We are now the proud owners of two carbon monoxide detectors. One downstairs:
and another upstairs:
Unlike smoke detectors, these sensors go on the wall instead of the ceiling. The downstairs one is supposed to be head-height while standing, so placing it next to the thermostat is about right. The upstairs one is supposed to be head-height while sleeping, so placing it by a light switch is a little high, but not too high. You can see the door in the background to get a sense of where the detector was installed.
The team from Advanced Home Energy (AHE) (blog article) installed Kidde Model KN-COB-B units. Kidde also makes a KN-COPP-B unit that has a digital display, but that was not necessary for my situation. The audible alarm that will go off in the event our gas furnace, tankless hot water heater, or fireplace accidentally vents back into the house will be sufficient. The cost was included in my AHE home improvement project, but a search of the web shows that these units cost between $18 and $44 dollars depending where you buy them. California building codes now require these detectors on each floor.
Breathing more confidently is alive in the lab.