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2015 C403.2.6.1 Demand Control Ventilation (DCV)

Discussion in 'Commercial Energy Codes' started by rgrace, Nov 20, 2019.

  1. rgrace

    rgrace Sawhorse

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    Keeping in line with baffling consistency questions brought on by ever changing energy code requirements, I will venture into this arena in an attempt to gain some logical explanation to specific demand control ventilation code language and try to decipher exactly what the intent is. Note the thread title code section reference; the code section in the 2018 IECC is C403.7.1 and in the 2021 IECC is not yet known. This requirement has not been in the same location since it's birth in the 2009 IECC and I suspect will continue to relocate every time we have a new edition.

    Without going into too many details, I am trying to apply Exception 4 and I am struggling with the term "supply airflow rate." My understanding of "supply" is that it is a combination of outdoor air mixed with recirculated air.

    That being said - scenario - A2, dining area and kitchen area only. RTU provides 8,000 supply air. 3,800 of this is outdoor air delivered to dining area (required by IMC). 3,500 is transferred into the kitchen for use as makeup air for the Type I hood system (permitted by IMC and encouraged by IECC). If my "supply airflow rate" is 8,000 and I subtract my "outgoing transfer air" being used for makeup air (3,500), my total is 4,500 which is greater than 1,200 and would require DCV.

    If the DCV reduces the outdoor air significantly based on the actual occupancy of the space (see DCV definition), there would not be sufficient makeup air for the hood system. This conundrum would require that the design professional provide a dedicated outdoor air makeup unit for the hood exhaust system. This is in direct conflict with what C403.2.8 is attempting to achieve, which is to eliminate the practice of providing dedicated outdoor air units which sole purpose is to provide makeup air to hood exhaust systems (and of course, waste energy).

    Soooooo, if the term "supply airflow rate" was actually meant to be "outdoor airflow rate," the equation would look like this; If my "outdoor airflow rate" is 3,800 and I subtract my "outgoing transfer air" being used for makeup air (3,500), my total is 300 which is less than 1,200 and DCV would NOT be required.

    Think it's just been wrong from it's birth and nobody (code officials that is) has paid any attention to it? Thoughts? Comments? Corrections?
     
  2. mp25

    mp25 Registered User

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    I do not have an extended knowledge of HVAC controls, but I could see this being accomplished by a combination of sensors/logic boards. The DCV would really only control the system during the opening and closing of a restaurant when the hoods are not operating and when the CO2 levels are relatively low.

    The controls could work based off several inputs
    1. a pressure sensor in the dining space to ensure that the amount of air supplied is approximately equal to amount of air exhausted or connected to the hood exhaust fan - so that the two modulate in sync
    2. CO2 sensor for DCV

    whichever requires a higher amount of outside air would control the OA dampers.
     

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