Creating a primary/secondary chilled water loop correctly..

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Hi all,

I am new to the forum and have done a few energy models before. I come to you today for a little help on creating my baseline system. I am getting results that I am not sure about and I'm seeking a sanity check and perhaps your opinion.

I am modeling a 1300 ton system and as per 90.1-07 App G it must be primary/secondary with VFD's on the secondary pump(s) and the primary set of pumps riding their curve for the baseline. The proposed system is variable primary and hence only 1 set of pumps with VFD's. On the condenser side, the proposed has VFD's, while the baseline does not (perhaps where we are gaining all our savings). I have used 22 and 19 W/gpm for the chilled water and condenser water systems, respectively, as per App. G.

The proposed model is about 10x as less in pump energy as the baseline and while I'll be happy to take the % and move on I do not want LEED to bite me in the @$$ later on. Traditionally, a primary/secondary configuration adds a little bit of pump energy (maybe 10%), but not 10x as much. The system pump energy required in both systems is going to be not that different. Am I modeling this correctly? I put the secondary chilled water pump under the 'Cooling Equipment Plant Controls'.

I've been messing with this all day so I may not have explained myself enough, but any help you may have would be great and I can supplement more info if you aren't quite sure about something.



Jimmy Woodard 

jwoodard's picture
Joined: 2011-03-18
Reputation: 1

Yes, there is definitely something fishy here. In order to get 10 fold savings, a VFD needs to run at about 20% load or so - 100% of the time.


  1. Are you on the latest version? there was an issue with the LEED report OVER-estimating energy on certain items in the baseline
  2. Also, may want to check your hourly loads - either in the Building Cool/heat demand report or using the visualizer (if operating in full year mode)

If there isn't an issue there, I'd take a look at a "by hand" estimate to see if your numbers are in the ballpark. If not, it indicates something missing.


Perhaps you can attach a .taf if that doesn't resolve the issue and I'll try to take a look at it.

Bob's picture
Joined: 2010-06-30
Reputation: 475