Each smoke and carbon monoxide detector that enters Consumer Reports’ labs is rigorously tested to make sure these devices keep you and your family safe. For smoke detectors, we test their response time to both flaming fires and smoldering, smokey fires. We’ve found that not all detectors can adequately detect both fire types.

For carbon monoxide detectors, we test them against low CO levels (at 100 parts per million) and high CO levels (at 400 ppm). We also test the accuracy of their carbon monoxide measurements if they offer readings through a digital display or audio announcements.

Combination detectors go through both sets of tests, while smart detectors receive additional testing for their wireless connectivity and smartphone alerts. All of these test results are then folded into our comprehensive smoke and carbon monoxide detector ratings. 

Smoke detectors are a must in all homes, and carbon monoxide detectors are needed for any home with fuel-burning appliances such as a furnace, water heater, range, cooktop, or grill. Even those living in all-electric homes should install carbon monoxide detectors, because CO can seep into the house from an attached garage or if a backup generator is used too close to your living quarters during a power outage.

You'll need smoke detectors that detect flaming and smoldering fires for each bedroom, with at least one detector installed on each floor, including in a finished attic and in the basement. You should also have a carbon monoxide detector on each living level, in the basement, and near (not inside) an attached garage.

Use the illustration below as a guide to where you need detectors and tally up the number of smoke and carbon monoxide detectors you need to buy. Keep in mind that your home might need more or fewer detectors depending on its size and number of rooms.

1. Check Local Regulations
Before you shop, check regulations in your area. Your town or state might have specific requirements (such as when you sell your home) about what you need, including types of detectors and their placement. Also, some insurance companies offer a discount for homes with smoke detectors.

2. Pick a Power Source
Hard-wired smoke and carbon monoxide detectors tie into your home's wiring, which requires professional installation, and usually have backup batteries. Battery-only detectors are the simplest to install, and they work during a power failure. Some models use removable batteries that require annual replacement, while others use sealed lithium batteries that last the life of the detector. Plug-in detectors are also available, but electric outlets are typically located low on the wall, while the optimal placement for the detector is on or near the ceiling.

3. Smart or Not
Smart smoke and carbon monoxide detectors give you the advantage of knowing if something’s wrong when you’re not home, but they also come with a big price tag. We also haven’t found a smart smoke and CO detector that successfully does it all. But if you do decide to outfit your home with them, we recommend also installing additional dual sensor smoke detectors. Smart detectors, whether hard-wired or battery-powered, will also interconnect with other detectors of the same model.

Another option is the Roost Smart Battery, which replaces the nine-volt battery in smoke and CO detectors that use them. It adds WiFi, smartphone alerts, and in-app hushing (for battery-powered models only) to existing detectors. A third option is a smart listening device like the Leeo Smart Alert or Kidde RemoteLync Smart Home Monitor, which listen for existing detectors’ alarms and send you alerts when they go off.

4. Look for the UL Stamp
Check the detector packaging to make sure it meets the Underwriters Laboratories Standard—look for the UL label. You should also look for the date of manufacture printed on the back of the detectors. Devices lose their sensitivity over time, so the fresher, the better. As a rule of thumb, replace carbon monoxide detectors every five years and smoke detectors every 10 years.

UL has also announced updates to its standards, which will require all manufacturers to make smoke detectors that can distinguish between smoldering fires and cooking smoke by May 2020. And as part of UL’s updated standard, it is expected that manufacturers will not be able to make “traditional, single sensor” (photoelectric or ionization) smoke detectors after May 2020.

5. The Truth About Interconnected Detectors
You can interconnect some smoke and carbon monoxide detectors so that all units in the house sound an alarm when any single one is triggered. Some newer homes may have wiring already in place to link the detectors. In a home without such wiring, you can buy detectors that interconnect wirelessly. These interconnected smoke and carbon monoxide detectors are an important safety feature in a home with multiple levels, although standalone detectors may be adequate for a small, single-level home.

While interconnected alarms can make your home safer, they do have some limitations. In many cases interconnected alarms can only interconnect with models made by the same brand. In fact, the National Fire Protection Association’s Senior Electrical Specialist Richard Roux told CR that you should always check with the manufacturer to see which specific models are compatible, even within the same brand, as some brands could change their interconnect system over time.

Despite the limitations, we still recommend using interconnected detectors. They can make your home much safer by alerting you to danger before the fire or CO spreads throughout your home. Roux adds that interconnected alarms (whether hardwired or wireless) are also becoming a requirement in many states when you try to sell your home.

6. Consider Voice Alerts
Children tend to sleep more deeply than adults and may not awaken to a conventional alarm. Some smoke and combination smoke and carbon monoxide detectors use a voice command, but it's not confirmed whether that's the most effective way to wake children. According to one study, a greater percentage of children ages 6 through 12 awoke to the sound of their mother's prerecorded voice than to a tone alarm.

7. Consider Your Security System
You can incorporate some smoke and carbon monoxide detectors into a system that sounds an alarm outside and inside the house. Such systems may also have a monitoring service that can notify the police or fire department, or even call your cell phone.

Smoke and carbon monoxide detectors are only protective when installed correctly and if their batteries are replaced annually. Smoke rises, so mount smoke detectors on the ceiling or high on the wall. Test smoke and carbon monoxide detectors weekly and vacuum them monthly. Follow the manufacturer's recommendations regarding battery replacement. Replace carbon monoxide detectors every five years and smoke detectors every 10 years.