Reply To: How can I use max tempareture as output parameter?

[Rob]

That's the maximum of the two values, so should return maximum of x or y.  

Try maxVal(Velocity v)@baffled   If you create a new Expression using the CFD Post editor then right click into the window there are a few options to choose from. Note, if you're using Fluent all of this can be found in the Report Surface (Volume) panel.