@bloodlust yeah i thought of something like that to cause this would ruin some awesome creations. indeed cause exact temp isn't going to work really good. maybe use it like this (!set temp wall -1/10) so you could use both exact and between.
It would be easier to set the wall filter to have a maximum and minimum that correspond to X and Y, so that we can use the floodfill code for setting fan tile velocity to set maximum and minimum filter temperatures.