The fan on my 13" MBP (2017) rarely kicks on unless I have an external monitor plugged in. I've been told that it has to engage the discrete graphics card to power my external monitor (only 24" / 1080p), and that's what causes the extra heat. Unfortunately, with the external monitor the fan is on all the time. I almost wish my work environment weren't so quiet, because then I wouldn't hear the fan kick on.