I've dealt with similar "inputs" and you almost always need to test (or read the instruction manual) to really know the answer. Interestingly, my first instinct

I've dealt with similar "inputs" and you almost always need to test (or read the instruction manual) to really know the answer. Interestingly, my first instinct was to think that the arrow was representing the numbers aka "lower number is colder" and apparently that's the solution. For these cases, the only way for a manufacturer to make sure that users (almost) always get it right is to either use orange/blue colors for the numbers (orange numbers means hotter) and/or a small ice/heat icon at each side.