Hi all

I realize that a new guidance document was released about a month ago
for dealing with District Thermal System, but I am in the process of
wrapping up a project that is using v1.0 of the Guidance for District
Thermal Systems, and I am getting a result that is really confusing me.
I apologize if this is repetitive, but I have browsed through the
archives, and while there was a lot of discussion on the DES Guidance
document, none of it seemed to address this particular issue I am
having. This project is registered under LEED NC v2.2.

For my project, I modeled the first step as outlined in the document,
with both the baseline and proposed models using the central steam plant
for heating (cooling is covered by DX coils). For this step, I am
seeing approximately a ~19% energy cost saving over the baseline model,
well over the minimum 14% required savings.

However, when I implement step 2 of the guidance, I set up the baseline
boilers as normal, and then set up a boiler for the proposed model using
the efficiency of the generating plant, accounting for thermal losses as
discussed in the document. The central plant has an efficiency of
approximately 85%, and coupled with the thermal losses of 15% for a
closed loop steam system, I am looking at an overall efficiency of about
73% for the virtual steam boiler in the proposed model. The boiler in
the baseline model has an efficiency of 80% per 90.1. So, since the
efficiency of the proposed boiler is lower than the baseline boiler, it
should come as no surprise that the energy savings are considerably
lower, and since the building has high heating and service hot water
loads, the overall energy cost savings reduces to about 9%.

My question is, does this make sense? I realize that the intention of
the document is to penalize buildings that use inefficient central
plants, but it doesn't make much sense to me that I am seeing such a
drop off in the overall result. Am I interpreting something in the
document wrong, or setting something up incorrectly? Or, is this the
expected result with a Central Plant operating at 85% efficiency? It
seems like a plant would have to be near 100% efficiency to see much
additional benefit from using a central steam plant if this is the case.

Thank you all for your help

Adam A. Gonthier