This question has been bugging me for years. I remembered it when I read this: Toilet flush buttons
In A/C most of the times is easy. Generally there is a snowflake for cooling mode and a sun for heat mode, or simply selecting the temperature:
But in some A/C and particularly other machines (like the heater) is not so easy or intuitive, at least when there is only icons without text. So you don't know if there is a sun for the summer mode or it represents heating.
I haven't seen a consensus on different interfaces. It can be both ways.
What are your thoughts on the matter? Have you ever seen a good, clear design, that is really intuitive? Do you know of any standards for this?