The detectors in my home are FireX; probably from a large contractor pack purchased by the builder. These alarms are direct wired, battery backed, with a traveler wire that coordinates all the alarms together. I looked for alarms that keep those features so that I don't reduce the safety or violate any city codes. The most obvious thing to do is replace with the same alarm, unfortunately my model (FireX 120-1070) is discontinued. I also chose to add carbon monoxide alarms into the mix. I am placing one of the combination smoke / CO alarms near both of my AC returns. My guess is that the furnace fan will be running if the furnace starts spewing CO into my home. Another good idea is to add a heat detector in the garage. Many home fires start with auto fires in the garage. However adding a new alarm location will require a qualified electrician or a long read through the National Electrical Code.

Some Possible Replacements
Kidde 900-0114 - Wire-in Combo CO / Smoke Alarm
Kidde 900-0121 - Wire-in Combo CO / Smoke Alarm

Kidde 1296 - Wire-in Smoke Alarm
Kidde i12040 - Wire-in Smoke Alarm
Kidde i12060 - Wire-in Smoke Alarm

FireX 4518 - Wire-in Smoke Alarm / Kidde 900-0114 replaces Firex 12000 and interconnects with FireX 4518
FireX 5700 - Wire-in Heat Alarm

I chose to replace with more FireX detectors because I don't want to rewire the quick disconnect AC plugs or purchase the ~$4 adapters. You can find the Kidde alarms at local retailers if you are handy with wire nuts.

Here's what I'm purchasing
Galesburg Electric
(6) FireX 4518 - Wire-in Smoke $11.08
(2) Kidde 900-0114 - Wire-in Combo CO / Smoke $44.63
(1) FireX 5700 - Wire-in Heat $24.70