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Thread: Duct Systems

  1. #1
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    Duct Systems

    I'm curious as to the proper way to do this.

    System description:

    Outside air through ERVs to bring it up to a minimum temperature for ventilation air to heat pumps which recirculate some of the room air. Room air gets mixed with the ventilation air prior to going back to the heat pumps. Exhaust air going back to the ERVs is equal in volume to the ventilation air.

    How many systems are needed? I'm thinking ventilation (conditioned OA), Supply, Return, and Exhaust at a minimum.

    Would you make a separate system for that mixed air (ventilation and return) going back into the heat pump? It's basically a plenum that has both ventilation air and return air going through to a common output.

    How would you mix the systems, ventilation and return? Does it need to be a plenum (or something else) that mixes the systems or can it be a duct?

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    System design needs to be done by an engineer, since requirements very enormously between one building and another.
    If you don't know the difference between plenum and heat pump, you should probably back away now.

  3. #3
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    As far as modelling this in Revit is concerned, I would just focus on getting the airflows right so it can be used for sizing. So I would probably just make a supply and return system with the heat recovery/mixing part between them as a Mech Equipment 'plenum' family to set the correct airflows.

    Recirculation is always tricky, especially if you also have a fresh air requirement per room that you want to be able to check.
    I have been trying to put all my Extract systems on Return instead, so that I can use the 'Exhaust' system types purely for recirculation. Then you can model the Fresh air supply, and return to central air handling separately from recirculation (eg. ducted fan coils). Then the net Actual supply air and Actual return air in the spaces will be correct and you can still use an air flow to size the recirculation ducts and air terminals.

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    Quote Originally Posted by josephpeel View Post
    As far as modelling this in Revit is concerned, I would just focus on getting the airflows right so it can be used for sizing. So I would probably just make a supply and return system with the heat recovery/mixing part between them as a Mech Equipment 'plenum' family to set the correct airflows.
    ^^This was my first thought.

    Quote Originally Posted by josephpeel View Post
    Recirculation is always tricky, especially if you also have a fresh air requirement per room that you want to be able to check.
    I have been trying to put all my Extract systems on Return instead, so that I can use the 'Exhaust' system types purely for recirculation. Then you can model the Fresh air supply, and return to central air handling separately from recirculation (eg. ducted fan coils). Then the net Actual supply air and Actual return air in the spaces will be correct and you can still use an air flow to size the recirculation ducts and air terminals.
    Is there any reason for using return for the air going back to the ERV and leaving the building instead of exhaust like you would for other exhaust systems?
    Last edited by RobDraw; January 10th, 2017 at 02:01 PM.

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    Outdoor Air will need to be a System Type, under the System Classification of Supply Air. New System Types can be created, but the 4 System Classifications (Supply, Exhaust, Return, and Other) are hard-coded. Don't use "Other Air" for Outdoor Air. Any View Filters you use to assign colors or control visibility can then use the System Abbreviation parameter for sorting.

    Would you make a separate system for that mixed air (ventilation and return) going back into the heat pump? It's basically a plenum that has both ventilation air and return air going through to a common output.
    If I understand you correctly (that your ERV system is dumping tempered outside air into the return plenums of your heat pumps, where it's then mixed with return air before going into the heat pump), I would handle this air mixing process inside the heat pump family. Make the plenums part of your heat pump family, with two connectors on them, one Supply (really Outside Air, per above), set to Preset/In, and one Return, set to Preset/Out. Then, inside your family, use formulas to calculate your mixed air temperature based on the amount of Outside Air and Return Air that you enter into separate Flow Parameters. This is how you get your Entering Air Temperature for your Schedule. The Discharge Connector on your Heat Pump will be Calculated/Out, so that the downstream supply air is totaled up and applied to your Supply Flow parameter.

    A few tips:

    You can use a generic Number parameter for Temperature, this will save you some hassle associated with "Inconsistent Units."

    Try to standardize the size of your plenums and the locations of the connectors, so that you can limit the graphical complexity of your family. Hopefully they're an accessory made by the heat pump manufacturer, so you can use their drawings to model the plenum accurately. If the plenums are field-fabricated, then you might want to schedule their dimensions on your Heat Pump Schedule to assist the contractor (and spare you the effort of calling them out on your plan!)

    Make your ERV Supply (Outdoor Air) Temperature and Return Air Temperature Type Parameters, since these shouldn't vary unit to unit.

    As for your ERV exhaust, I would make this an Exhaust system. Your ERV family can use a similar formulas to calculate its own entering and leaving air temperatures. You will need to manually match the air temperatures between the ERV and the Heat Pumps (which isn't a big deal if you use Type Parameters.)

    The take away here is that almost all of the work will need to happen inside the families, and not "out in the model." If you don't have a firm grasp of how connectors and duct systems work, you're in for an education! I recommend the Sybex books "Mastering Revit MEP 2016." I learned a ton from that one. It has a whole chapter on duct connectors that I still refer to.

    Good luck and feel free to ask more questions!

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    Thank you for all the info.

    I had already decided to stop the OA at the ERV and use SA for the tempered air being distributed from the ERV. I also chose to go with RA going into the heat pumps. It turned out to be a good choice as they will have an unoccupied mode where they won't be getting any ventilation air.

    It's funny you mention calling out the plenum on the plans. I started out making a separate family for each one of them. After the first one, I tried to figure out how I was going to tag it. I decided to make it a duct and tag it as such. So yeah, I have a supply duct connected to the return "plenum". I understand that I may be creating a fragile situation and am prepared to fix it if it breaks. I've done a few rooms and might try connecting them back to the ERV, tomorrow, to see if systems start changing.

    I don't have to worry about calculations, or scheduling for that matter, at this point. This company is in it's Revit infancy with minimal training or experience. I just joined them as they are doing their first two Revit projects and it's a basic modeling exercise at this point. Floor plans are the priority and everything else is being done manually in AutoCAD but I really do appreciate the added info. I had been contemplating what I would have to do if we were utilizing smart schedules and calculations, which was the motivation behind my question.

    I saw a similar question elsewhere and the suggestion to work the plenum into the equipment family was also there. Unfortunately, a vast majority of the units will have a unique plenum. Some similar ones but most likely not identical.

    Thanks again for the very informative post.
    Last edited by RobDraw; January 11th, 2017 at 10:20 PM.

  7. #7
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    Glad I could help!

    It takes a few projects before a firm is ready to make the jump from the hybrid CAD/Revit approach to going full Revit. But once you get it set up and working, you'll never want to go back to the manual schedules. Not only can you automate the airflow calculations, but you can also automate the electrical loads.

    No one in my company was interested in getting this stuff working, so over time I got it working on my own and started using Schedules without telling anyone. By the time management found out, I already had sheets full of working schedules. :P
    Last edited by Necro99; January 12th, 2017 at 02:38 PM.

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