Modeling Laboratory DCV and ASHRAE 62?

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Hi All,

Does anyone have clarity on modeling OA flow rates in laboratory spaces,
specifically how they relate to the LEED baseline requirement for OA
flow when the proposed model has DCV?

We have a client that wants a specific ACH rate (100% OA, once-thru
system) that is well above the ASHRAE 62.1 OA requirements for
laboratory space in Table 6-1. We would like to use VAV hoods,
occupancy-based exhaust modulation, etc. But since this is considered
DCV, is GBCI really going to require us to bring the lab OA volume(flow
reduction) in the proposed system?

Thanks for any advice!

James Hansen, P.E., LEED AP

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Sorry, that last sentence didn't make any sense (thanks Bill). Should
read:

We have a client that wants a specific ACH rate (100% OA, once-thru
system) that is well above the ASHRAE 62.1 OA requirements for
laboratory space in Table 6-1. We would like to use VAV hoods,
occupancy-based exhaust modulation, etc. But since this is considered
DCV, is GBCI really going to require us to bring the lab OA volume down
to ASHRAE 62.1 minimum requirements for the baseline model? Or will we
be allowed to set the baseline OA flow for the laboratory at the minimum
prescribed rates (say 8 ACH), and take credit for flow reduction in the
proposed system?

James Hansen, P.E., LEED AP

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Hi James,

In a recent project we used a combination of CFD, the 62.1 IAQ procedure,
and short-term exposure limits from OHS regulations to verify the ACH rate.
The VRP rates were comparatively low. For an acetone spill event we arrived
at a figure of 8 ACH. The baseline, then, would be the same. We had DCV on
the system, but not for lab spaces where ventilation flow was simply turned
down at night.

Shaun

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James,

See this thread for some background on the subject:
http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/2011-October/010327.html

Most likely, you are correct in thinking that you will have to model the Baseline with ASHRAE minimum requirements if you model the Proposed with DCV. Potentially, you could justify modeling the baseline using a local (or campus?) OA requirement if it is more stringent that ASHRAE 62.1.

If you keep the OA requirement identical between the baseline and proposed models, there are still opportunities for energy savings:

* from energy recovery (if it is not required in the Baseline per G3.1.2.10)

* from increased baseline fan power due to pressure drop adjustments (Return and/or exhaust airflow control devices, others in Table 6.5.3.1.1B)

* from VAV hoods (this is trickier, but if the baseline system type is not variable, such as System 3 or 4 per G3.1.1 exception c., the baseline flow might be autosized at a higher CFM that the proposed design, and will not be 100% OA. If you model a reduced OA requirement at night, which is typical and also required in G3.1.1 exception d., the baseline will still have to run at 100% design flow but with a lower OA%, while the proposed model can ramp down to save fan energy.)

Regards,
Bill

[Senior Energy Engineer 28Jun2012]

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Also - when you switch from a standard VAV system to a chilled beam system in a heat gain driven lab there is some reduction. For instance if your EHS mandated ACH is 6ACH, and thermal loads require you to go up to 8ACH to meet the cooling needs, that 8ACH has to be 100% OA (sine you cannot return lab air). A Chilled beam system would allow you to hold the air changes at 6ACH.

Vikram Sami, LEED AP BD+C

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Thanks for the replies everyone!

I forgot to mention that this is a LEED v4 project, and the verbiage in
90.1-2010 has changed a little bit.

With lab exhaust systems > 5,000 CFM (which ours has), you have to
either have a 50% effective heat recovery device, or the ability to
throttle back non-regulated zones to 50% of the exhaust / makeup air
when not used (there i a fancy equation if you want to combine partial
heat recovery and partial flow reduction). I think you also have the
option to provide direct makeup air that is simply tempered to within a
few degrees of the space setpoint.

So I guess this answers the question for me - I'll end up modeling DCV
for both the proposed and baseline (to 50% of peak ACH for baseline, and
whatever we decide for the proposed), along with heat recovery for the
proposed. I won't have to bring the OA for the baseline all the way
down to ASHRAE 62 values per G3.1.2.11 and 6.5.6.1/6.5.7.2. The other
option is to model 100% full ACH flow for the baseline system along with
a 50% effective heat recovery device, and see if that is more/less
efficient than the other baseline option.

Since G3.1.2.11 in 90.1-2010 now references Section 6, and 6.5.6.1
references 6.5.7.2 which gives multiple options for compliance, it
sounds like there are multiple baseline options for laboratory systems
with exhaust > 5,000 CFM in the new LEED v4.

I wonder if GBCI will provide a ruling on this to clarify down the
road...

James Hansen, P.E., LEED AP

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