with time the coils will form a layer of dirt around them, i assume mostly carbone.
this will make it less efficient and also run hotter. you can just dry burn the coil to burn that off, but it will kill your head rather quickly, and getting yocan replacement heads in europe is very difficult - i had to order mine in china.
this cleaning procedure works best for me and will be much more gentle on the wire:
boil in naphtha for a while (water bath, you know the drill). dry with cotton, mount to battery. add enough polyglycol to cover the coils. heat up, use cotton while heating to soak up the hot pg. repeat a few times. before the last cleaning, close the vaporizer and draw on on it to suck in the pg that has flown into the air channels.