I need to add an DSC WL904 to my existing system which need to be disable on "Stay" mode. But i am new to this and the system was installed by ADT. They are trying to charge me $224 to have it done, please help.

I have several rental units - most have natural gas furnaces & stoves. I work with fire dept and there have been 2 instances where co2 detectors have saved lives in the past 2 months (heating season). BTW all my units have at least one smoke detector on each floor - newer ones direct wired. I recently ordered several Night hawk detectors - I received first alerts. Is there something I should be looking for with these detectors? both plug in w/ battery backup - both have digital readout both have test button & low battery alarms. I'd like to get them in to my units (and my own home) asap. Is there much difference in detectors/ alarms? Are they worth installing? SHould I be looking at direct wired units?