Where to Put Carbon Monoxide Detectors at Home

Why the bedroom hallway comes first

Carbon monoxide is dangerous, and it is most dangerous to people who are asleep and cannot notice anything wrong. An alarm mounted in the hall outside the bedrooms is there to wake them while there is still time to walk out. New Jersey's fire safety guidance describes the intent in the same terms: alert occupants in the area outside the bedrooms before the hazard reaches those rooms.

If bedrooms sit at opposite ends of a floor, or a top-floor bedroom has its own stair, each separate sleeping area gets its own alarm. A unit inside the bedroom as well does no harm, especially where someone sleeps with the door shut.

Every level, matched to the house you live in

Federal consumer safety guidance recommends alarms on each level and outside sleeping areas. How that plays out depends on the layout. Older houses in this region stack rooms vertically, and the fuel-burning equipment is usually at the bottom of the stack, two or three floors away from the bedrooms.

In a home with a nursery or a family member who sleeps downstairs, treat that room as its own sleeping area. The goal is simple: no one sleeps more than a closed door and a short hallway away from a working alarm.

• Brownstone or rowhouse: the garden level or cellar with the boiler, the parlor floor, and each bedroom floor above
• Philadelphia-style rowhome: the basement with the heater and water heater, the first floor, and the bedroom hall upstairs
• Center-hall colonial or split-level: the basement, the main floor near the family room fireplace, and the upstairs hall
• Two-family house: each unit's sleeping area and living floor, plus the shared basement
• Any house with an attached garage: near the door from the garage into the house, at the distance the manual allows

High, low or in between

Height is the question people argue about most. Carbon monoxide weighs about the same as air and mixes with it, so it does not pool on the floor or collect at the ceiling the way smoke does. The correct height is the one printed in the manual or on the back of the alarm. Some models are built for the ceiling or high on a wall, some list a range of wall heights, and plug-in models are designed to sit in a standard outlet near the floor.

Combination smoke and carbon monoxide alarms follow the smoke rule and go on the ceiling or high on the wall, because smoke rises and the smoke sensor needs to be up there. Mounting one low to suit the carbon monoxide side would slow its response to a fire.

How close to the boiler, stove or fireplace

Fuel-burning appliances can release a brief puff of exhaust when they light, before the flue warms and draws properly. An alarm mounted right next to the boiler or the fireplace opening may catch that puff and sound, and people who get a few false alarms start pulling batteries. Most manuals give a minimum distance from fuel-burning appliances for exactly this reason.

Put the basement alarm in the same space as the boiler and water heater but at the distance the manual lists, on a wall or ceiling with open air around it. In a room with a fireplace or wood stove, mount it across the room rather than on the chimney breast, where heat can also shorten the alarm's life.

Spots that cause trouble

Manuals list places where an alarm will either sound when it should not or stay quiet when it should sound. These are the ones that turn up most in older homes.

• Inside a bathroom or laundry room where steam and humidity build up
• Beside a window, an exterior door, a ceiling fan or a heating or cooling vent that dilutes the room air
• In an unheated attic, porch or garage colder or hotter than the alarm is rated for
• In direct sun or above a radiator
• Behind curtains, inside a closet or blocked by a sofa pushed against a plug-in unit

Interconnected, plug-in or hard-wired

The same federal guidance recommends interconnected alarms, so that when one sounds they all sound. That matters in tall, narrow houses, where an alarm in the cellar may not be heard three floors up with bedroom doors closed. Interconnection can be wired or wireless, depending on the product line.

It also recommends battery-operated alarms or alarms with battery backup. Winter storms knock out power in this region, and a plug-in or hard-wired unit without a battery goes silent exactly when people start running generators and space heaters. A portable generator belongs outdoors, well away from doors, windows and vents.

What NYC, New York State and New Jersey rules say

New York City's housing agency tells building owners they must install at least one approved carbon monoxide detector in each dwelling unit, within 15 feet of the primary entrance to each sleeping room, with an audible end-of-useful-life alarm. Tenants are responsible for maintaining the devices. New York State law requires an operable carbon monoxide detector in one- and two-family dwellings, condominium and cooperative units and multiple dwellings, where the unit has appliances, devices or systems that may emit carbon monoxide or has an attached garage.

New Jersey's fire safety bulletin says one- and two-family homes with a fuel-fired appliance or an attached garage need carbon monoxide alarms in the vicinity of the bedrooms, and they may be battery-operated, plug-in or hard-wired. The bulletin notes that the appendix to NFPA 720 describes in the vicinity of as within 10 feet. Philadelphia and Delaware have their own requirements. Rules change, so confirm with your local building or fire department, and treat this summary as a starting point rather than legal advice.

Testing, replacing and keeping a record

Test each alarm as often as its manual says, replace batteries on schedule, and replace the whole unit when it reaches the end of its rated life. Write the install date on the back with a marker so nobody has to guess later. A chirp after a battery change is often the end-of-life signal.

When you buy a house or move into an apartment, assume the alarms are as old as the paint until proven otherwise. Look for a manufacture date on the back, replace any unit without one, and walk each floor to confirm that every sleeping area and every level is covered. Older brownstones and rowhouses often have one alarm per floor mounted wherever an outlet happened to be, which may leave a top-floor bedroom hall uncovered.

An alarm warns you about exhaust in the house. It does nothing about why the exhaust got there. The national chimney standard, NFPA 211, sets a yearly inspection for chimneys and vents, and that visit is where a blocked or broken heating flue is caught before an alarm has to catch it.