Plumbing Systems for Well-Documented Flow

April 2nd, 2012

Plumbing in Autodesk® Revit® MEP has often received a bad rap, and some of it is deserved.  When you are new to this platform, plumbing can be very daunting.  You have to tackle several new tasks all at once in order to be productive.  The tasks, printing, modeling, annotating, and engineering are all very different then previous AutoCAD®-based workflows.  This article is intended to help with some of those last two tasks, annotating and engineering.

All of the plumbing tutorials offered for Revit MEP by Autodesk cover the basics of modeling and defining logical systems.  These cover most of what you need for documenting flow for domestic water supply and sanitary sewer.  The basic process is to place your plumbing fixtures, assign them to a logical system, pipe and tag as needed.  The flow in the pipe is specified in your family and tracked in the pipe (see Figure 1).  There are a few basic requirements for allowing this flow tracking to function.  

Figure 1

First, all the connected families must be consistent in the way they define flow.  The connectors must have the same Flow Configuration and System Classification (see Figure 2)—in this case, Fixture Units and Domestic Cold Water.  They must also be consistent in defining the direction of flow (this is not related to the direction the pipe connector arrow points).  Think of this in the sense of fresh water flowing IN to the fixture from the pipe, and sanitary drain water flowing OUT of the fixture to the pipe.  If you mixed fixtures with some flowing IN and some flowing OUT, you would not be able to properly collect flow in your pipes.

Figure 2

Second, there can be only one possible path for water to flow.  This is different from real life.  For instance, it is perfectly acceptable in plumbing construction to loop the domestic cold water supply around a building floor with periodic taps out to fixtures.  However, Revit does not understand how to calculate this.
So long as you have these two items, Revit can track your flow.  Now let’s look at what is needed to apply this to the more difficult piping options.

Sanitary Vent

Beginning with Revit MEP 2012, there is increased control over systems, as well as some additional System Classifications including Vent.  We also have the ability to connect pipe of different classifications and have them retain that classification setting.

However, connecting pipe of different classifications doesn't suit our need because it gives us two separate possible paths for flow, which means that Revit fails to track our flow past one of these junctions.  The Vent classification doesn't work in any case, because it doesn't track flow at all (see Figure 3).

(Figure 3: Flow (1 FU) is tracked in the first pipe segment, but not after the tee (0 FU); the properties of Vent pipe does not include flow.

The best option for tracking flow in both sanitary drains and sanitary vents is to separate the two at the fixture family.  Create enough model geometry in your family to host both connectors.  Classify them both as Sanitary with the Flow Configuration set to Fixture Units (and don't forget to give the Fixture Units a value).  The flow direction is only important in the sense that all of your families must match. 

Once your families are set up to separate your drain and vent and you pipe them accordingly, you can use pipe tags to parametrically track and document flow.  This will make pipe size easier to calculate (see Figure 4).

Figure 4

Note that you do not need to explicitly define any logical systems for this to work.  Even if all the pieces are left on whatever default system Revit chooses to give them, you still have full documentation for flow. 

One missing piece is the ability to use View Filters to change your color, as seen in Figure 4, or line type to visually distinguish between drain and vent pipes.  The most flexible and robust method for filtering your views is to use System Name.  This requires that you set up user-defined systems.  The name that you select can be used for filtering, and will be displayed in the pipe tags. 

Since systems do not affect flow collecting in pipes, you do not need to have a separate System Name for each pipe tree.  For example, most single-story buildings have a single sanitary drain outlet to which every fixture is connected, but many vents through the roof. 

Just because the vent pipe tree in the North wing of the building is not connected to the vents in the South wing does not mean that they have to have unique logical systems.  I generally assign only one logical system with a name that is convenient to display for each of these pipe systems; Domestic Cold Water (CW), Sanitary drain (WASTE), Sanitary vent (VENT), and Sanitary condensate drain (COND).   If there are multiple domestic hot water heaters in a building and you wish to document them separately, then you can append the heater callout to the system name, Domestic Hot Water (HW-1).

Figure 5: Once both the System Name and flow are well-defined, you can use tags and View Filters to demonstrate both in your plans.

Natural Gas

Tracking and documenting flow for natural gas piping (fuel gas) adds a few complications to our strategy.  We still need exactly one possible path for Revit to track flow, and it's still best to use System Name to drive View Filters and tags.  But what flow are we tracking? 

Gas pipe size calculations (in my part of the world) are based on cubic feet per hour or CFH.  Revit does not have CFH as an available unit, so we will need to choose a placeholder.  The simplest answer is to use gallons per minute (GPM).  We also need to choose a System Classification that will allow us to specify flow in GPM.  You can use one of several options; in this example I used Hydronic Supply.  You should plan this choice carefully—you will have to keep your classification consistent across your families and projects.

Next, we need to specify the demand.  Gas is usually specified in mechanical equipment and in water heaters as BTUH under heating input.  If you trust your equipment specifier to enter the BTUH correctly, then the most flexible option is to use a formula to calculate CFH directly from BTUH.  This is not quite as simple as it may seem.  You will need to edit the family and add a shared parameter (we will be scheduling this later).  Title it descriptively. For example, my parameter is ACH (for my company's name) CFH as GPM, all condensed to ACH CFHasGPM, the Discipline is Piping, Type is Flow.  The rough formula is 1,000 BTUH per CFH.  Since the parameter specifying BTUH is likely formatted in Revit to use that unit type, you will have to not only calculate the number, but also convert the units from BTUH to GPM. 

Revit makes this complicated.  The formula shown in Figure 6 should reasonably work;  however, when you press OK, Revit internally converts the units of energy from BTUH to VA, and units of flow from GPM to CFM. Because of rounding settings, we end up with 0 GPM as our value instead of 120 GPM (see Figure 7). 

Figure 6

Figure 7

Figure 8

To fix our rounding issue we can arbitrarily enlarge both the divisor and multiplier (see Figure 8), and now we have correct calculated flow (see Figure 9).

Figure 9

Now assign this calculated flow to the pipe connector in the family and it's ready for use in your project.  Your gas flow CFH (substituted with GPM) will properly calculate in the pipe in your project.  The next complication comes from your pipe tag that shows the flow.  If you add the text “CFH” to your tag family, in the project it will still show GPM as well (see Figure 10).


Figure 10

To fix this, go to the project units settings and set the Unit Symbol for piping flow to None (see Figure 11).

Figure 11

Finally, because we used a shared parameter to calculate CFH in our families, we can schedule this value and document our total building gas load as well (see Figure 12).

Figure 12

Cost / Benefit

There is significant effort required to set up these methods for use in your office.  Plumbing and mechanical equipment families, pipe tag families, project unit settings, and view filters will all need to be adjusted.  However, this effort can be paid back when you make full use of the parametric capabilities in Revit MEP.  This method allows you to change the heating input value on one water heater and be 100 percent confident that every pipe tag and schedule on every sheet in your set will remain up-to-date. 

Like most things in life, what you put into this process is what you will get out.

Joel Londenberg is one of the co-authors of Mastering Revit MEP 2012 and is currently the BIM manager for ACH Mechanical, an HVAC contractor. He has been an independent training and implementation consultant specializing in Autodesk® Revit® MEP, and has many years experience with leading mechanical and plumbing design, and supporting electrical design for a wide range of projects. He has also worked with several major HVAC equipment manufacturers to develop their Revit content for distribution.

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