In Iowa Green Streets Criteria version 4.0, Section 7-16 specifies that "One hard-wired carbon monoxide (CO) detector shall be installed for each sleeping area, minimum one per floor."  This section is labeled Combustion Equipment (includes space and water-heating equipment).  Under "Intent", this section mentions the difference between direct vent and power vented equipment.  With either type of equipment, a minimum of one detector per floor is mandatory to meet the criteria of Iowa Green Streets.  In contrast, IFC 2012 requires CO detection on floors that use gas-fired appliances, floor above and floor below, as well as any floor with a garage.  CO detection is here to stay.