How many smoke and carbon monoxide alarms does a rental need?

Guests sleep in a home they don't know, often after a long drive, and they won't know where the stairs or the exits are in the dark. That's why alarms are the first purchase in the safety part of HostCalmly's planner, ahead of anything decorative.

The US Fire Administration (opens in a new tab) says to install smoke alarms "in every bedroom, outside each separate sleeping area" and "on every level of the home, including the basement." For carbon monoxide, the Consumer Product Safety Commission (opens in a new tab) says "CO alarms should be installed on each level of the home and outside sleeping areas."

Counts by home size

Our planner counts 1 smoke alarm per bedroom + 1 per floor, and 1 CO alarm per floor. In most layouts, the alarm on each floor sits in the hall outside the bedrooms, so it covers "outside each sleeping area" too. A home whose bedrooms are at opposite ends of a floor needs one more alarm outside each group of bedrooms.

Our planner also counts fire extinguishers: one on each floor plus one for the kitchen. The CO count assumes the home has fuel-burning appliances (a gas stove, furnace, water heater or fireplace) or an attached garage, the usual sources of carbon monoxide. An all-electric home with no attached garage may not need CO alarms under your local code, but many hosts install them anyway because they cost little.

Which kind to buy

• Sealed 10-year battery. The battery lasts the alarm's whole life, so there's no low-battery chirp at 3 am in the middle of a stay. The USFA says to replace any smoke alarm "when they are 10 years old" or "if they do not sound when tested."
• Combination smoke and CO. One device on the hallway ceiling does both jobs, so there's one less thing to mount. Check that your local code accepts a combination unit where it asks for CO.
• Plug-in CO alarm. It sits in an outlet near the bedrooms with no drilling, which suits a condo where you can't make holes. Check that it has a battery backup for power cuts.
• Interconnected or smart alarms. When one sounds, they all sound, and some notify you by phone. They cost more, and some local codes require them in new builds.

Keeping them working between guests

1. Test monthly. Both the USFA and the CPSC say to test alarms monthly. A turnover is a good moment: put "press test on every alarm" on the cleaner's checklist once a month.
2. Replace batteries yearly in alarms that take them. The CPSC recommends replacing smoke and CO alarm batteries every year. Sealed 10-year units skip this step.
3. Write the install date on the alarm. That way you know when the 10 years are up.
4. Show guests where things are. The emergency info sheet lists where the extinguisher, first-aid kit and shutoffs are, and the house manual repeats it.

Hypothetical example: a two-floor, three-bedroom cabin

Three bedrooms upstairs, a gas furnace and a kitchen downstairs.

• Smoke alarms: 3 bedrooms + 2 floors = 5
• CO alarms: 2 floors = 2, one in the upstairs hall outside the bedrooms and one downstairs
• Extinguishers: 2 floors + 1 kitchen = 3

If you use a combination smoke and CO alarm in the upstairs hall, it covers that floor's smoke and CO, so you'd buy 4 smoke-only alarms plus 1 combination unit, plus 1 CO alarm for downstairs.