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Extract fan sdding up airflows incorrectly

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    Extract fan sdding up airflows incorrectly

    Hi good people of the internet. I have a problem with an extract fan I have sent to a client and am unsure of why it is happening (mainly due to the fact I have very little experience with the project-building side of Revit and only focus on making families).

    Basically if you see the image below the fan is extracting from two internal grilles (50l/s and 60l/s) and then extracting all 110l/s through the external grille. the problem is that although the airflow on the fan is set to 110l/s the airflow is showing as 220l/s in the project browser. Both the intake and discharge connectors on the fan are set to exhaust and calculated, and changing the discharge to Preset (still don't understand what it really should be, shocking I know) doesn't change anything.


    Can anyone shed any light on this please? I'd appreciate any constructive help.

    Last edited by mrgrotey; April 26, 2016, 01:46 PM.

    Calculated = Connector will take whatever flow rate is connected to it. The duct system tells the fan what the flow should be.
    Preset = Connector has a flow rate set by the family (Link a parameter to the Flow in the connector). The fan puts this flow rate into the duct system.

    I assume you want to just total up the flow rates from the extract grilles and show this in the fan. So use 'calculated' in the fan connector on the room side.
    It looks to me like the grille in the outside wall also has a flow rate of 110 and this is also being added to the total from the extract grilles in the rooms, so you have 220 in the fan. Check the flow direction in the outside grille (It should be IN to the grille from the duct) or try setting the flow of the outside grille to -110.

    I think really you should have the fan connector on the room side set to IN, Calculated (To get the total flow). Then have the fan connector to the outside set to OUT, Preset. Link 2 parameters to the flow on these connectors and use 'Airflow out = Airflow in' to set the flow to the outside. You dont actually need to give the outside grille a flow rate, just make that with IN, Calculated, to pick up the flow rate from the fan/duct.
    I made an outside air grille that also includes a calculation for 'Net Area' (Usually (Area-Frame)*0.5) and the velocity of the air (Air Volume/Net Area). That way you can check if the outside grille is large enough too.
    "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


      Thanks very much for the in depth description. I have put these suggestions to my customer and will report how it goes. Much appreciated.


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