DES Option 1 Pump Power

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Hello, I am reviewing an energy model and associated LEED comment for a DES project that is modeled using Option 1 (stand-alone scenario), there is no specific reference to pump power within the guidance document. The LEED comment states:

"Since the project follows the DES guidance Option 1, only the Baseline secondary distribution loop from Appendix G, with the relevant pump and loop parameters from that secondary loop must be modeled. Additionally, since the distribution loop for hot water is a primary-only loop that includes both upstream and downstream equipment, this option is not available for purchased steam or hot water, and the Baseline Case hot water pumps must be modeling identically in the Baseline and Proposed Cases based on the Proposed Case values.

I believe the directive here is to model the baseline pumps identical to the proposed and equal to the proposed values. The LEED review comment states that the loop includes upstream equipment when Option 1 is specifically to exclude upstream equipment and model stand-alone. The energy source is purchased heat.

I think the problem is that I see that the steam coils in the roof-top units are modeled in the same loop as the HW terminal units. If I separate these into two loops I should be able to claim energy savings for the pump power associated with the HW terminal units. Is this correct or is it still required to model the HW terminal unit pump power identical in baseline and proposed cases?
Thank you.

Kevin J. Kyte, PE BEMP LEED AP

Kevin Kyte2's picture
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Hey Kevin (Doug from LaBella). I'm sorry, I only have 90.1-Appendix G '07 in front of me, but it was just 19W/gpm <120ksf, or add VFD's if >120ksf. What is the language now in ASHRAE 90.1-'10?

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Bobba_Fett's picture
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Kevin,
Here's my interpretation of the LEED comment: An Option 1 heating loop is equivalent to a primary loop upstream with a secondary loop downstream. However, Appendix G requires heating loops to be modeled as primary-only. The baseline pump power allowance is based on a primary loop only, and there is no way (according to the LEED comment) to separate the primary loop pump power (which is excluded) from the secondary loop pump power. Therefore, the reviewer wants to see identical heating loop pump power in baseline and proposed.
However, in your case, your district/campus loop is a steam loop which you could argue has no pump power, which could potentially justify modeling 19W/gpm for the baseline heating loop and your actual separate steam and HW loops (from a steam-to-HW heat exchanger, correct?) in the proposed. Unless you get a more thorough review comment or find a CIR explaining it better, I would try that approach and explain it in detail with the revised submission.

Regards,
Bill

Bill Bishop's picture
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The review comments seems to imply that a secondary distribution loop is necessary because of the DES guidance.
Stating, "..loop parameters from that secondary loop must be modeled."
It also states that, "..hot water pumps must be modeling identically in the Baseline and Proposed Cases based on the Proposed Case values."

Anyways, the review comment is confusing because it seems to be quite poorly written.
"..this option is not available for purchased steam or hot water". What option? Option 1?
Separate the upstream and downstream equipment and make primary loops out of everything it is then.

Kevin Kyte2's picture
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I've been pondering this one as well. I'm imagining that if we say "DES = purchased heat = boiler" then Appendix G would want to have 19 W/gpm pumps primary to the building. But on the other hand, that 19 W/gpm comes from an assumption of a boiler which may have a higher pressure drop than the HX or secondary loop connection in your case. If your proposed building has a secondary loop off of a campus distributed hot water system, then the allowance for 19 W/gpm is probably generous.

The wording is also ambiguous - the building could be considered "primary" if the connection to DES was a heat exchanger rather than a secondary pumping configuration. If you did go to Option 2, then the losses of the steam system or heating hot water system would be accounted for there.

I'd leave the steam vs HW argument out of it - you'd be claiming a pumping energy "benefit" in model space that ignores some real world losses for condensate recovery, losses in the piping, etc. Where the proposed is steam all the way to the coils, I think the baseline should also neglect pumping energy.

The interesting part of this project is that it sounds like it has district steam and primary heating hot water provided to the building? If so then stick with what the reviewer suggested.

If I'm not grasping the scenario and the building receives steam that is used directly in preheat coils and through a heat exchanger to create hot water for use in terminal units, then I think the 19 W/gpm should be allowed for the pumped portion only. I think this would also apply if the heating hot water was through a heat exchanger to allow for pressure drop of the heat exchanger.

What was your W/gpm for the proposed case? If the proposed is way less than the baseline I think we are missing something and the reviewer may be correct.

David

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The last scenario applies, just as stated, the steam coils were modeled in the same loop as the terminal units. I revised each into separate loops and separate meters to make sure they could be modeled as "primary loops". As you know, a portion of the building contains spaces that have minimum air change requirements. These spaces are modeled as CV per guidance using system type SZRH. Therefore only the spaces that do not have minimum air change requirements may claim energy savings on pump power for zone equipment. The W/gpm for the Proposed case is 11.7. Is this excessively low?

Kind Regards,
Kevin

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So I'm not sure if you are modeling this in Trace or eQUEST or... since this is in the general discussion forum, but in Trace I have modelled the secondary pumps in the Misc Accessories of Create Plants. Here you could model a duplex condensate pump (if it applies) and associate its use with equipment of your choosing (boiler, chiller, controls, etc...). Also, you can create a second boiler to account for just the boiler capacity and pumps for the HW side, and then have a second boiler for the steam-side with no pump, but use the Misc. Accessories as it fits your situation, but here's some advice...Only put in the portion of the capacity of the boiler size that fits that load. So if your HW loop is your primary heating source, as this capacity is used up, it will kick on the second boiler to accomodate any load above the primary heat source capacity. I've developed this method when I had multiple geothermal type heatpumps (W2W, W2A) on a couple LEED projects and it was considered by my LEED reviewer as the most conservative way to model my situation. I think that is the lesson here is to proceed in the most conservative fashion that you can defend and explain to the reviewer to get them to believe you've presented your model in the best light.

I am currently training myself in eQUEST, so I do not have any insight with that software, yet.

Doug Breese

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[edit-double post]

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