Electronic Alarms and Home Security Devices - Plug in CO2 & explosive gas detectors

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?

Actually, those plug in units work pretty well. If anything, they drift to overly sensitive. The toughest thing is placing them well. The websites for both First Alert and Kidde have pretty good guidelines for CO detector placement.

Thanks overly sensitive I can deal with - rather be called out by tenant to check unit then worry that they aren't safe....thanks

Problem is I can't find any info on the units I received (First Alert explosive gas & carbon Monoxide alarm) Not even on the F A web page....wonder if they were discontinued or if they're really new products

If they aren't on this FA webpage, (http://www.firstalert.com/CarbonMonoxideAlarmsCatalog.aspx) then they may well be a very new product.