I recommend a standard 9V battery powered ionization-type (vs. photo cell type) smoke detector for each level but, at least one, in or near the bedroom. The ionization-type provides a quicker response with fewer false alarms. Additionally, you need a 110V low level (less than 30ppm threshold) digital display CO detector on each level. You can forego battery power on the CO detector units since, far-and-away, the greatest source of CO production will be a malfunctioning home heating plant and most modern heating units can't function without 110V. One caveat, understand that the sensors employed in both smoke and CO detectors have a finite life: most less than 10 years, at the end of which time, simply discard and replace with fresh units. Current production detectors are imprinted with the factory-recommended replacement date.