Kidde KN-COSM-B Review

The Kidde KN-COSM-B is a combined Carbon Monoxide and Smoke Alarm in a single unit.

I had deliberately not included any of these type of alarms on this website previously because although they are perfectly acceptable units, the optimal position in which to place a smoke alarm and a carbon monoxide detector are different - the CO Alarm works best when positioned somewhere between eye level and two feet below the ceiling, while a smoke alarm works best attached to the ceiling itself.

A detailed explanation of why this is the case can be found here.

That being said, after receiving several emails suggesting that I should include a combined CO & Smoke Alarm, I've decided to present the facts to you, and let you make your own decision.

KN-COSM-B Features

  • It uses 3 AA batteries - it warns you when the batteries need replacing and it has a fail-safe measure which prevents it from being attached to the ceiling without the batteries installed.

  • Because you will attach these to your ceiling, the Kidde KN-COSM-B has a built in voice which tells you if you have an emergency due to smoke or carbon monoxide to help you take the appropriate action in response to the alarm sounding.

  • It remembers the peak CO readings it's taken so when you press the test button it will tell you if levels have exceeded 100ppm - you can then have your fossil fuel appliances such as gas cookers or heaters checked to see if they are malfunctioning.

  • The smoke alarm can be temporarily silenced in the event of a non-emergency such as burnt cooking setting it off.

  • It comes with a 5-year warranty.


  • Because you need multiple units throughout your home, the combined Smoke & CO Alarm saves you money.
  • As far as combined Smoke & CO Alarms go - this is one of the most popular on the market.
  • It had over 1,000 customer reviews with a high star rating on Amazon.


  • It may not detect low levels of Carbon Monoxide Gas in some situations due to its placement on the ceiling.


Although the ceiling is not the optimal place to put a CO Detector, it is certainly a better place than none at all. For those of you on a budget it is much safer to get a combined alarm like the Kidde KN-COSM-B than to chose to install only a Smoke or CO alarm separately.

Get the updated Kidde KN-COSM-BA and the latest customer reviews at Amazon.

Please ensure you have enough alarms to protect your entire home - more than one are needed in the vast majority of dwellings - see Places within the home where CO Detectors must be installed.