Another DCV question - when to claim credit per G3.1.2.5?

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[This is a LEED/Appendix G question - not an eQUEST-specific issue. My apologies if this has been answered before. I didn't see it in the archives.]

Can a proposed design claim credit for DCV if DCV is not required for the proposed design per 6.4.3.9, even though DCV would be required for the baseline under the same criteria?

Proposed designs modeled for LEED and/or Appendix G can claim credit for reduced ventilation rates per the exception to ASHRAE Standard 90.1, G3.1.2.5.
The exception is "When modeling demand-control ventilation in the proposed design when its use is not required by Section 6.4.3.8." [The typo appears in App. G. 2007 - should be "6.4.3.9".]

My proposed design does not require DCV per 6.4.3.9, because all of the systems that serve spaces meeting the DCV requirement also have energy recovery, which is exception (a.) for 6.4.3.9.
Therefore, I would argue that the proposed design can claim credit for DCV.
However, none of the baseline systems that serve the same spaces meet the G3.1.2.10 requirement for energy recovery. The baseline systems do not meet any of the exceptions to 6.4.3.9.

Does the exception to G3.1.2.5 imply "...when its use is not required in either the proposed or baseline design"?
I'm hoping not, otherwise this is a blue Monday. Tell me how you feel.

Thanks,
Bill

William Bishop, PE, BEMP, LEED AP 

Bill Bishop's picture
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Bill,

I interpret this exception as "G3.1.2.5..Exception: When modeling
demand-control ventilation in the proposed design when its use is not
required by Section 6.4.3.8" for the baseline design (section G3.1.2 is
titled General Baseline HVAC System Requirements")

If I understand your situation correctly, the baseline systems in your
project are not subject to G3.1.2.10, and do not meet any of the exceptions
to 6.4.3.9. If this is the case, then I believe you have to model DCV for
these systems in the baseline. Proposed systems do have energy recovery, and
thus do not require DCV due to exception to 6.4.3.9, so DCV does not have to
be added in order for the project to meet G1.2 prerequisite.

To summarize, you have DCV in the baseline and no DCV in the proposed
design, which is reverse condition compared to that described in Exception
to G3.1.2.5. This means that the exception does not apply to your project
and you should model the same ventilation rates in the baseline and proposed
designs. Following this logic, if you add DCV to the proposed design, you'd
just break even with the baseline which also has DCV. The proposed design
might have an edge because it also has energy recovery, as long as the
associated fan power penalty is not too punishing.

Thanks,

Maria

Maria Karpman's picture
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Hi Bill,

You can model DCV for credit in the Proposed design. I believe the procedure is to model the Proposed as designed and to model the Baseline to ASHRAE 62 in those zones that have DCV. Also, you need to make sure that the DCV sensors are in the zones and not in the return ducts for multi-zone systems.

Cheers,
Dan

?
Daniel Knapp, PhD, P Phys, LEED? AP O+M

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The proposed design has DCV. I would like to use the G3.1.2.5 exception to take credit for it. Maria, you seem to be saying that I should model DCV in both the proposed and baseline, unless I didn't have DCV in the proposed, in which case I should model it in neither. Do I understand you correctly?

Thanks,
Bill

Bill Bishop's picture
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Hi Bill,

Ah, I understand your question better now. My reading is that DCV is required in the baseline design, unless energy recovery is also required by 6.4 I would model the proposed design with DCV + Energy Recovery as per the design, and the baseline design with DCV as per 6.4.3.9. It doesn't seem to me that the requirement that the baseline design meet the prescriptive requirements would be obviated by having the proposed design include energy recovery.

Cheers,
Dan

?
Daniel Knapp, PhD, P Phys, LEED? AP O+M

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I believe that whether or not you have DCV and energy recovery in the
baseline is independent on whether or not it is specified in the proposed
design. Since you have DCV in both baseline and proposed designs, I don't
see how it can qualify for credit. I don't believe that exception to
G3.1.2.5 intends to change the requirements on when DCV should be included
in the baseline. Instead I think it is just trying to say that ventilation
rates may differ between baseline and proposed models if there is DCV in one
and not the other.

Thanks,

Maria

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