My house came equipped with Firex Mod. FADC alarms. It appears that this is only a smoke detector. If so, my house has no carbon monoxide detectors. Since all the FADC detectors are hardwired, if I want to replace one of two of them with combination smoke/CO2 detectors, can I use any model of trhe smoke/CO2 detectors, or can I use any hard wired model? Am I right about the FADC being only a smoke detector?