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If in between samples you go from above max to below min you'll have both heat and ac on.

The take-away is that like programming lift controllers[0] it's not quite as simple as it might first seem - many many edge cases.

[0] http://play.elevatorsaga.com/ (for example)

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I think that's wrong, because all 4 if-statements are evaluated for each sample.

But yes agreed, in general there are lots of edge cases. I actually wrote a finite state machine specification for an elevator in a logic class once. Without having really thought about it much, it looks like the biggest problems with thermostats are dealing with scalars, sampling frequencies, and sampling error.

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