District Thermal System

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Dear All,

I'm now working on a project using Districted Heating System provided by
a municipal system. The municipal system is working for a very large
areas providing steam for about hundred buildings. And my building is
one of them.

According to the latest update issued by USGBC
(https://www.usgbc.org/ShowFile.aspx?DocumentID=4176), the modeling will
be implemented by two steps. My question is about Step-2 (Aggregate
Building/DES Scenario). In step-2, the energy source of Proposed
Building is virtual on-site chiller representing upstream DC system. The
document also issues "The DES central plant itself shall always be
modeled as a total, entire unit." My understanding is that it requires
to model the central plant with the full capacity that is able to
provide heating for all of buildings in whole district. However, if
doing so, the energy consumption of the proposed building may be quite
quite high, since it includes the energy consumption of the whole
heating plant!! But my building is only one of the 100 buildings that
are heated by the central plant. It is weird and I don't think it is
what USGBC asked for. Does anyone know what shall I do for the proposed
building in Step-2?

Thank you.

May Xu, LEED(r)AP

May Xu's picture
Joined: 2011-10-02
Reputation: 0


The Laboratory has spent quite a bit of time modeling the thermal plant on the Texas A&M Campus with good success. Unfortunately, we ended up using some rather complex, interacting loop models with chiller on/off, staging algorithms, etc.

We have a number of papers and thesis on this at our web site "www-esl.tamu.edu" look under publications.

Jeff S. Haberl, Ph.D., P.E

Jeff Haberl's picture
Joined: 2011-10-01
Reputation: 0

Hi Jeff,

Thanks, I found many useful information there, it is a very good
resource! However, I didn't get relative answers regarding my question,
which is about the simulation rule updated by USGBC recently about the
district thermal system. Could anyone give me any suggestions?

Thanks a lot!


May Xu's picture
Joined: 2011-10-02
Reputation: 0

- the energy accounting should be based on the annual efficiency of the entire plant as it will be operated
But also that:
- the energy accounting does not need to include the energy consumption of the entire plant
- the energy modeling does not need to include discrete modeling of every component in the plant

Notice that while Appendix G dictates number, size, type, and efficiency of baseline equipment, this document offers defaults that are simple system level efficiency values.

While the document anticipates refinement and invites feedback to leedinfo at usgbc.org, my emails have received only administrative acknowledgments after 8 weeks.

So if you cannot confidently decide how to apply this document to your project, you probably need to do a CIR. They are not free but neither is time spent responding to the reviewers, remodeling, or both.

Good luck,

Paul Riemer

Paul Riemer's picture
Joined: 2011-09-30
Reputation: 0

Hi Xu,
I think everyone is wrestling with the new rules trying to figure out how to apply them?especially with respect to step 2.? You can query USGBC directly (at no cost) at leedinfo at usgbc.org.? I've found that the front line customer service people are not that familiar with the new rules yet either; but, they can elevate your question to the appropriate parties.? It just may take a little longer than usual to get an answer.? If your question is too project specific, they may tell you to send in a CIR instead.
Do you know the cost of your chilled water/hot water?that the municipality is going to charge you?? If so, I don't see the new Step 2?modeling requirements as a whole lot different from the old modeling rules.? The NEW?rules require a virtual chiller plant/boiler plant?to be modeled that is reflective of the actual plant's efficiency.??Under the?OLD rules,?if you knew the cost of the chilled water/hot water,?that?cost was?supposedly, inherently, reflective of the plant's efficiency as long as it incorporated all production costs (i.e. chillers, cooling towers, pumps, boilers, as applicable per utility) and could be used directly in your proposed model.??I would say this approach?still holds true for the?NEW?Step 2 model.? You may just need to do a little investigation with the utility and provide some back up data or a letter to LEED as a supporting documentation stating that all the relevant production costs are reflected in the cost
of chw/hw (assuming that's the case).?? If you wanted to dig a little deeper, you may be able to get (electrical and chw/hw?water production) metering data from your utility and confirm/determine an actual?cost from that.? The NEW rules mention monitoring?data as an allowable?means of determining the virtual central plant's efficiency.....it seems like?metered electrical input and production output?would fit this description.

----- Original Message ----

Julia Beabout's picture
Joined: 2011-10-01
Reputation: 3

Dear Bldg-Sim Community,

Keep in mind that "Price" and "Cost" always include non-energy items like
maintenance, overhead, profit, etc. That means using those values does not
often give an apples-apples comparison with energy efficiency. For that
reason, the new USGBC guideline seems to be clear that you may not mix the

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0

Yes, agreed, the new rules seem to imply that maintenance costs, overhead, profit etc?do not need to be included in the analysis (in contrast some old CIRs).??These costs?would typcially be included in?a commercial or municipal district system.? However, local campus central plants may or may not have?those costs?figured in.? Either way, it's worth inquiring about with the chw/hw provider.??If they?have?information regarding their maintenance costs etc it would be good to extract those from the cost.??It would certainly work in you?proposed building's favor and be more of an apples to apples comparison with the baseline building.?

----- Original Message ----

Julia Beabout's picture
Joined: 2011-10-01
Reputation: 3

As I understand it, under the new rules, you should never be comparing district CHW or steam to utility elec and gas. Correct me if I'm wrong.

District Vs. district, (inc all costs as given by provider) with only changed bldg systems (BUT baseline DX/furnace systems become CHW/HW types), envelope, lighting
District plant(s) modeled to get elec & gas Vs. un-modified PRM baselines (elec & gas).

Yes, agreed, the new rules seem to imply that maintenance costs, overhead, profit etc do not need to be included in the analysis (in contrast some old CIRs). These costs would typcially be included in a commercial or municipal district system. However, local campus central plants may or may not have those costs figured in. Either way, it's worth inquiring about with the chw/hw provider. If they have information regarding their maintenance costs etc it would be good to extract those from the cost. It would certainly work in you proposed building's favor and be more of an apples to apples comparison with the baseline building.

----- Original Message ----

Fred Porter's picture
Joined: 2011-09-30
Reputation: 0

Hi May,

I interpret the update differently. It does not state that the capacity must
be the same, only that the efficiencies must be the same. I believe it is
expected that you create a virtual onsite plant sized to replace the loads
normally covered by the DES. The virtual plant must have the same
efficiencies as the actual plant, but not the same capacity. In addition
you must implement a calculation or approximation to account for losses in
the distribution system. I believe that the statement "the DES central plant
itself shall always be modeled as a total, entire unit" is to prevent the
model from combining the virtual plant (to cover DES loads) with any
supplementary onsite plants that may exist. I don't think it is intended
that you model the DES at its actual capacity.

Anyone else have a different opinion?

Craig Simmons

Craig Simmons's picture
Joined: 2011-09-30
Reputation: 0

It seems that the intention here is that if a credit is taken for an exemplary LEED building for energy efficiency, it will have to overcome district energy system (DES) inefficiencies if they are present. But at the same time, you are able to get the minimum two EAC1 credits, to some degree regardless of how bad the DES performs - so not excluding most projects, but limiting the options for those projects with inefficient infrastructures. This recognizes some of the constraints that a campus project might encounter.

Likewise an efficient DES that may have efficient equipment, cogeneration, heat recovery or other efficiencies will benefit the LEED project and the team is rewarded for having built onto this type of system.

Step 1: Building stand-alone scenario - for EAP2 compliance as well as documenting up to two EAC1 credits. In both cases will be purchased from district energy systems for heating and cooling as appropriate. This I think was the discrepancy between Fred and Julia's posts - the USGBC document states on page 8 that the actual prices are to be used, so it would include the overhead and profit components for a commercial DES. For campus situations without a "price" you must calculate the equivalent value and it isn't specified if this includes the overhead components.

If your building used DES for only one source, then the other source would be modeled as proposed with the baseline equipment per App G.

So as Fred mentioned, it will be DES vs DES, chiller vs chiller, or boiler vs boiler, etc.

If only pursuing EAP2 and two or fewer EAC1 credits (and achieving them?), stop here.

Step 2: Aggregate building - for pursuit of additional EAC1 credits - here the DES will be modeled for electricity and fuel usage, and compared to a building with baseline heating and cooling equipment per App G.

The key point of the text is "virtual on-site equipment representing upstream" systems. So not exactly that system, but representing the LEED project's share of that system.

Putting some numbers to a cooling plant for example purposes, the intention is not for you to model a 500 ton load building being served by a plant with four 5,000 ton chillers. Neither is the intention that you should model the other buildings making up the 19,500 ton difference.

The simulation programs are generally able to use efficiency calculations based on the total plant, but imposed on a theoretical smaller "virtual" plant serving the load of the new project. This is a reasonable approximation to the share of the plant energy for the new project assuming the cooling/heating loads are following the profile of the DES in general. The virtual 500 ton chiller will have capacity specified to match the LEED project's demand, but with the efficiency curves and EIR of the existing 5,000 ton chillers.

Obviously the capacity of everything in the plant would have to be adjusted, not just the chillers and boilers - include pumps, cooling towers, etc.

There is a statement about Step 2 only providing up to four additional EAC1 credits over Step 1, up to a maximum total of ten credits (like normal). But my interpretation of the document is that even if you documented six EAC1 credits in step 1, you can only claim two of them unless you unlocked that potential by proceeding to Step 2. Is this everyone else's interpretation also?

Hope this helps!


David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

Thanks so much for all your replies. They help me a lot. Thank you!

May Xu's picture
Joined: 2011-10-02
Reputation: 0