Temperature Difference in Revit 2017

Tips, tricks & tutorials

This info is a bit dated, but the question came up this week, and I had to scratch my head a bit to remember the technicalities behind it to explain.

In the Revit 2017 release, we added a Temperature Difference parameter type, to simplify the process of defining certain calculations in families.. for example, users will use the Tin and Tout to compute a DeltaT on a coil, to then relate the fluid flow rate to the energy requirements based on this DeltaT.

Prior to the 2017 release, there were some mathematical gymnastics that had to happen to subtract two temperatures… the reason being that unlike most other numerical values (such as length, volume, etc…) that share a common 0 point, temperature has a unique characteristic wherein 0 in one temperature unit doesn’t necessarily correlate to 0 in another temperature unit… it depends if you are trying to convert a temperature or a temperature difference.

For example, if you wanted a value of 23°F converted to °C… are you asking for a an absolute -5°C or a relative 12.8°C?

  • Absolute temperatures convert like this:  0 °F = 32 °C = 273.15 K = 491.67 °R
  • Temperature differences convert like this:  e.g. 0 °F = 0 °C = 0 K = 0 °R

Since Revit only had a Temperature unit type, it was the user’s responsibility to force conversion to temperature difference by jumping through some hoops.  Now, you can explicitly define your intent by using the appropriate parameter type (Temperature vs Temperature Difference).

During upgrade Revit tries to ensure that whatever parameter values were set before upgrade are retained after upgrade.  You may find you want to revisit your formulas and parameter types to take advantage of this simplification.

See this brief video demonstrating the new Temperature Difference unit type.


Martin Schmid

Martin Schmid joined Autodesk in 2005 as a project consultant for MEP customers, both for Revit MEP and AutoCAD MEP. Martin holds a BS in Architectural Engineering, and MS in Engineering from Kansas State University, and a Masters in Management of Technology from the University of Texas at San Antonio. He worked as an electrical designer, engineering coordinator, and application developer before joining Autodesk. Martin has co-authored books for Revit and AutoCAD MEP, and is a licensed engineer in the state of New Hampshire. Martin worked in the product globalization and customer success teams prior to joining the product management team, where he now guides direction related to the extended MEP product portfolio, including MEP functionality within Revit, CADmep, ESTmep, CAMduct, and AutoCAD Plant 3D. In his spare time, Martin enjoys reading, travelling, and attending his kids’ activities.

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1 comment

  1. technostructllc

    Dear author, thank you for penning down such an informative article that not only clears doubts but also imparts new knowledge.

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