Without a long discussion, I recently tried a temp gun, and I am not that confident in it. I know from work experience that IR guns are uncertain without accurate emissivity settings which are hard to determine and vary depending on the surface you are measuring. Even the cleanliness of the surface can change the reading. I believe the temp gun is a good relative measurement (it's hotter now than it was last time) but not to be counted on for precision. But I must say that is just me. Lots of people use them reliably and like them. What is certain is that the boiling temp of water will not change much. Get some spit on your finger and wipe it on the header. If the spit dances and takes a few seconds to evaporate, temo is good. If it vaporizes instantly, you are too hot.