[genewitch]

My house has hard wired smoke detectors and they all have batteries and all get to chirping on their schedule every 3 years or so.

I'd be willing to bet the NEC says "must be able to perform without mains power" for any hard wired setup.

[hinkley]

The battery only CO monitors by Kidde, if I recall correctly, are cheaper than the price of the combined smoke/CO detector minus the price of a Kidde smoke detector. I recently bought a combined unit (ironically because I had a second wireless unit I didn’t recall purchasing that was complaining about low battery near one of the wired units and could not figure out why it was chirping an undocumented signal) and almost put it back on the shelf when I saw the price.

By all means put one in your house but I think it’s a racket to have all four detectors in your house also be carbon monoxide monitors when you can just put a battery unit at the far end of the house from the combined one.