Friday, January 07, 2011

Revit MEP - Sanitary Venting and Fixture Units

Revit will push drainage/waste fixture units downstream to the end of the system. If three water closets are assigned four (4) waste fixture units each (WFC is the parameter in stock content), the pipes that are attached directly to each water closet will show four (4) WFC as a read only parameter value. As you check each pipe that is closer to the end of the system you'll find the fixture units incrementally increase after each water closet. If you assign the water closets to a sanitary system you'll see that they also add up in the System Browser (you can show Fixture Units). So far so good.

Venting is often required at each fixture and each vent is then routed over to the associated sanitary riser. This is where things go awry. The fixture units calculation inside Revit can tolerate a closed system where the contents of the pipe and the air in the vent can't circulate via a loop (see later images). As soon as you create a loop condition fixture units cease to accrue to piping layout.

Most of the engineers I've met want to use a fitting that has an integral vent connection. The one you see in the images in this post was created by Scott Johnson and he shared it with me a couple years ago and it works nicely. Attempting to simulate this isn't a problem until you try to connect them all together.

The solution in the images here involves placing a cap at the top of each Vent riser to close the system. To complete the venting (that isn't really venting anymore) you just run additional piping above the cap elevation. The different appearance for the vent pipe and fittings is done with a filter. The vent pipe you see is a separate Pipe Type so simpler fittings can be used as it is sketched. A Type Comment parameter value of "Vent" makes it easy to use a Filter. In plan views I'd be inclined to hide the vent pipe using the same Filter so they don't compete with the sanitary pipe graphics, the fittings above tend to obscure the fittings below. If you place these caps as high as possible it might not be necessary to fuss with a filter. This image shows the System Inspector results after using this technique.

System Inspector won't even work until you create a situation that doesn't allow for this loop. By loop I mean this condition.

I've experimented with changing the nature of the connector in the wye fitting itself but unfortunately as soon as Revit detects an alternative path that creates a loop, no fixture units past that point.

I suppose you could argue, "What's the point? Since you can't use the sizing tool on sanitary pipe anyway?". It might not really matter to some, but if the pipe knows what fixture units are present it should help you do the sizing in-canvas. It will help because you can touch a pipe and see the value (in the Properties Palette). You can then refer back to your cheat sheets to choose the best diameter for the pipe.

Made a video too, watch at OpEd Videos or You Tube or listen and watch here (my graphics card goes haywire at the end a bit).


Steve said...

Joel Londenberg wrote to me to mention that another solution is to incorporate the "piping" that connects directly to the plumbing fixture so all the connections required are actually part of the plumbing fixture instead.

It seems that the bi-directional nature of the fittings in my example contribute to the "loopiness" issue I'm writing about.

None of the stock content is done this way however so it will take some effort.

RevitGarage said...

I agree with Joel's suggestion of making some of the pipe part of the family and that is how I set up plumbing fixture families. It also aids in reducing the amount of modeling time for the Revit user.

Steve said...

Yes, it is a bit more elegant to incorporate such things into the plumbing fixtures directly. It doesn't represent the "kit of parts" actually involved but perhaps that is less important than being efficient/effective.

It does take more work on the part of the person creating the plumbing content. Since there are myriad choices for connecting fixtures it might result in many versions of the "same" fixture.

All things to consider ultimately.