Sizing AS3500 Compliant Pipework Using Revit – Part 2

An important part of domestic water design is calculating flow and pressure losses, a common complaint I have from fellow Aussies though is that Revit’s pressure loss report is it’s not in a familiar format and is not something that they’re comfortable including within their design folders. Not to mention, with the AS3500 pipe sizing work(around)flow that I posted last week, the flow rates still don’t quite match up due to the IPC vs AS3500 conversion factor.

So with a powerful tool like Revit at your disposal, why not utilise it to generate data outputs that are in that familiar format and that you are happy to drop into your design notes?

There is always old faithful – the Excel spreadsheet that performs all your calculations..


..because it’s the way you’ve always done it, so why change right? Not a bad solution if you want to spend the rest of your career manually measuring pipework on drawings to verify your design; ultimately though this is how we came to the true “She’ll be right” Australian method of pipework sizing. After all, doing it properly takes far too long, so just go with your gut and prove your design with actual calculations later, but only if someone questions it down the track.

Did you know you can recreate basic Excel calculations within Revit schedules using calculated values? Of course you did! OK so maybe you didn’t. The problem I have with this method though is due to needing to neutralise Revit units your formulas will start looking a little.. weird.

Take the following example, comparing a simple Excel formula to it’s Revit equivelent



Becomes this

=((AS3500_PSD / 1 L/s) * 0.001) / ((0.7854 * Diameter ^ 2 * 0.001) / 1 m²) / 1000

Which how you’d explain that to another engineer I’m not quite sure. They’d probably look at you like you’re a bit odd. So what’s the solution then? Well, maybe you’ve heard of this thing called Dynamo? It’s frequently touted as the best method to model a facade made from old chairs, but did you know that Dynamo can be used for actually useful things too?

In Dynamo, our formula is quite clean and simple, in fact if you squint a little, it looks just like our original Excel formula.


Armed with that knowledge and a bit of time, that trusty old Excel sheet you’ve been using your whole life can be converted into Dynamo. Once you’ve got the calculations working off your Revit model in Dynamo you can generate all the data within your schedules for every single piece of pipe in your model within seconds. So do you still want to spend the rest of your life measuring pipework? I thought not..

And here are the fruits of my labor. My graph was developed in 0.9, but I can confirm it will work in 1.0 and the 1.0.1 pre-release.


Stepping through from left to right, this is how it all works:

Starting off simple, I’m selecting all the pipework within the model.


From there, we’re feeding the All Elements of Category node into the Element.GetParameterValueByName node. Filter out the pipework elements based on their system classification using a combination of String.Contains and List.FilterByBoolMask. This gives a set of true and false results for all of the selected elements, which in this case is our pipework. We take the “in” output of the List.FilterByBoolMask which gives only the elements marked as true.

In this instance I have used String.Contains and checking for the string water. What this means is that it will select anything with water in the name, which would include cold water, hot water, rainwater and so on. You can adjust this to filter out your pipework as you need for as many different systems as needed.


From this point onward is where all the good stuff happens. We push our pipework out to three separate groups of nodes. A Barrie Smith lookup, velocity calculations and head loss calculations. The Barrie Smith lookup was the most difficult part as it required custom python code to perform the lookup.


In this group, our results from the previous step feed into the Element.GetParameterValueByName node. We’re picking up the calculated fixture units from our pipework and multiplying it back out by our 3.5x multiplier from my previous post to give us our actual AS3500 compliant fixture units. The code block in the upper left generates a list which is Table 48 from Barrie Smith transcribed to Dynamo. The fixture units and Table 48 are then fed into the custom python node


The output of the python node then feeds into a math divide node and we divide our flow rate figures by 28.317 and pushes the result to a flow based parameter that I’ve named AS3500_PSD. The reason for division is that Revit stores all units of measurement in imperial rather than metric. When inputting data through Dynamo, Revit doesn’t know that you’re inputting metric figures. What this means is that when you review your data in Revit, you have input 0.23 gallons per minute, not 0.23 litres per second.

Heading back to our second (green) step, the List.FilterByBoolMask node feeds into another Element.GetParameterValueByName node. This time we’re picking up the diameter of our pipework and we’re finding the diameter in imperial and converting it to metric using a formula within a code block.


The metric diameter from the code block and the probable simultaneous flow rates from the third (blue) step combine first to calculate our velocity. The calculated result is converted back to imperial and populates to a velocity based parameter named AS3500_Velocity


Our last fork from the second (green) step is to calculate our head loss for our section of pipework. The brighter pink section first takes our length. Now for whatever reason this is taken in millimeters,  so no conversion from imperial required. In the purple section we’re feeding the code block with our pipe size and probably simultaneous flow from previous steps to calculate headloss per 100m, then another code block to calculate headloss for the section of pipe. Finally we push our headloss calculations into their respective parameters AS3500_Headloss and AS3500_HeadlossSection.


When you’re finished, you end up with a schedule you can drop on your drawing or export to Excel for design verification, and it’s all calculated as you model and far more accurate than you would have ever provided before.


The example that I’ve used here is highlighting flows where the velocity falls outside the acceptable range of AS3500, so you have a simple visual prompt that you need to check your design. I have found though that most of the time, the design is correct and the sections of pipe highlighted are either 15dia supplying a single fixture with a velocity greater than 3.0m/s or 20dia with two fixtures connected with a velocity of less than 1.0m/s.

Give it a go yourself, once the Dynamo portion is humming along your return of investment is incredibly quick.

Getting Started With Hydraulic Pipe Sizing in Revit

I’ve been poking my nose around Revit Forum a little more lately and a post in the MEP forum prompted me to write a little bit about hydraulic pipe sizing. I speak with a lot of guys in MEP locally and they give me the impression that they think pipe sizing is some kind of dark art, in actual fact it’s not that difficult at all.

In my response to the original post, I just created some generic plumbing fixtures to show the example, but I thought I’d take some time to go into a little more depth on sizing domestic hot and cold water services, my examples are going to be somewhat based around AS/NZS3500 Parts 1 and 4.

The Basics of Pipe Sizing

Before you begin, to perform pipe sizing for a plumbing system, you need to have a good set of families that have either flow rates or fixture unit ratings applied to the piping connectors. A good set of families are the building blocks of good pipe sizing results.

In the past I’ve mentioned that I have these basic families for plumbing fixtures that are simply place holders for the pipe connections. There is no need for elaborately modelled fixtures in an engineering model. If you like, you can even copy/monitor the architect’s fixtures and then map them inside your model to your engineering fixtures. The idea of these plumbing fixture families is explained really well in an AUGI article that Dave Benscoter wrote back in early 2013.


The placeholders for the piping connections are controlled by parameters for height, spacing, diameter and offset from the wall (for drainage). A lot of these adjustments may not be required on most jobs, but the ability to take the modelling further is there ready to go when needed. The family allows for automated tagging of fixture abbreviations, pipe sizing calculations and if you go to the effort of making a family type per specified fixture, you can generate accurate penetration drawings as well.

Another alternative that I have used is rather than model all the pipework up to and including the plumbing fixtures is that I will have an isolation valve box family where I fill out the fixture units of the fixtures it is serving. Just remember, if you’re doing this and you’re entering a flow rate it should be probable simultaneous flow – also called diversified flow – for which I usually rely on the Barrie Smith ‘Blue Book’.

If you’re using out of the box (OOTB) Revit families, check the loading units (which are called fixture units in Revit) and check that they match the requirements of AS/NZS3500.

In my example, the plumbing fixture connectors have their direction set to in and the hot water unit has the cold water set to in and the hot water set to out. The cold water inlet of the hot water unit also has an instance parameter where I can enter the downstream probable simultaneous flow.

You need to make sure that there are no open ends in your system, in 2014 and newer, you can use the Cap Open Ends tool if required.


The small box in my example is to cap off the end, it is set to out as it is essentially the water connection to the site.



For the purpose of the demonstration, I’ve created all the distribution pipework at 50mm. The connections on the basins are 15mm and the hot water unit I’ve used is a Rheem 613050 so both the inlet and outlet are 32mm.

For the cold inlet to the hot water unit, you can set it up to work on either fixture units or a litre per second rating. If using a litre per second flow rate, make sure that it is based on a probable simultaneous flow value.I’ve used probable simultaneous flow in my example, which based on the Barrie Smith Blue Book is 0.21l/s



Once you have all your pipework and fixtures connected, select the pipework that you want to size and you will see the Duct/Pipe Sizing icon on the ribbon.



The next step is to input the data of how you want to size your pipework.

The options available to you are to size by maximum velocity only, or you have the option to select either and/or maximum pipe friction in pascals per metre.

You can also set additional constraints using the drop down box, the options are calculated size only, match the connector size, or to use the larger of either the calculated or connector size.

Finally, you can also restrict the maximum pipe size that is allowed in your system.

I am simply going to size the piping based on a maximum velocity of 3.0m/s (AS/NZS 3500.1 Clause 3.4).

In the constraints section, restricting the sizing to that of the connector allows you to achieve the 15mm branch to a single fixture rule (AS/NZS 3500.1 Clause 3.5.2) however keep in mind that Revit will size the pipework as 15mm until the next branch (tee) fitting so if your fixture is further than 3m from a branch you will need to adjust the pipe size manually yourself. Revit will also oversize branches of pipework connected to a larger fitting as shown below with the HWU. You should always check your sizing results and never rely on Revit to do all the work for you.

As you can see, other than our oversized pipework from the HWU highlighted in red, the results are quite good.

You could argue that my 25mm incoming connection is too large to serve only 5 hand basins. The reason it sized to 25mm is because of my flow value on the cold water inlet to the HWU. If I change the cold water connection to use fixture units, and manually enter 5 fixture units, the incoming pipework is reduced to 20mm.


Analysing Your Pipework Velocities With Revit

You can create a view in Revit that analyses the velocities of your pipework (amongst other things). This can assist you when reviewing your pipework sizing with easy to follow graphical displays of your pipework velocities. You can find the Pipe Legend tool on the Analyze tab of the ribbon.


The results give an easy to follow display of your pipe velocities by colour, you can clearly see that the 32mm pipe from the hot water unit has a velocity of 0.9m/s



Manually adjusting the pipe size to 20mm, you can now see that the velocity of the pipe exiting the hot water unit is now 2.7m/s or changing to 25mm, the velocity is 1.5m/s. Based on velocity, either pipe size is acceptable under the Australian Standards.



Creating a Pressure Loss Report

Finally, you can also run a Pipe Pressure Loss Report which is found on the analyze tab.


The pressure loss report is generated in HTML format, I am simply going to select the cold water system only and I’m going to display the system information and the critical path.




The resulting report shows that we have 51.1kPa of pressure loss along the critical path. Referring back to AS/NZS3500.1 we require a minimum pressure at the outlet of 50kPa (Clause 3.3.2) which tells us that we need a minimum of 101.1kPa at our connection point.

Conversely because the maximum allowable static pressure at an outlet is 500kPa (Clause 3.3.4) if we had 600kpa available at the connection point we would need to install a pressure limiting valve to comply with Australian Standards.