Also you can decouple the handling of ac and heat, and check the current state to prevent a redundant transition into the current state.

  if (temp > max + error)
    if (off (ac))
      turn_on (ac);

  if (temp < max - error)
    if (on (ac))
      turn_off (ac);

  if (temp < min - error)
    if (off (heat))
      turn_on (heat);

  if (temp > min + error)
    if (on (heat))
      turn_off (heat);