My Firex units timed out in our 11 year old house. All 5 started chirping within a few months of each other. These units at Wal-mart were almost direct replacements. I used the included adapter so my old wiring plugged into the new units. The only thing I had to do was remove the old base plate and install the new one (a 5 minute job with just a screw driver). I did have to find some A/C wired CO detectors at a different retailer, but it all worked out.