Cooling Increase DCV and ERV Measures

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Hi eQuesters,

The next two questions have been asked on here multiple times, but I have scourged the list and haven't found a resolution. My building is a school in New England.

I have a DCV measure that shows an increase in cooling. My thought is that DCV is taking precedence over the economizer. Is there a way to force eQuest to have economizer priority or is there another issue going on?

The other issue is in the Heat Recovery. It also shows an increased cooling load for the building. I tried setting it on an ERV Schedule and limiting it to the heating season. I tried playing with the different controls. I know it's not due to the increase in fan energy needed since the majority of units already have ERVs, but the efficiencies are better than code (and I also tried messing with the fan energy). Someone on the bldg-sim list even suggested controlling the ERV with Exhaust DT and increasing the delta-T to 20F.

I couldn't get any suggestions to work. How can I avoid overheating? Or is there another issue?

Thanks in advance for any help you can give me.

Sincerely,

Netty

Netty Nina, LEED(r) AP BD+C

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DCV - For min OA - make sure you have some non-zero value - I think that
triggers the economizer. If you are using the min OA schedule - make sure
it is either .001 or -999 for economizer to work. Let me know if helps.

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R B
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I had been using -999. I just tried it with 0.001, but still overcooling. Maybe it's a different issue?

Netty Nina ERS

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I don't know if this is relevant to your situation, but we recently ran into a problem where turning on DCV increased the OA intake for a VAV system. The model is at an early design stage for core and shell and the OA hasn't been specified yet so we were using eQuest OA autosizing. The OA without DCV was sized to meet the critical zone requirement at the full design supply flow rate. When using DCV, the OA is sized to meed the critical zone on an hour-by-hour basis, and as the supply flow rate drops, the outdoor air requirement can go up if the OA fraction for the critical zone approaches 1 leading to an increase in overall OA. Lesson learned: don't rely on eQuest to autosize the OA as it does the OA sizing against the full design supply flow rate which may not be the worst case condition.

Cheers,
Dan

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

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Nina:

Below is a GBCI review comment on a project where I provided Enhanced
Commissioning services, but not the energy model. It might help.

7. Table 1.4.2 and the system Input Data reports for the Proposed model indicate
that demand control ventilation (DCV) was modeled for credit in the Proposed
case; however, it is unclear how the outdoor air ventilation rates were
determined in each case. Table G3.1.4 in the Baseline building column allows
schedule changes for DCV as approved by the rating authority, and as the rating
authority, GBCI requires that, for spaces taking credit for DCV in the actual
design, the outdoor air ventilation rates in the Baseline model must be modeled
using the minimum ASHRAE Standard 62.1-2007 rates during periods of peak
occupancy, and the outdoor air ventilation rates in the Proposed model must
reflect the outdoor air ventilation rates from the actual design during periods
of peak occupancy. For example, if credit for DCV is reflected for AHU-3 in the
Proposed model, the minimum outdoor air ventilation rates must reflect 497 cfm
in the Baseline case and 1,000 cfm in the Proposed case, based on the values
indicated in IEQp1: Minimum Indoor Air Quality Performance. For spaces taking
credit for DCV in the actual design, revise the Baseline model to reflect the
ASHRAE Standard 62.1-2007 minimum outdoor air ventilation rates and revise the
Proposed model to reflect the outdoor air ventilation rates from the actual
design. For all other spaces (no DCV control or DCV control modeled in both the
Proposed and Baseline model), confirm that the outdoor air ventilation volumes
are modeled identically, using the actual as-designed ventilation rates (not the
ASHRAE Standard 62.1-2007 minimum ventilation rates) in the Proposed and
Baseline models, as required by Section G3.1.2.5. In addition, verify that all
HVAC systems in both the Proposed and Baseline cases are modeled with zero
outdoor air flow when the HVAC fans are cycled on to meet unoccupied setback
temperatures, unless health or safety regulations mandate an alternate minimum
flow during unoccupied periods (in which case, the unoccupied minimum outdoor
air ventilation rates must be modeled identically in each model). Further,
provide the revised total outdoor air ventilation volume (in units of cfm) for
the Proposed and Baseline models in Table 1.4.2, and provide a revised Input
Data report for each system in the Proposed and Baseline models reflecting the
changes. Note that the total minimum outdoor air ventilation volume in the
Baseline model must never be greater than the Proposed model.
Regards,

Paul Diglio, CEM, CBCP

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