The Interconnectedness of All Things
Jon Atkinson – LKPB Engineers, Inc.
A holistic approach to Mechanical families, parameters, and formulas.
Revit families were created to communicate with each other; data can be pushed or
gathered via their connectors, which allows us to create some very powerful families and
schedules. Using this data we can coordinate across disciplines and decrease errors and
design time. Mechanical can input values and provide electrical the necessary information to
properly design their systems, hydronic piping can be sized based on the CFM input from the
HVAC designer, lighting designers can look at architectural wall finishes for lighting calculations,
and architectural wall assemblies can be used to accurately determine heating and cooling
loads for spaces and zones.
Parameters can be viewed as discreet bits of information you plug into your equipment
to populate your schedule, an arduous and tedious task. But if you look at how all those bits of
data can affect all the other bits of data, and you can get them talking to each other, now you
have a useful design tool. Since engineering is chock full of formulas, we’ll look at how we can
use these formulas to drive our parameters and streamline the design process.
We’ll look at how load calculations for a Space can determine the Air Terminal CFM,
which in turn tells the VAV what size it needs to be, which then sizes the supply ductwork, it also
tells the hydronic reheat piping what size it should be, or the KW if it’s an electric reheat coil.
This is all done with connectors, parameters, and formulas.
Learning Objectives
At the end of this class, you will be able to:
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Learn how to get data flowing from one family to another.
Learn how to use formulas to automate tedious calculations.
Learn how to use schedules to maximize productivity.
Learn the interconnectedness of all things Revit.
About the Speaker
Jon Atkinson is the BIM Manager for LKPB Engineers. He started in the architectural field in
1996, but moved to engineering in 2010 (so he could wear flannel shirts). Jon has been a CAD
Manager, BIM Manager, Revit Developer, even a lowly drafter, and holds a BSTM in CAD
Management.
The Interconnectedness of All Things
An holistic approach to Mechanical families, parameters, and formulas.
During the course of a project there are many tasks that devour our time, some of which we can
do nothing about, but others we can pass off to Revit. We’ll look at areas where Revit can help
us to save time and effort so designers can design instead of wasting time on tedious tasks like
calculations, or manually resizing elements. We can take it one step further and implement
cross discipline coordination, getting mechanical equipment information to electrical as it is
placed and defined in the project.
Current workflow:
Let’s take a look at our current workflow and identify areas where we can save time.
Define system strategy
Load calcs export and import
Airflow and layout
Equipment Sizing and Selection
Documentation
Cross Discipline Coordination
Where can we save time?
New workflow:
Define system strategy – initial design intent, required equipment, etc.
Define Spaces and zones – this is time well spent. Yes, it’ll probably change, but do it anyway.
Load calcs export and import - by defining spaces and zones you will save huge amounts of
time when you export to Trane Trace. Import the results from Trace and get to work.
Air terminal CFM – using schedules we can quickly define, and size, all of our air terminals.
VAV sizes – automatic and based off CFM you shouldn’t need to do much here.
Size ductwork – Revit is really good at this, though there may need to be some adjustments.
VAV calcs reheat coil GPM or KW – more free work!
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The Interconnectedness of All Things
GPM and CFM data back to AHU and pumps – manually size equipment. Boo!
Electrical requirements for equipment – even more free work!
Size pipe – again, fairly easy.
To get it all working smoothly you need families that can handle the data and have them send
the data upstream, and schedules to help you work more efficiently with that data.
Spaces, Zones, and Load Calcs
By defining your spaces and zones with ASHREA definitions you will save a lot of time when
doing load calulations. Export a gbXML to Trane Trace (or other load calculation software) and
all of your spaces and zones will have the needed information to quickly set up and run the
calcs. Save an updated gbXML and import it back into Revit and you now have your calculated
CFM.
I recommend making Arch ceilings non-room bounding and pushing spaces up to the level
above. This way you can add the plenum space back in inside your calc program. It also saves
time in not modeling plenum spaces in Revit and then needing to delete them in Trace.
Now on to the good stuff!
Families
When creating a new family, or editing an existing one, consider all of the information, where it
comes from, where it needs to go, and what it’s going to do when it gets there.
Flow of Data
You define your Air Terminal CFM from the information Load Calculations.
You define your VAV size based upon your Air Terminal CFM
You calculate Q and MBH based upon your CFM, EAT, and LAT
You base the GPM or KW for a reheat coil based upon Q, and/or EWT and LWT.
You base your Pump HP based upon GPM
Furthermore:
You define your MOCP based upon KW, KVA, FLA, MCA, HP, Volts, and Phases.
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The Interconnectedness of All Things
Connectors
Now that we have our formulas working we can link the parameters to our connectors and push
that information up to the equipment. There are three settings for duct and pipe connectors that
are very important but aren’t linked to a parameter; Flow Configuration, Flow Direction, and
System Classification.
The Flow Configuration for duct and pipe connectors is very important for getting the
information to move properly.
Calculated – is used when information is being pushed to the connector from a connector on
another family.
Preset – is used to push information upstream to another connector.
The Flow Direction is also important in determining which direction the information is moving.
The flow direction for an Air Terminal is IN.
System Classification - defines the type of system to which the connector will be attached.
Using Global will allow the connector to acquire the system type that is being attached to it, like
a pump being connected to a hydronic supply / return, domestic water, or fire protection
Simplify your life
Make your Air Terminals and VAV boxes size themselves based on the manufacturer’s CFM
range tables using IF statements, several parameters, and lookup tables.
Formulas
These are the number one most powerful tool you have when creating families. IF, AND, OR,
NOT, Arithmetic, and YES/NO, all of these can be used similarly to Excel. However, you may
need to convert your parameters to be Unitless before they can be used in a formula by dividing
by 1. Revit will automatically determine you are dividing by whatever the default parameter unit
is and add it to your formula. Similarly, you can match the parameter’s unit type by multiplying
your formula by 1. The length and complexity of a formula is only limited by your imagination
and proper syntax.
Once we have GPM, and our systems are properly connected, we can use this information to
size our reheat pump and provide Electrical the information they need to size a breaker and
service requirements.
Once any single parameter for a motor family; KW, KVA, FLA, MCA, or HP has been input we
can determine the MOCP. We can use industry standard formulas in our parameters to convert
all of the above information into a single value such as KW.
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The Interconnectedness of All Things
KW = If([KVA] > 0VA, [KVA] * [Power Factor], If([FLA] > 0, [FLA] * ([Volts] * sqrt([Phase])),
If([MCA] > 0, ([MCA] / 1.25) * ([Volts] * sqrt([Phase])), If([HP] > 0, [HP] * 746, [KW
Override]))))
To get Max Over Current Protection to the nearest 5A.
MOCP = (roundup(([KW] / ([Volts] * sqrt([Phase]))) * 1.25) / 5) * 5
I highly recommend reading the Revit Forum post titled “Revit Formulas for "everyday"
usage” and the Revit Help section “Conditional Statements in Formulas”.
*Disclaimer: As accurate as they are, some of these calculations are ballpark figures
used to aid in the planning stages of a project and may change later depending on
equipment selection, system design, and model changes.
Lookup Tables
Another powerful tool is the Lookup Table. In the past these tables were primarily used to return
a dimensional values, now we can look up or return ANY unitless number, but we still can’t
return text. In order to use a lookup table we need to remove the unit, get the value from the
table and add a unit back in.
Ex. – 8” Neck CFM = (size_lookup([Lookup Table Name], "8NECK", 0, [Length], [Width],
([Design FPM] / 1 FPM))) * 1 CFM
Note the Design FPM is divided by 1, and once we have the value from the table we multiply by
1. Revit automatically adds the appropriate unit designations to the formula.
The appropriate header syntax in the lookup table’s CSV file is:
<Column Name>##length##feet (or inches) to return a length value
<Column Name>##number##feet to return a numeric value.
Schedules
Schedules are incredibly powerful and can be used for any number of functions other than
placing them on sheets. You can use them to schedule Zones (you did set up your logical
zones, didn’t you?) and embed an Air Terminal schedule to quickly review the calculated vs. the
actual supplied CFM for each space. You can schedule all of your circuits and embed a
Mechanical Equipment schedule to review load, wire size, MOCP, etc. You can add Calculated
Fields that combine parameters into a formula. You can add Conditional formatting to highlight
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The Interconnectedness of All Things
fields that fall outside a specified range. You can even adjust the Borders to mimic
concatenation.
One drawback to using families that have formulas is that they can’t be edited in a schedule. A
way around this is to have override parameters that allow the user to define a parameter to
something other than the calculated value.
Ex. – MOCP = If([MOCP Override] > 0, [MOCP Override], (roundup(([KW] / ([Volts] *
sqrt[Phase])) * 1.25) / 5) * 5))
Utilizing override parameters is especially useful if a calculated value is something that doesn’t
exist in the real world. In the example of MOCP, what happens if it calculates 55A for a breaker
size?
Air Terminal example
A standard Titus TMS diffuser has several parameters that define its size; Length, Width, Neck
Velocity, and Max CFM for the desired neck size. This makes it relatively easy to create a
lookup table and a set of parameters to control the Neck Size.
8” Neck = (size_lookup(Lookup Table Name, "8NECK", 0, Length,
Width, (Design FPM / 1 FPM))) * 1 CFM
Lookup Tables have been mainly used to return Length data, and it was near impossible to
return any other type of value. There are now 2 ways to return a NUMBER value;
ParameterName##number##feet
ParameterName##other##number
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The Interconnectedness of All Things
VAV Box example
The VAV box is similar but no lookup table is needed, just string a few IF statements together.
“Inlet Diameter = if(Airflow < [4" Neck] + 1 CFM, 4", if(Air Flow < [6" Neck] + 1 CFM, 6"…”
From this starting point we can begin adding more capabilities, like calculations for hot water or
electric reheat coil. We can turn to the article by Autodesk “Determining Water Flow
Requirements based on Air Flow and Design Assumptions”.
To simplify the article it boils down to:
Delta T Air = [LAT] – [EAT]
Delta T Water = [LWT] – [EWT]
Q = [CFM Min] * 1.08 * [Delta T Air]
GPM = [Q] / (480 * [Delta T Water]
To make the VAV box family useful for cross discipline coordination we can calculate the
requirements for an electric reheat coil using the formula for KW.
KW = [Q] / 3410
Temperature
What is this -459.67°F (or -273.15°C) that appears whenever I add a temperature parameter?
Temperature conversion can be a tricky thing, and one of the least understood, as Revit uses
Kelvin as its base unit. Revit will convert your Fahrenheit units to Kelvin, perform the calculation,
then convert it back to Fahrenheit.
Ex. 180°F - 160°F = -439.67°F
What Revit sees:
355.37 K – 344.26 K = 11.11 K
The conversion from Kelvin to Fahrenheit is: °F = 9/5(K – 273.15) + 32.
The conversion from Fahrenheit to Kelvin is: K = 5/9(°F – 32) + 273.15.
The Conversion from Kelvin to Celsius is: K + 273.15.
Hence, 9/5 (11.11 K – 273.15) + 32 = -439.67°F. As far as Revit is concerned this is an
accurately calculated value. But we need it to return an expected useable value.
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The Interconnectedness of All Things
So, how do we get back to Fahrenheit, or another useable value (unitless)?
MORE MATH!
The formula for a unitless value, so it can be used easier in other formulas:
(459.67 * (Temp1 – Temp2) / 0 °F) Yes, you can divide by zero!
What Revit sees:
(459.67 * (Temp1 K – Temp2 K) / (5/9 (0°F -32) + 273.15) or (255.37 K)
= 459.67/255.37 * TempX
= 9/5*TempX = TempY (unitless)
The formula that maintains the temperature unit:
Temp1 – Temp2 + 0°F
What Revit Sees:
Temp1 K – Temp2 K + 255.37 K
= TempX + 255.37 K
= 9/5(Temp X K - 273.15) + 32 = TempY°F
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