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    Outside Air System

    There is a bit of controversy here as to what an Outside Air system is to be based on. This is the air that goes from a louver via plenum/duct to the unit, be it an air handler or just a fan supplying OA directly to a space.

    It's my understanding that Revit has the three core systems, Exhaust, Return, and Supply. Additional systems can be added but they have to be based on one of those three.

    Right?

    The consensus is that in a 100% OA system, that it should be Supply Air. If there is actual Return Air being mixed with the OA, it is then considered RA.

    Another opinion is that it should be RA all the time.

    Is Revit going to have an issue with SA going into and out of an air handler?
    RobDraw
    Senior Member
    Last edited by RobDraw; August 27, 2014, 02:28 PM.

    #2
    Is there not a recommended practice for this?

    Are there any adverse affects from having Supply Air going into and out of an air handler?

    Comment


      #3
      I use Return Air for extracted air being blown out of the building (because it is usually connected to other return air systems via a fan or AHU) and Supply Air for outside air going to other supply air systems.
      I do make separate Duct System Types for this so that no one connects into them with a normal extract or return duct.

      In AHUs the outside air connectors should be linked with the inside air ones, so you dont have to worry about defining the flows, its just the same as what comes in/out of your duct system (Unless you have recirc in the AHU?)
      josephpeel
      Forum Addict
      Last edited by josephpeel; August 28, 2014, 10:59 AM.
      "One must imagine Sisyphus happy." Albert Camus - "The innovator has for enemies all those who have done well under the old conditions, and lukewarm defenders in those who may ​do well under the new." Nicolo Machiavelli -"Things that are too complex are not useful, Things that are useful are simple." Mikhail Kalashnikov

      Comment


        #4
        I'm surprised that there aren't more responses to this.

        Is there something about the wording that makes it difficult respond?

        Or is it that the answer is so obvious that it is blowing in the wind? (Pun intended.)

        Comment


          #5
          Originally posted by RobDraw View Post
          I'm surprised that there aren't more responses to this.

          Is there something about the wording that makes it difficult respond?
          I'm not surprised at all. Your original question is a Revit question, but it's a question that really needs to be answered by a mechanical engineer who understands air systems AND Revit. Josephpeel is such an individual.
          Tannar Z. Frampton ™
          Frampton & Associates, Inc.

          Comment


            #6
            Originally posted by josephpeel View Post
            Supply Air for outside air going to other supply air systems.
            One engineer said that he would make it RA because it is negative pressure in the duct.

            The ones that said they would use SA had to agree that in anything other than a 100% OA system (one that recirculates some of the conditioned air) that same duct would have to be RA.

            I understand both sides. If you think about the system, it could be considered a closed circuit, even if it is 100% OA. The outside could be the duct that closes the system. Now I think it's really a matter of opinion as to where that circuit starts and stops. The obvious choices are at the AHU or where the circuit goes outside. If it is where the circuit goes outside then that's kind of an arbitrary location for that line. Is it then the circuit exits the building or when it enters? I tend to think in tangibles, so I like putting that line at the fan powering the system.

            The question that remains is what happens in Revit? Will it make any difference if I make the OA from the native SA or RA systems?

            Either way, in the end it will be called OA from the louver to the unit.

            Comment


              #7
              Consider adding Mixed Air to your duct systems and make it Return Air. That would help differentiate your two scenarios. Make Outside Air as Supply Air.

              It does matter to Revit when it comes to connectors in families. Most AHU mfr I've seen designate the outside air connector as supply air. AHU with just an inlet and outlet are typically return air and supply air connectors. Check out your mfr of choice and see how they build their families. Keep in mind that connectors are pretty simple to change if and when needed.

              You might also consider systems such as Relief Air, High Pressure Supply Air and Low Pressure Supply Air, etc.. The thought being you could differentiate quantities where different gauges, construction and insulation types may be applied. Useful in estimating. Also nice to use different colors for each of them. May not be necessary for the master template but could be useful for certain projects. Keeping it simple is better for most projects.
              John Karben | IMEG Corp.

              Comment


                #8
                I like the Mixed Air idea. The additional systems are probably more than we need but I will keep that in mind for our more complicated systems.

                I have very limited experience with manufacturer's families for AHUs as we don't usually get anything from them until well after I have created a simple family as a place holder. By that time, we usually just use the one I made rather than going through the process of getting the vendor supplied family approved for use.

                Can you please explain why you use supply air connectors for outside air beyond that is what manufacturers are using? At this point, I have little faith in those families because the few that I've taken a look at vary in quality quite a bit.

                Comment


                  #9
                  Of the three choices supply air most closely fit the bill as it is "supplying" air to the building. One way to think of it is with the AHU in 100% economizer mode. Supply air is from the OA louver to the space. Return and exhaust air is from the space to the exterior of the building.
                  kubsix
                  Senior Member
                  Last edited by kubsix; September 3, 2014, 01:59 PM.
                  John Karben | IMEG Corp.

                  Comment


                    #10
                    Yeah, thanks for that. I'm starting to get my head around that way of thinking about it. I just had to make an intake louver that just dumps combustion air into a boiler room. No fan. The only logical choice for the connector for the short duct attached to the louver was supply air. Why make a differentiation between that and one that is connected to an AHU?
                    RobDraw
                    Senior Member
                    Last edited by RobDraw; September 3, 2014, 12:05 PM.

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