You can't smell carbon monoxide, so put an alarm on every floor

The one

A carbon monoxide alarm that works when the power doesn't, and enough of them to cover every level of your house.

The spec profile:

• Listed to the standard. In the U.S., look for UL 2034 on the box and on the alarm. In Canada, look for CSA 6.19. An unlisted alarm is a decoration.
• Power that survives an outage. Either a plug-in or hardwired model with battery backup, or a battery model with a sealed battery the maker rates for the alarm's full life. Plug-in with no backup is the one to avoid.
• A printed replacement date. The maker sets a service life for the sensor. It's on the box and usually on the back of the alarm. Find it before you buy.
• An end-of-life signal. The alarm should tell you when it's worn out. Read the manual to learn what that chirp sounds like.
• A test button. Every listed alarm has one.

Digital readouts and wireless interconnect are nice. They're not the one thing you need.

Why this one

Carbon monoxide comes off anything that burns fuel: a gas furnace, a water heater, a stove, a fireplace, a car idling in the garage, a generator. You can't see it and you can't smell it. The CDC says hundreds of Americans die from accidental CO poisoning every year, and many more end up in the emergency room.

The early symptoms are headache, dizziness and nausea. That reads as the flu, or a bad night's sleep. People in the house go to bed feeling lousy and don't wake up. An alarm is the only thing in the house that notices.

Every level, because that's the guidance. The Consumer Product Safety Commission and the NFPA both say to put alarms on every level of the home, including the basement, and outside each sleeping area. Take a two-story house with a finished basement and three bedrooms upstairs. That's an alarm in the basement, one on the main floor, and one in the upstairs hall outside the bedrooms. Three alarms. Say they run $30 apiece, an example and not a quote, and you've spent less than a tank of gas for a pickup.

Follow the maker's placement instructions on the box. Don't tuck one right beside the furnace or the stove, where a brief puff can set it off for no reason.

The outage is the point. Think about when CO shows up in a house. It's the ice storm, the hurricane, the three-day outage. The power's out, somebody drags a generator closer to the garage than they should, somebody lights the camp stove indoors. That's the night the alarm matters most, and it's the night a plug-in with no battery backup is dead on the wall.

If you run a generator, the placement rules in Twenty Feet From the House and Other Generator Rules come first. The alarm is your second line. Don't make it your only one.

What to skip

• Smart alarms with a subscription. An app that texts you is fine if you own a vacation house. Don't pay a monthly fee for something a siren in the hallway already does.
• One alarm "in the middle of the house." CO doesn't care about your floor plan. A basement furnace leaking into a basement with no alarm is a problem that one upstairs unit may notice late.
• Alarms with no date. If you can't find a manufacture or replace-by date on the box, put it back. Same goes for anything on a clearance shelf past its shelf life. Check the date before it goes in the cart.
• Aerosol "test" gimmicks. The test button is what the maker tells you to use.
• Extended warranties on a small device. Read the maker's warranty instead. Look at the length and whether it covers replacement of the whole unit.
• Exotic sensitivity claims. Alarms are built to a standard so they don't sound off at low, brief exposures. A box shouting about "ultra sensitive" isn't telling you anything the standard doesn't.

If your situation is different

You're in an all-electric house with no attached garage. Your risk is lower, but it isn't zero. A generator, a charcoal grill dragged inside, a car left running, or a gas stove at a neighbor's can all change that. One alarm outside the bedrooms is a sensible minimum. Check your local code.

You rent. Rules about who supplies and maintains the alarms vary by state and by city. Read your lease and ask the landlord or property manager. If there's none, buy a battery-powered one and put it up. It's yours, and it goes with you.

You've got a combination smoke and CO alarm. They're fine. Just know the two sensors can wear out on different schedules. Go by the earlier replacement date the maker prints, and read the manual to see which chirp means what.

Someone in the house is hard of hearing or sleeps like a rock. Look for models made for that, with strobes or a connection to a bed shaker. For kids who sleep through everything, look at interconnected alarms so one going off sounds all of them.

You've got a camper, an RV or a boat. Those need a CO alarm listed for that use. Check the maker's spec sheet and don't assume a household one is rated for the vibration or the temperature swings.

Before you buy

• Find the UL 2034 or CSA 6.19 listing on the package.
• Check the power: battery backup if it plugs in or is hardwired, or a sealed battery if it doesn't.
• Find the sensor's service life and the replace-by date.
• Count your levels, then add one for each separate sleeping area.
• Read the warranty document. Look at the length, whether it replaces the unit, and what voids it.
• Check the current price.

When you hang them, write the install date on the back with a marker. Press every test button once a month, the same day you check the smoke alarms. If one ever goes off for real, get everyone outside into fresh air, call 911 from out there, and don't go back in to grab anything. The house can wait. Nobody in it can.