Demand control ventilation Co2 vs. occupancy. Which is more versatile?
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  1. #1
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    Demand control ventilation Co2 vs. occupancy. Which is more versatile?

    I have a number of buildings that have large internal heat gains like gyms, weight rooms, dance studios, pools, etc. Currently we run our ventilation controls solely on time schedules. We put co2 controls on a few units last year and are seeing pretty good reduction in energy use and cost.

    I'm thinking that co2 is more appropriate then occupancy sensors because of the elevated co2 release during physical activity. Also I feel that occupancy can lead to excessive cycling with areas like locker rooms and break rooms where people walk through often, but don't stay too long.

    Are the co2 sensors accurate enough and responive enough to handle all my zones?

  2. #2
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    We have tried both ways as well. IMHO, the CO2 does work better, but you still need to run the units and have the sensor in the return air. On smaller zones, like a classroom, we have had success with a sensor in the room.

    Occupancy sensors, we have programmed in a delay so the area has to be occupied for say ten minutes, to alleviate the problem of people just walking through.
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  3. #3
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    So it looks like CO2 works well with VAV systems, whereas an occupancy sensor would be better in a constant air volume system.

    As usual every building is a unique puzzle.

    Thanks!

  4. #4
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    Check your local codes, some areas require occ sensors to use Co2 reset of outdoor air.

    Personally I think occ sensors are useless, and Co2 is the way to go.
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  5. #5
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    Quote Originally Posted by orion242 View Post
    Check your local codes, some areas require occ sensors to use Co2 reset of outdoor air.

    Personally I think occ sensors are useless, and Co2 is the way to go.
    The issue we have is when it is a single zone area, say a high school gym, in order to use just Co2 you would have to put them in multiple locations and then decide to use a high select or average. If all the people are at one end of the area and the sensor is at the other the unit would be extremely slow to respond if at all.

    Do you have another method you would use in this instance?
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  6. #6
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    Trane has put together a good document explaining the fundamentals and the ASHRAE standards. http://www.trane.com/commercial/Uplo...-APG001-EN.pdf

  7. #7
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    We use demand ventilation on large zone and single zone applications. In California we have high ventilation requirements so it really pays off. We use, at the zone level, a two stage sequence where we will increase the space airflow first then increase the osa at the unit. Most cases the demand, in an office application, is an increase of occupancy load so moving air from around the building will take care of the demand ventilation requirement. We have been running some tests with various manufactures of co2 sensors and have found some interesting results. The dual beams have been best but are also the most expensive. We have had good results with the new ALC Rnet one we are playing with currently.

  8. #8
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    Quote Originally Posted by Ithos View Post
    Trane has put together a good document explaining the fundamentals and the ASHRAE standards. http://www.trane.com/commercial/Uplo...-APG001-EN.pdf
    Good find and thanks for posting it.
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  9. #9
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    just a thought, I went to a large gym with complaints about humidity. They used CO2 sensors but but failed to properly dehumidify. so in the winter they had humidity issues and in the summer same. reheats not working correctly, improper HRV set up...this was a designed build and along the way i think someone forgot how to design it...

  10. #10
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    Mar 2005
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    If you have a Mixed Air Unit with multiple zones, in my experience you are better off placing the CO2 sensor in the supply air stream. This will ensure you are delivering just the right amount of Outdoor Air. Let's use a school gym as an example, normally there are instructors offices, storage rooms and the like sharing the Gym Ahu. If you use the return air as placement for your CO2 these unused or less used rooms are going to dilute your CO2 concentration at the CO2 sensor.

  11. #11
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    Quote Originally Posted by silversurfer View Post
    If you have a Mixed Air Unit with multiple zones, in my experience you are better off placing the CO2 sensor in the supply air stream. This will ensure you are delivering just the right amount of Outdoor Air. Let's use a school gym as an example, normally there are instructors offices, storage rooms and the like sharing the Gym Ahu. If you use the return air as placement for your CO2 these unused or less used rooms are going to dilute your CO2 concentration at the CO2 sensor.
    Interesting concept. Wouldn't the supply air be just as mixed as the return air and even more so once you mix in outside air? Possibly with VAV's closing down/going to a minimum position there may be some strategy, but I would think you'd have to sense at those locations as well. I'll have to read the manual, but if you put it in the return air you have a mix of everything that is coming back from the building. Now ideally your return air CO2 sensor location can also sense the exhaust air so you're always sensing what's coming back from the building. As to using one sensor I'd still think the return air/exhaust air location is best. If it's mixed enough to be low by the time it get's to the return, then it'll be supplying low to the space. Granted it may not be low enough to dilute the space, but that's no different than mixing in outside air which I would think it's low CO2 content would actually dilute the reading more than a mix of return air with higher CO2 content, but not enough to start the control strategy to purge.
    If you don't have a common return/exhaust location you can sense from then I would have at least 2 CO2 sensors, one on the return and one on the exhaust and control to the highest reading.
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  12. #12
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    Briefing the document and going back to the OP, IMO the most versatile setup would be CO2 and IAQ or just VOC.

    http://www.greystoneenergy.com/webcu...ir300_spec.pdf
    http://www.bapihvac.com/content/uplo...hite_Paper.pdf

    I was really impressed with the Greystone sensors. I recall being in a building and seeing that the OA damper was wide open and sensor was recording high. Took awhile to find but I found they were painting in a room and although I could barely smell the paint in the room being painted the sensor picked it up and was purging the air.

  13. #13
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    Jun 2011
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    The honeywell jade economizers can incorporate co2 sensors as well as enthalpy. The enthalpy sensors can actually pull in mechanical cooling to control SA humidity. They are pretty easy to set up too. Most places sell them for comparable prices to the older analog econos.

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