in a series arrangement (upstream/downstream). I have a single air-side system attached to this loop and have night cycling disabled (system should operate ~12 hours/day).

series chiller setup

The chilled water pumps function as expected, but what I've noticed is that the condenser water (cw) pumps run continuously during all operating hours. I have discovered that this is entirely driven by the series arrangement of the chiller evaporators. When I switch the chillers to a traditional parallel arrangement the cw pumps work properly. Here is the cw pump operation when in series:

cw pumps

I have the cw pumps operation set to 'intermittent' and I have a minimum flow of '0'. Adding a bypass pipes does seem to impact this result in anyway. The impact on the cw pump seems be be defined entirely by the evaporator side of the system with the chiller condenser arrangement having no impact. I have verified that AHU coil has no load and also verified that the chillers shutoff:

chiller operation

Has anyone else seen this issue?

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@jt1234 You refer to "cw pumps", so I am guessing that the condenser loop is configured with the chillers in parallel with separate branch pumps serving each chiller condenser? What specific chiller type? What is the chiller flow mode? It may be helpful to post the full idf file.

MJWitte's avatar MJWitte  ( 2018-08-08 09:30:58 -0500 )edit

The condenser water pumps ("cw pumps") are the where I'm seeing the issue. This issue isn't specific to one particular project as I've been able to replicate it for every project with a series configuration. The chiller type and chiller condenser configuration oddly enough do not seem to play into this. For reference, the chiller condensers are configured as series counterflow, but the issue appears to be driven entirely by the chiller evaporators arrangement for some reason. The Chiller Flow Model is set as 'Not Modulated.'

jt1234's avatar jt1234  ( 2018-08-08 09:46:33 -0500 )edit

It would be helpful to see the idf files. If you don't want to post them here, you can send to

MJWitte's avatar MJWitte  ( 2018-09-06 14:20:51 -0500 )edit