The units-aware Quantity system of Mathematica is a dream come true (time to catch up SysML, yes, I'm looking at you).
But there are a few "gotchas" when dealing with them. Because of how Mathematica chooses to implement Units and Quantities, it distinguishes between absolute temperatures and relative temperature differences, and you can't just mix and match them and hope they'll somehow cancel out.
Sometimes Wolfram Alpha units lookups work as expected w.r.t. temperatures, sometimes they don't. For example, the lookup for "W/(m^2 K)" (as used for heat transfer coefficients) gives in fullForm:
Quantity[1, ("Watts")/("KelvinsDifference" ("Meters")^2)]
That is not in fact convenient if other quantities in an expression are in Kelvin! So one sometimes has to tediously force the absolute temperature unit:
Quantity[1, ("Watts")/("Kelvins" ("Meters")^2)]
The external links provide plenty of detailed discussion.