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 in the event of 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 a finished attic and the basement. You should also have a carbon monoxide detector on each living level, in the basement, and near (not inside) an attached garage.

Before you shop, check regulations in your region. What you need, including types of detectors and their placement, can vary. Also, some insurance companies offer a 5 percent discount for homes with smoke detectors.

Fires burn differently: Some flare, some smolder. Make sure you purchase a smoke detector that can detect both types of fires.

Ionization Smoke Detectors are best at detecting the small particles typical of fast, flaming fires but in our tests, all tested poorly for detecting smoky, smoldering fires. Ionization units are prone to false alarms from burnt food and steam, so don't mount them near a kitchen or bathroom.

Photoelectric Smoke Detectors are best at detecting the large particles typical of smoky, smoldering fires but poor at detecting fast, flaming fires. Photoelectric units are less prone to false alarms from burnt food and steam, so you can install them safely around the kitchen or bathroom.

Dual-Sensor Smoke Detectors combine ionization and photoelectric technology to save you the hassle of installing two separate smoke detectors. But you will still need to install carbon monoxide detectors.

The latest smoke and carbon monoxide detectors have added features to better protect you and your family. Here's what to consider when tailoring the safest combination of detector options to your household's needs.

Power Source: Hard-wired smoke and carbon monoxide detectors tie into your home's wiring and require professional installation. Battery-only detectors are simple to install, and they work during a power failure, but most batteries require annual replacement. (Lithium batteries may last the life of the detector.) Plug-in detectors are available, but electric outlets are typically located low on the wall, while the optimal placement for the detector is on or near the ceiling.

Battery Backup: A backup battery for hard-wired smoke and carbon monoxide alarms offers security in case of a power failure. All battery-powered smoke and carbon monoxide detectors warn you when the battery is low. Some provide warning chirps, a low battery voice message, or a visual display.

Smart Smoke and Carbon Monoxide Detectors: You can link 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 have wiring already in place to link the detectors. In a home without such wiring, you can buy detectors that interconnect wirelessly. These smart smoke and carbon monoxide detectors are an important safety feature in a home with multiple levels. A standalone detector may be adequate for a small, single-level home.

Digital Carbon Monoxide Display: This feature displays carbon monoxide concentrations in parts per million, even when the concentrations are below the level that triggers the detector. Carbon monoxide detectors certified by UL must go off at no less than 70 ppm, but as little as 30 ppm can harm heart patients, pregnant women, and children. The display can alert you if the carbon monoxide level is inching up or is higher than usual. Some also show the peak level since they were reset, warning you of any spikes that occurred while you were away. Expect to pay a little extra for this feature: $5 or $10 for standalone detectors, slightly more for smart detectors.

Hush Button: To silence a nuisance smoke detector, pressing a hush button is more convenient than disabling the unit, and it avoids the possibility of forgetting to turn it back on. All the smoke detectors we tested had this feature. Some carbon monoxide and combination CO/smoke detectors can work with a remote control to silence a nuisance alarm.

Special Detector Types:
• Strobe light alarm are the best warning for the hearing impaired. Some smoke detectors have an integral strobe light, and some accept add-on strobes.

• Voice alarms. Children tend to sleep deeper than adults and may not awaken to a beeping sound. Some smoke and combination carbon monoxide and smoke detectors use a voice command, but it's not confirmed whether that's the most effective way to wake children. According to one study, many pre-teenagers who slept through tone detectors awoke to the sound of their mother's prerecorded voice.

• Safety lights. Some smoke detectors provide path illumination, a potential life-saver in the dark.

Overall Security Systems: You can incorporate some smoke and carbon monoxide detectors into a system that sounds an alarm outside and inside the house. It can also have a monitoring service notify the police or fire department or even call your cell phone.

Carbon monoxide detectors should be installed outside of each sleeping area of a house, each level of the house, and in the basement. If your carbon monoxide detector is a plug-in model without a cord, it needs to be plugged directly into an outlet. Make sure the outlet is out in the open and not behind furniture, curtains, or other objects that could restrict air flow.

Carbon monoxide detectors with a digital display should be mounted at eye-level so they can be read easily. If the CO detector is battery powered and doesn't have a display, it can be mounted anywhere on the wall or ceiling except within four inches of where the wall meets the ceiling. Air doesn't circulate freely at that level, which will delay the alarm response.

According to First Alert, and similar to other manufacturer's instructions, you should not install a carbon monoxide detector in the following locations:

• Within 5 feet of any cooking appliance.
• Outside or in direct sunlight.
• Garage, kitchen, furnace room, or any extremely dusty, dirty or greasy area. 
• Within 20 feet of a fuel-burning heat source (furnace), or fuel-burning appliance (water heater).
• Humid areas: Away from sources of high humidity, such as a bath or shower, sauna, humidifier, vaporizer, or dishwasher. If there are fuel-burning appliances in the laundry room or utility room, mount them in the room but as far away from the appliances as possible.
• Where temperature is colder than 40˚ F or hotter than 100˚ F, including unconditioned crawl spaces, unfinished attics, uninsulated or poorly insulated ceilings.
• In turbulent air: Near ceiling fans, heat vents, air conditioners, fresh air returns, or open windows. Blowing air may prevent CO from reaching the sensors.