What does "piping" have to do with the temperature of anything?

The sense of piping relevant here is the one for making a musical sound, as by playing the pipes. The idea of a dish that’s piping hot is one so hot it makes a sizzling or hissing noise, perhaps not closely-similar to the sound of the pipes, but at least audible. It’s first recorded near the end of the 14th Century, in Chaucer’s Canterbury Tales. In the Miller’s Tale it says (in modernized spelling): "Wafers piping hot out of the gleed," where a wafer is a kind of thin cake, baked between wafer-irons, and gleed is the hot coals of a fire.

(World Wide Words, http://www.worldwidewords.org/index.htm)