EQ Credit 1 (LEED NC 2009)

[grey25]

Hello, I'm not a mechanical engineer so forgive the low level of this question. I'm reading through the proposed changes in the new LEED 2009 (NC) Rating System. For EQ Credit 1 (Outdoor Air Delivery Monitoring), the requirements have changed. Actually a part was added.

Here is the old text:

"Install permanent monitoring systems that provide feedback on ventilation system performance to ensure that ventilation systems maintain design minimum ventilation requirements. Configure all monitoring equipment to generate an alarm when the conditions vary by 10% or more from setpoint, via either a building automation system alarm to the building operator or via a visual or audible alert to the building occupants."

Now, they added this:

"All densely occupied spaces must include CO2 sensors. If the AHU predominantly serves densely occupied spaces, outdoor airflow measurement at the AHU is not required. If the AHU predominantly serves non-densely occupied spaces, outdoor airflow measurement at the AHU is required, in addition to CO2 sensors in the densely occupied spaces."

I'm confused by the newly added material. I would think that one should monitor the airflow at densely occupied spaces because there is more CO2 being expelled by more people breathing and it more critical to measure airflow with the larger number of people.

However, they have it the other way around. They say to measure the less densely occupied areas.

What basic logic am I missing?

Best,
Patricia

I've uploaded the pdf containing the new redlined 2009 NC version if you want to look at the whole thing. The piece I'm asking about is on page 69.

Thanks in advance

 

[mauricestoker]

I think the idea is that the CO2 sensor will be more sensitive to shortage of outside air for densely occupied areas. The LEEDS provide "one size fits all" guidance, which is no excuse for exercising good engineering judgment.

I would never drop outside air measurement, and generally start design load calculations with the ASHRAE 62.1-2007 standards, whether densely crowded or not.

Unless LEEDS gives additional guidance on where the CO2 will be measured, and the system it is applied to, I would not follow it blindly. If you have several zones, and one is consistently occupied, especially when other areas are not, a CO2 sensor in the geneal return means nothing.

I think ASHRAE 62.1 gives a good explanation of how and why the CO2 standard was developed, not quite as dramatic as a canary in a coal mine, but as a good indicator of air quality. However, for an ME to go with this interpretation and ignore minimum required outside air requirements cannot be recommended for occupied areas. It would make sense with an unoccupied area, such as with a variable air volume system with night setback or occupancy sensor, but as an ME I would not be willing to stand in front of the customer to say I was doing it for LEEDS credit.

I would also never place an audible or visible alarm for occupants to show an alarm if outside air or CO2 did not meet requirements.The building system, through trending or commissioning will hopefully show that. Just my opinion, but I would not expect a general occupancy to react any differently to a low CO2 alarm than to a fire alarm. Maybe its because I have been doing bio safety labs for too long, but an alarm to a general, and unprepared audience, that says "be alarmed" is not always a good thing, like suddenly yelling "fire".

Bottom line is this appears to require design of densely occupied area(s) each as a single zone, and as a single zone have no CFM basis for verification of proper outside air. While densely occupied zones are generally treated as a separate zone for supply purposes, ignoring ASHRAE 62.1 requirement is not recommended. If several densely occupied zones adjoin each other, such as several conference or auditorium or assembly areas, that could be expensive. If you were to design a chuch, even a small one, you might have an additional air handling unbit to install.

It's a voluntary standard, ASHRAE 62.1 and local code/IMC are a different matter. If you are ever in a store or mall, you would not want outside air cutting back because CO2 is okay; idiots darting out and spraying "sampler" stenches or a few persons with spastic colons will not register for CO2. When it comes back to the ME, I would rather be able to support that outside air was met than say CO2 emissions were considered.

As a matter of preference, I use outside air as much as possible by economizer. I would not recommend cutting back on outside air, or monitoring outside air, unless the space is unoccupied. On the other hand, I've never lived in Jersey.

 

[GMcD]

Grey25: My take, as a mechanical building services engineer and a LEED AP (note "LEEDS" is a nice town in England), I understand the densely populated spaces are assumed to have inherently high minimum ventilation (fresh air supply) rates anyway, based on the design occupancy, so the measurement requirement is relaxed. In a less densely populated area where the fresh air component may be small relative to the overall air circulation rate, the measurement and verification of the minimum fresh air is more reliable to insure that adequate minimum fresh air is being supplied to the fewer # of occupants.

That being said, I think its just as important to have some kind of measurement and verification on high occupancy spaces so the right amount of fresh air can be supplied without penalizing energy use. Why dump in 15cfm per person when only half the design occupants are there?

I'm in agreement with Mauricestoker above- the ASHRAE standard specifies "minimums" - below that your are likely breaking the law, but the only limit on how much more fresh air you can bring in is dictated by energy requirements to treat that fresh air (humidify/de-humidify, heat/cool).

 

[grey25]

GMcD,
Thanks! I understand which is amazing.

Maurice,
Thank you as well!

Have a great rest of your week, both of you!
-Patricia