How to Model Parkade CO2 Levels for Exhaust Requirements

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Dear building-sim,

I'm currently working on a LEED NC sports facility with an underground
parkade for approximately 200 cars.

The parkade exhaust system (100% OA) turns on if the CO2
levels detected in the parkade exceed a particular threshold. Has anybody modeled such a system? What
assumptions can be made for the CO2 profile? As it is in a northern climate, I
do not want to overestimate the usage of the system which would skew the heating
energy required to heat the OA to the required supply temperature.

Your help is greatly appreciated.

Thanks

Donal

Dónal OConnor's picture
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Hi Donal,

You'll want to control outside air off CO (not CO2) in a parking garage and be sure the controls sequence utilize the correct ppm setpoints. Generally there is a minimum ventilation/exhaust rate to maintain even when CO levels are in check for code compliance.

For modeling the system, a specific ventilation schedule correlating to expected usage of the car garage has worked for me in the past. I would imagine similar schedules for this facility have been created for occupancy and lighting/plug loads.

Regards,

Bob Quitadamo

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Donal,

Yes, the fans are controlled based on CO, not CO2.

You would just create different schedules for the exhaust. The baseline is constant flow during the occupied hours plus 1 hr before and after occupancy to capture people arriving early and leaving late. ASHRAE 62.1 requires 0.75 cfm/ft2 of outside air flow. You'll have to determine what flow rate your local code requires.

For the proposed model, create a new air flow schedule that gets you approximately 80-95% energy savings. There are several reports online that provide these ranges. Look for the ones from building owners or researchers, not manufacturers that claim 98-99% savings. The 80-95% range is based on a number of studies of these systems, as the fans spend most of their time at minimum flow, ~0.15 cfm/ft2. In general, if the building operates many hours you'll be closer to 95%, and 80% if the building operates few hours.

I like to create a profile so I capture demand charges properly rather than a flat average curve. For an office space, you'd have peaks from 7-9, a bump from 12-1 for those people going out to lunch, and another peak from 4-6 as people go home. For a hospital parking garage I just modeled, I had peaks from 11pm - 1am, 7-8am, and 4-6pm to capture shift changes. You'll have to decide what makes sense for your specific project.

Hope that makes sense.

Fred Betz PhD., LEED AP

P.S. CO demand control ventilation will be included in the Baseline model for ASHRAE 90.1-2010 per section 6.4.3.4.5 Enclosed Parking Garage Ventilation. Enjoy the model savings while it lasts!

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Be prepared to argue your case with the LEED reviewer. On the past several projects for which I tried to take credit for CO control of garage exhaust fans, the reviewer wanted me to provide a list of newly constructed buildings in my area that had non CO-based control of parking garage ventilation systems. They want you to prove that it is not common practice in your area. In the DC area, almost every engineer I know is doing CO control of garage exhaust fans, so we've had to abandon this ECM for all of our LEED projects. Would love to hear of someone arguing this point successfully, and what tactic they used!

James Hansen, P.E., LEED AP

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Donal,
We have discussed this previously so I suggest reviewing both the Bldg-Sim and equest-users archives. I vaguely recalled posts about GBCI pushing back on crediting CO-based control, so I found one and pasted it below.

As for 'while it lasts' - if addendum bm succeeds in rolling Appendix G back to align with 90.1-2004 then it seems like CO-based control of garage fans could be back in play as we simulate to demonstrate 40%-50%-60%-? savings vs. 90.1-2004 baseline.

Paul Riemer, PE, LEED AP BD+C

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Hi all,

Thanks for all of your responses! They're greatly appreciated!

Cheers

D?nal

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