economizer low limit
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  1. #1
    Join Date
    Jul 2008
    Posts
    31

    economizer low limit

    What is the recommended temperature on the low end, to use the minimum quantity of outdoor air, when controlling with outdoor air dry bulb temperature?

  2. #2
    Join Date
    Nov 2007
    Posts
    388
    I am not sure what your asking. Why do you care about the low end for the economizer changeover? The lower your setpoint, the less opportunity there is to use the economizer cycle.

    The minimum quantity of OA is based on the code requirements at the time the system was designed and has nothing to do with the economiser cycle, but does determine the minimum position of the OA damper. If unsure, I would recommend a 10-15% OA. Remeber, you need to take airflow readings to set this.

    If you're using a small tonnage RTU (< 15 TR), I would use a 55*F drybulb changeover temperature for the economiser. For larger sizes, I would look at look at OA enthalpy changeover or integrated differential enthalpy changeover.

    Here's some info on the different strategies:

    http://www.achrnews.com/Articles/Fea...00f932a8c0____

  3. #3
    Join Date
    Jul 2008
    Posts
    31
    Per the California Energy Code, the economizer should shut off, and minimum outdoor air should be used at 76 degrees OAT DB and above. The design heating temperature for the area is 30 degrees, and I can't find in the code at what temperature in the winter at which I need to use minimum outdoor air. You would not want to use 100% outdoor air at an outdoor air temp of 30 degrees.

  4. #4
    Join Date
    Jul 2008
    Posts
    31
    I'll answer my own question. At an outside air temperature of 55 degrees or lower, modulate dampers to maintain a supply air temperature of 55 degrees, for coolilng. For heating, drive outside damper to minimum, close relief damper, and open return damper.

    Not sure why I asked the question.

  5. #5
    Canada has a code for 15% OA for min pot. OA is set for 55F and mixed air 52F.

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