How Many Carbon Monoxide Detectors Do I Need?

You just moved into a two-story house with a basement furnace and an attached garage. The previous owner left a single CO alarm in the upstairs hallway — and now you are wondering: is one enough, or do I need more? Most people do not want a "smart home" lecture; they want one clear answer. This page gives a practical checklist based on two ideas that cover most real-world layouts: coverage by level and coverage near sleeping areas. Think of a CO alarm as a last line of defense — if it cannot be heard or is too far from where people sleep, it will not do its job.

This is general safety information — not medical advice. If a CO alarm sounds or you suspect exposure, move to fresh air and contact emergency services.

Key Takeaways

One CO alarm per occupied level (including a finished basement) is the baseline minimum.
At least one alarm near sleeping areas so it can wake people up at night.
Add extra alarms for long layouts, separate sleeping wings, attached garages, or distant fuel-burning appliances.
You do not need a detector in every room — strategic placement beats total room count.

Baseline Rule: One per Level + Sleeping Areas

For most homes, the safest minimum is simple: one CO alarm on every level of the home (including a finished basement) and at least one alarm near sleeping areas so it can wake people up at night. In many layouts, the "bedroom hallway" alarm and the "level" alarm are the same unit.

A quick way to count the minimum

Count the levels you live on (basement, main floor, second floor, finished attic).
Add one for each separate sleeping wing or long hallway where an alarm might not be heard clearly.
Add one more if you have an attached garage or a fuel-burning appliance area that is far from the nearest alarm.

When You May Need More Than the Minimum

Long homes or split layouts

If your home is ranch-style, split-level, or has thick doors between areas, add another alarm so no sleeping room is "around the corner" from CO protection. This is the practical meaning of carbon monoxide detectors near bedrooms: keep at least one alarm close to where people sleep.

Multiple sleeping wings

Homes with bedrooms on opposite ends, multi-suite houses, or a main bedroom on the first floor plus kids upstairs often need more than one "near bedrooms" alarm. If you cannot clearly hear the test tone from the hallway alarm in a closed-bedroom situation, add coverage.

Attached garage

An attached garage is a major reason people ask "do I need a carbon monoxide detector in every room?" You do not — but you do want reliable coverage in the living space connected to the garage, plus a sleeping-area alarm as backup.

Distant fuel-burning appliances

If your furnace, boiler, water heater, or fireplace is on a level or in a zone that is distant from the closest alarm, add coverage so a problem is detected early. This is especially helpful when appliances are in a finished basement or utility area separated by doors.

Apartment Considerations

Small/medium apartment: 1 alarm near the sleeping area is often a reasonable minimum.
Large apartment or long hallway: 2 alarms — one near bedrooms and one closer to the main living area.
Unit above/next to a garage or mechanical room: consider an additional alarm on the side nearest that risk.

If you rent, check whether the building provides alarms and whether you are allowed to add battery-powered units. Even when a landlord provides the minimum, adding an extra alarm near bedrooms improves real-world safety.

Combination Smoke/CO vs Separate Units

Combination smoke/CO alarms reduce device count and work well when placed for both hazards. The practical approach: use combinations where they naturally fit your smoke-alarm plan, and use a dedicated CO unit when you need better placement flexibility for CO coverage.

Battery Backup and Power Outage Planning

Power outages are when people run generators or use alternative heat — which can increase CO risk. That is why "hardwired only" is not ideal unless it includes a working battery backup. Make sure at least one alarm near sleeping areas runs during outages. For generator-specific rules, see generator distance from the house.

Testing and Replacement

Buying the right number of alarms is step one. CO alarms have sensors that age, so they need periodic replacement. Test monthly using the test button, replace at end-of-life per the manufacturer label (commonly 5–10 years), and never ignore chirps — low battery and end-of-life signals are different, but both need action. See how long do carbon monoxide detectors last.

Quick Home Checklist

One alarm on each level you live on.
At least one alarm near sleeping areas (hallway outside bedrooms).
Extra unit for long layouts or separate sleeping wings.
Coverage on the level connected to an attached garage.
Battery backup so alarms work during outages.
Test monthly; replace at manufacturer end-of-life date.

Frequently Asked Questions

Is one CO detector enough for an apartment?

For a small or medium apartment, one alarm near the sleeping area is often a reasonable minimum. For large apartments or units near garages or mechanical rooms, a second alarm closer to the living area improves coverage.

Do I need a CO detector in the basement?

If the basement is occupied (bedroom, family room, office) or contains fuel-burning equipment, yes. Do not rely on an upstairs alarm to cover a basement — stairwells and closed doors delay detection.

Do I need a CO detector in the garage?

Typically not in the garage itself (harsh conditions damage sensors). Instead, place a unit inside the home near the door leading to the attached garage.

Can I use a plug-in CO alarm?

Yes. Plug-in models with battery backup work well, especially in apartments. Make sure the outlet is not hidden behind furniture and the alarm sound can reach sleeping areas.

Do combination smoke/CO alarms work as well?

Combination units can work well when chosen and placed for both hazards. They reduce device count. Use a dedicated CO unit when you need better CO-specific placement flexibility.

Sources & References

Published: January 15, 2024