Where a House Needs CO Alarms Under the 2021 IRC

Why this matters

Most customers put their CO alarm next to the furnace, because that is where they imagine the CO comes from. The code puts it somewhere else: outside the bedrooms, because the danger of carbon monoxide is that it reaches people while they sleep and cannot notice it. A plug-in CO alarm in the basement beside the furnace will sound, but two floors below a closed bedroom door it may not wake anyone. A tech who knows where the code actually wants CO alarms can tell a customer in one sentence why theirs is in the wrong place, and can spot the house that needs one even though it has no furnace at all.

The trigger: which houses need CO alarms

In the 2021 IRC, R315, as adopted by your jurisdiction, a dwelling unit gets CO alarms when either or both of these is true:

• It contains a fuel-fired appliance. A gas or oil furnace or boiler, a gas water heater, a gas range or cooktop, a gas dryer, a gas or wood fireplace or stove. One is enough.
• It has an attached garage with an opening that communicates with the dwelling unit. A door from the garage into the house is the usual opening. The reasoning is the car: a vehicle idling in an attached garage produces CO that a door, even a closed one, does not reliably keep out.

The second trigger is the one people miss. An all-electric house with a heat pump, an electric range and an electric water heater still needs CO alarms if it has an attached garage with a door into the house.

That is the new-construction rule. For an existing house, the CO requirement reaches back when permitted work is done, with its own exceptions, including the asymmetry that a permitted combustion appliance replacement triggers CO alarms while a non-combustion one does not. That trigger is owned by its own card; see related: Smoke and CO Alarm Requirements Your Permit Triggers. Read your adopted edition for any local exceptions or amendments, and note that many states also require CO alarms by statute, independent of the building code, sometimes in more locations or on resale or rental.

Where the alarms go

Under the 2021 IRC R315.3, as adopted, paraphrased:

• Outside each separate sleeping area, in the immediate vicinity of the bedrooms. One alarm per sleeping area, placed so it covers the doors of the bedrooms in that area.
• Inside a bedroom, where a fuel-fired appliance is located in that bedroom or in its attached bathroom. A bedroom with a gas fireplace, or a primary suite whose bathroom has a gas water heater closet, gets its own alarm inside the room.

A "separate sleeping area" is a group of bedrooms served by one hallway or vestibule. Three bedrooms off one upstairs hall are one sleeping area. A guest bedroom on the main floor across the house from them is a second one. A basement bedroom is another. Each needs its own alarm outside it.

"Immediate vicinity" has no number in the IRC. Read it as the hall or vestibule the bedroom doors open onto, close enough that a sounding alarm is heard through a closed bedroom door. Where a state statute or the alarm maker gives a maximum distance from bedroom doors, use that figure. Mounting height also comes from the maker's instructions: CO mixes through room air rather than pooling at the floor or ceiling, so makers allow a range of heights, and the instructions say which.

Note what the IRC does not require: an alarm near the furnace, in the garage, or on every story. Those may be good additional locations, and the alarm maker's instructions often recommend some of them, but they are not the required ones. In the garage itself, a CO alarm is usually the wrong device, because exhaust, temperature and humidity there are outside most household alarms' listed environment; read the instructions before placing one.

NFPA 72 asks for more in some editions. NFPA 72 absorbed the former CO standard, NFPA 720, and its household CO provisions have called for alarms in more places than the IRC, including on each occupiable level in some editions. Where your jurisdiction adopts NFPA 72 for this directly, or a state statute sets locations, the stricter set governs. The alarms themselves are listed to UL 2034; a combination smoke and CO alarm is listed to both UL 217 and UL 2034 and is permitted in place of separate units.

The diagram: one house, three required locations

A two-story house with a basement furnace, a gas range, an attached garage with a door into the mudroom, a main-floor guest bedroom, and a gas fireplace in the upstairs primary bedroom.

SECOND FLOOR
+-------------+-------------+-----------------+
| BEDROOM 2   | BEDROOM 3   | PRIMARY BEDROOM |
|             |             | gas fireplace   |
|             |             | [CO 3] inside   |
+---- door ---+---- door ---+----- door ------+
| HALL  [CO 1] outside the upstairs sleeping   |
+---------------------------------------------+

FIRST FLOOR
+-------------+----------+----------+---------+
| GUEST BED   | HALL     | KITCHEN  | MUDROOM |
|             | [CO 2]   | gas      | door to |
|             |          | range    | GARAGE  |
+-------------+----------+----------+---------+
                                    | GARAGE  |
BASEMENT: gas furnace, gas WH       | attached|
(no required location here)         +---------+

Three required alarms: CO 1 in the upstairs hall outside bedrooms 2, 3 and the primary; CO 2 outside the guest bedroom, which is a separate sleeping area; CO 3 inside the primary bedroom because of the fireplace. The furnace, the water heater and the garage each make the house need CO alarms; none of them is a required location.

Worked example: an HVAC crew's walk-through

An HVAC tech is in the house above to service the furnace, with no permit involved. The customer has two plug-in CO alarms: one beside the furnace in the basement, one in the kitchen. Before leaving, the tech checks the house against R315 the way an inspector would on new work, so the customer knows where they stand.

Trigger check. Fuel-fired appliances: furnace, water heater, range, fireplace, four of them. Attached garage with a door into the house: yes. Either condition alone triggers CO alarms; the house meets both.

Sleeping areas. Upstairs: bedrooms 2, 3 and the primary share the upstairs hall, so that is one area. Main floor: the guest bedroom is off a separate hall on another floor, a second area. That makes 2 sleeping areas, and 2 alarms outside them.

Bedrooms with a fuel-fired appliance in the room or its attached bath. The primary bedroom has a gas fireplace: 1 alarm inside it. No other bedroom qualifies.

Required total: 2 + 1 = 3. Of the customer's two existing alarms, the basement one and the kitchen one, neither is in a required location. The house has 0 of 3 required locations covered.

What the tech tells the customer. In about two minutes, and on the job note: the alarms you have are not where the code wants them; the alarm that matters most is the one outside the bedrooms, because CO is a danger while you sleep; you need one in the upstairs hall, one outside the guest room, and one inside the primary bedroom because of the fireplace. The kitchen unit can move to the guest-room hall today. The basement unit can stay as an extra if the customer wants it; it does no harm.

Check the dates while you are there. Both plug-ins have manufacture dates on the back; record them and their replace-by dates (see related: Reading the Manufacture Date and the Ten-Year Replacement Rule).

What changes the answer.

• Detached garage, all-electric house. No fuel-fired appliance, no attached garage: the 2021 IRC does not require CO alarms. A portable generator or a charcoal grill used near the house is still a CO source, and some states require alarms regardless, so the advice stays "recommended," but the code does not reach it.
• The furnace is replaced under permit. Now R315's retrofit provision applies, the inspector expects the three locations above, and the alarm work belongs in the HVAC quote. See related: Smoke and CO Alarm Requirements Your Permit Triggers.
• A basement bedroom is added. That is a third sleeping area and a fourth required location, and if the bedroom has a gas water heater in an attached bath closet, a fifth inside the room.
• More than one CO alarm is required. The 2021 edition also wants required CO alarms interconnected, with relaxations for existing areas similar to the smoke alarm ones. Listed combination or wireless-interconnect units are how that is usually met without opening finishes.

Mistakes worth catching on a walk-through

One alarm for a house with split bedrooms. A single CO alarm in the upstairs hall leaves the main-floor guest room uncovered. Count sleeping areas, not floors.

The fireplace bedroom. A gas fireplace in a primary suite is the case most often missed, because the hall alarm outside the door feels close enough. The code puts one inside.

Behind furniture or in a cabinet. A plug-in alarm in an outlet behind a couch, or a battery unit in a drawer, is in the right room and still not working. Follow the maker's height and clearance guidance, and keep it out of dead air.

The CO alarm as a combustion test. An alarm in the right place is the occupants' warning, not a measurement. If a tech suspects a CO problem, it is measured with an instrument, by the trade that services the appliance. See related: Carbon Monoxide Testing + Response Reference.

References

• International Residential Code, 2021 edition as adopted by your jurisdiction: R315 (carbon monoxide alarms), including R315.3 (location)
• NFPA 72, in the edition your jurisdiction adopts, which absorbed NFPA 720 household CO provisions
• UL 2034 (CO alarms) and UL 217 (smoke alarms), for combination units listed to both
• See related: Smoke and CO Alarm Requirements Your Permit Triggers; Carbon Monoxide Testing + Response Reference; The CO Alarm End-of-Life Signal and How Long a Sensor Lasts