Is it a rule to use "in" after "disappoint"? Can I replace it with "at" or "with"?
Great question. Generally, we use disappointed + preposition, therefore you can be disappointed with/at/by/in etc. However, in this case while you can replace in with "with", you can't replace it with "at". That is because "disappointed at" needs to be followed by something and not someone. Since he is treating the thermostat like a person you could say "disappointed at your behaviour" but not "disappointed at you". Ex: We were deeply disappointed at the result. Ex: His parents were disappointed in/with him.