I have long advocated against the use of combination alarms because the most effective position for a smoke alarm is typically on the ceiling, while carbon monoxide detectors are usually best mounted at about eye level on a wall.
However the growth in popularity of combination smoke and carbon monoxide alarms can't be ignored, so with my 'disclaimer' above I'll present some of the reasons this category is now growing so rapidly.
Combined alarms cost only slightly more than the individual detectors of either type with the result being that you save a lot of money having to buy only half the number of units. Installation is also much easier when you only have to mount half the number of alarms as you have to when installing separate smoke and CO alarms.
Smoke and Carbon Monoxide Detector Reviews
Kidde P3010K CO ***Editor's Best Pick***
The Kidde P3010K CO is our most highly recommend combination smoke & carbon monoxide alarm due to its extended 10 year life, its 10 year battery which never needs replacing, and its overall low cost of ownership when compared to most other models.
This battery powered unit is a great budget option for those who want to save money on the purchase price.
This is the most expensive option currently available but it does have a range of features not yet available in other models.