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.