Does the canned smoke simulate a smoldering fire? Just kidding.
The detector won't save a life if it doesn't go off. The videos on the Barre fire dept. site concerning smoldering fires, which are more common compared with flaming fires, are compelling.
I think the key is to replace the existing ionization only sensors with dual sensor units - oh wait - that's what I'm doing - just kidding again.
THEN, you have to slap up the CO detectors on each floor. THEN have them interconnected so you can be warned in sufficient time when something downstairs goes off.
CO detectors have to be replaced every 5 years, with the new ones beeping at that time, so if you have a combo unit, say photo and CO, the entire unit has to be replaced.
Anyway, I think it's a little complicated for such a serious subject, if one wants to get the optimum (current code? whose code?) solution.
The amount of bucks spent compared with potential to save you life isn't that much.
The wireless photo electric/CO sensors that BRK makes in its OneLink line also seems to have a lot of potential for remodeling, example:
http://www.brkelectronics.com/product/SC7010BV . It communicates to an existing hardwired interconnected system.
I just got into this when I changed my batteries a couple of days ago.
I never realized there was any controversy on the subject.