Sid 03 expressed precisely :
Looking to install some new smoke detectors. I was going to install
Ionization type smoke detectors in the lower floors and then a combination
smoke and CO detector on the 3rd floor where the bedrooms are located.

I noticed that most of the combo detectors do not use ionization to detect
smoke, but use photoelectric type sensors. Is this a a big deal or should I
stick with the most popular ionization type ? Years ago I was told by a
contractor to not install combo detectors, but to keep the CO and Smoke
detectors separate ? Does anyone have an opinion on that ?

If the CO detector has an interconnection wire for other alarms, should you
connect it to the interconnection wire for the fire-Alarms ? All this stuff
is manufactured by Kidde, so I am assuming that is all compatible ?

Thanks
Sid.

CO is heavier than 'air'. Smoke is generally in hot air and rises.--
follow their instructions for placement.