Modelling Naturally ventilated Parking

Same as the proposed in almost all cases.

I am assuming by naturally ventilated this means open, above ground floors. It would not be appropriate to have mechanical ventilation in the baseline case without mechanical ventilation in the Proposed case.

If you had an underground garage and somehow replaced the mechanical ventilation system with an engineered natural ventilation system then you might be able to make that comparison.

Marcus, followup question for the below grade garage:
Condition: I have a completely unconditioned garage located in the cellar. There is no heating. No cooling. Only an exhaust fan and an opening in the entry gate to the garage to provide unconditioned fresh air. This garage fan operates via VFD as there are carbon monoxide monitoring controls that allow fan speed to modulate based on those levels. I am NOT modeling this as a system as I am not required to given the fact that there is no cooling or heating (unconditioned spaces have neither heating nor cooling systems). The exhaust CFM and the fan power remains the same (as shown in the design dwgs) in the baseline and proposed cases. The only difference is that fan operating schedule: baseline is 100% on during occupied hours and the proposed case fluctuates based on a similar type of people schedule throughout the day.

The LEED reviewer has indicated this as a DCV system (?!) and has noted that I should be treating my exhaust fan according to the baseline fan power calcs AND the ASHRAE minimum ventilation rates. The reviewer is treating my system as DCV even though the ASHRAE application of DCV applies to CO2 sensors and modulating occupancy, not CO (related to toxins in the air; as a result of increased intermittent occupancy).

In my opinion, this should be treated as an exceptional calc following the method I described earlier because fans that ventilate only, ie garage exhaust fans, are NOT qualified as a fan system, meaning that the fan power remain equal in both modeling cases. Additionally, the rules for modeling DCV do not apply in this case, for the reasons listed above.
Please let me know your thoughts.

Technically it is a DCV as I understand the term but demand here is cars, not people. Sounds like you are using people as a proxy for cars.

It is definitely an exceptional calculation. You can find good guidance regarding garage usage in one of the ASHRAE Handbooks (can't recall which one right now). You will need to make sure this is not a local requirement (if it is you can't claim savings) and provide a good justification for the schedule you are using.

The Baseline fan volume should not exceed the minimum required ASHRAE 62.1 parking ventilation rates of 0.75 cfm/square foot. The Baseline fan power should be the same as the Proposed (unless you are using premium efficiency motors then the Baseline fan power can be a bit higher based on standard motors). I agree that G3.1.2.9 does not apply to this ventilation only system.

Thanks Marcus,
Technically yes, but in every place DCV is mentioned in ASHRAE it is only used in the context of CO2 monitoring based on people occupancy.
It is not a local requirement to provide CO monitoring. It does mandate minimums when using automatic detection devices and for continuous operation. The minimum ventilation rate is 1.5 CFM/SF (for continuous operation), twice that of ASHRAE. The User's guide indicates that the local code can be used in lieu of the ASHRAE when the code is more stringent (from IAQ prereq). Wouldn't that apply in this case?

It could. What is the minimum when using automatic detection devices?

It is not allowed to go below 0.05 CFM/SF. I never get down to that number because of limitations to the fan. The min VFD setting is 30% fan power which equates to a CFM on the fan curve greater than the .05/SF. . It reads that automatic operation of the system cannot go below that .05 cfm/SF and that garage exhaust systems shall be capable of producing 1.5 CFM/SF.

Sounds like you should be able to use the 1.5.

Since this crosses over into your EQp1 submission which must be consistent with these calculations I would recommend that you submit a clarifying set of questions to GBCI. Explain the situation and ask them about the baseline fan power and ventilation rate. You should also propose how you think it should be handled and seek their approval.