Done some work in Wisc., typical scenario for IL and WI is each bedroom, hallways off bedrooms within 15' of door openings, long hallway you get 2 smokes, 1st floor of two story house- hallway/common area by stairs, got a basement-bottom of stairs close to door, then the furnace room/area.
Some inspectors want them by stairs no matter what, some don't get too upset if in the basement it's just in the furnace area.
Typically following the print approved by the municipality is the safest bet, but if in doubt, call the AHJ of the area.
Not all locales require the CO detectors as of this date. But to the electricians advantage, and the disadvantage of the homeowners pocket book, Fyrnetics has a combo CO/Smoke that was approved finally.The alerts had to be seperated. The unit can be installed in just the locations required and work with the smoke only units.
