IRC 2024 Expands Carbon Monoxide Alarm Locations to All New Homes

Quick Answer

IRC 2024 R315 requires carbon monoxide alarms in all new dwellings that contain any fuel-burning appliance or that have an attached garage. The 2024 edition broadens the trigger: prior editions focused on dwellings with fuel-burning appliances in a sleeping area or directly attached garages connected to a sleeping area, but the 2024 language captures any fuel-burning appliance anywhere in the dwelling and any attached garage regardless of its relationship to sleeping areas. CO alarms must be interconnected when smoke alarms are required to be interconnected, and 10-year sealed-battery units are permitted to serve as the battery backup just as they are for smoke alarms under R314.

What IRC 2024 Actually Requires

IRC 2024 R315.1 states the general application: carbon monoxide alarms shall be installed in new dwellings that have fuel-burning appliances or have attached garages. The triggering conditions are written with an “or” — either condition independently requires CO alarm installation. A home with only an attached garage but no fuel-burning appliances still requires CO alarms. A home with a gas range but no garage also requires CO alarms. A home with both, of course, requires CO alarms.

R315.2 specifies where the alarms must be located. Carbon monoxide alarms shall be installed outside each separate sleeping area in the immediate vicinity of the sleeping rooms. Additional CO alarms are required on each story of the dwelling. The code does not require a CO alarm inside each individual bedroom in the same way smoke alarms are required inside each sleeping room, but the alarm outside the sleeping area must be positioned to be audible inside the bedrooms with the doors closed.

R315.3 addresses interconnection. Where smoke alarms are required to be interconnected by R314.4, the carbon monoxide alarms required by R315 must also be interconnected, either with the smoke alarms or among themselves, so that activation of one CO alarm causes all interconnected alarms to sound. Listed wireless interconnection satisfies this requirement under IRC 2024 on the same basis as it satisfies the R314.4 smoke alarm interconnection requirement.

R315.4 covers power supply. Carbon monoxide alarms in new construction must receive primary power from the building wiring and include battery backup. The battery backup must be a sealed, non-replaceable battery with a minimum 10-year service life, consistent with the battery upgrade in R314.6 for smoke alarms. Combination smoke/CO alarms listed for both functions are explicitly permitted and will satisfy both R314 and R315 when installed in a location that meets the placement requirements of each section.

Why This Rule Exists

Carbon monoxide is odorless, colorless, and lethal at concentrations that produce no sensory warning. Occupants can become incapacitated while asleep before they detect any sign of a problem. The Centers for Disease Control and Prevention estimates that carbon monoxide poisoning causes more than 400 unintentional non-fire deaths in the United States each year and tens of thousands of emergency department visits. The actual exposure toll is believed to be higher because CO poisoning symptoms — headache, nausea, dizziness — mimic common illnesses and often go unattributed.

The expansion of the trigger in IRC 2024 to cover any fuel-burning appliance addresses a gap that existed when the rule was limited to appliances in or near sleeping areas. A gas furnace in a mechanical room, a gas water heater in a garage, a gas fireplace in a living room, or a wood-burning insert in a den can all produce CO under malfunction, improper venting, or spillage conditions. Limiting protection to homes where the combustion source is directly adjacent to sleeping areas misses a large share of real-world CO events, where the source is in a mechanical space and the gas migrates through ductwork, wall cavities, or pressure differentials to reach sleeping areas.

The attached garage trigger covers a separate pathway: vehicle exhaust. Running an engine in an attached garage — even briefly — or leaving a car idling in a garage that shares a wall or ceiling with living space can produce CO concentrations that infiltrate the dwelling quickly. Modern attached garages are often better sealed than older ones, which reduces passive air exchange and increases the risk of CO accumulation when a combustion source is present.

What the Inspector Checks at Rough and Final

At rough-in inspection, the inspector confirms that CO alarm circuits are roughed in outside each sleeping area and on each story where required. If the project uses hardwired interconnection, the interconnection conductors must be in place. If wireless interconnection will be used, the inspector should be informed of the product selection and interconnection method before drywall is closed so there is no dispute at final.

At final inspection, the inspector checks that CO alarms are installed in every required location: outside each separate sleeping area and on each story. If the dwelling has an attached garage, the inspector confirms CO alarms are in place regardless of whether fuel-burning appliances are present. Devices must be powered, listed for carbon monoxide detection, and seated on their bases. The inspector will test an alarm and verify that all interconnected CO alarms and interconnected smoke alarms sound together as required by R315.3.

The inspector will also review whether combination smoke/CO units are used and confirm that the placement satisfies both smoke alarm and CO alarm location rules. A combination unit placed to satisfy only the CO rule may not satisfy the smoke alarm location rule, and vice versa. If a combination unit is being used to cover both requirements at a single location, the inspector will verify that the location works for both.

Product listing is a specific inspection point. CO alarms must be listed in accordance with UL 2034 or a comparable standard. Smoke alarms listed to UL 217 alone do not satisfy the CO alarm requirement. Confirm the product carries the CO listing before installation.

What Contractors Need to Know

The broadened trigger in IRC 2024 means that CO alarm planning is now a standard part of every new home, not a conditional add-on for homes with particular appliance types. Before the building plan is finalized, confirm whether the home has any fuel-burning appliance and whether the garage is attached. In practice, nearly every new single-family home will have at least one of these conditions and often both.

Layout planning for CO alarms should begin alongside smoke alarm layout because the interconnection requirements link the two systems. If smoke alarms are hardwired and interconnected, CO alarms must also be interconnected. If wireless interconnection is used for smoke alarms, the CO alarms must also be part of a listed wireless interconnection system. Running both systems through the same planning process avoids the situation where the smoke alarm system is finalized and the CO alarm system is added as an afterthought with incompatible interconnection.

Combination smoke/CO alarms can simplify layout and reduce device counts, but the placement rules must be reviewed carefully. CO alarms are not required inside each sleeping room the way smoke alarms are, but they are required outside each separate sleeping area. A combination unit placed outside the sleeping area satisfies the CO outside-sleeping-area requirement and the smoke outside-sleeping-area requirement simultaneously. The smoke alarm inside each bedroom, however, must still be a separate device unless the bedroom unit is also a listed combination alarm and placement works for both functions.

Sealed 10-year battery backup is required for CO alarms in new construction under IRC 2024. Order devices that are specifically listed with this battery type and verify the listing label. The same procurement discipline that applies to smoke alarms under R314 applies here.

What Homeowners Get Wrong

The most common homeowner misconception is that CO alarms are only needed near the furnace or in the mechanical room. R315.2 places CO alarms outside sleeping areas and on each story, not directly at the appliance. The alarm near the furnace might detect a high concentration event quickly, but it does not warn sleeping occupants, who may be separated from the mechanical room by multiple closed doors, floors, and hallways. The sleeping-area placement is the primary life-safety location.

Many homeowners believe that if their home is “all electric” with no gas appliances, they do not need CO alarms. Under IRC 2024, this is true only if there is also no attached garage. A home with an electric heat pump, electric water heater, electric range, and electric dryer still requires CO alarms if it has an attached garage, because vehicle exhaust from the garage is the triggering condition, not only fuel-burning appliances inside the dwelling.

Homeowners also assume that a CO alarm that is several years old continues to meet code requirements. CO alarm electrochemical sensors have a finite service life, typically five to seven years for the CO sensing element, even if the device appears to function and passes the test button check. The test button on most CO alarms checks the electronics and alarm circuit, not the sensor’s ability to detect low-level CO. Replace CO alarms according to the manufacturer’s stated service life, which is printed on the device or in the instructions.

Finally, homeowners often do not understand the interconnection requirement. A CO alarm that beeps on its own when CO is present provides warning only to occupants near that device. Interconnection ensures that every alarm in the dwelling sounds, waking occupants throughout the home and giving those in distant sleeping areas the same response time as those closest to the triggering device.

State and Local Amendments

Many states adopted CO alarm requirements years before the IRC addressed the issue, and those state-level requirements may be more stringent than IRC 2024 in terms of locations, device standards, existing-home applicability, or point-of-sale triggers. California, New York, Illinois, New Jersey, and numerous other states have comprehensive CO alarm laws that apply to existing dwellings, rental properties, and resale transactions, not only new construction.

In states with active CO alarm legislation, the state law typically controls over the IRC where the state law is more stringent. For example, some state laws require CO alarms in every sleeping room, not just outside the sleeping area, which goes beyond the R315.2 placement rule. Other state laws require CO alarms in homes that are more than a specified number of years old, trigger CO alarm upgrades at change of occupancy, or require landlords to certify CO alarm compliance at each lease term.

Local amendments can also restrict or clarify the IRC 2024 CO alarm requirement. Some jurisdictions require CO alarms on each story where there is a sleeping area rather than on every story, while others add requirements for CO alarms in specific locations such as near attached garages or fuel-fired appliances. Confirm the applicable local and state requirements before finalizing device placement and product selection for any permitted project.

When to Hire a Professional

Hire a licensed electrician for CO alarm installations in new construction that require building-wiring primary power, interconnection conductors, or circuit work. CO alarm circuits are life-safety wiring and must meet the same installation standards as smoke alarm wiring under the adopted electrical code.

A professional is also appropriate when a remodel adds an attached garage to an existing home, converts a space to an attached garage, adds a fuel-burning appliance to a dwelling that did not previously have one, or triggers a permit that requires CO alarm compliance as a condition of approval. These changes can require not only new CO alarm locations but also coordination with existing smoke alarm interconnection systems to ensure the combined system meets R315.3.

For existing-home battery replacement where no new wiring is required, a homeowner can typically replace CO alarms independently, but the replacement devices must meet current listing and battery requirements for the applicable jurisdiction. Confirm whether the state or local law requires sealed 10-year batteries before purchasing replacement devices.

Common Violations Found at Inspection

• Omitting CO alarms entirely in a home with an attached garage on the assumption that no fuel-burning appliances means no requirement.
• Installing CO alarms only in the mechanical room or near the furnace rather than outside the sleeping areas and on each story as R315.2 requires.
• Using smoke alarms listed only to UL 217 as the CO alarm, failing the product listing requirement for carbon monoxide detection.
• Failing to interconnect CO alarms when smoke alarms are interconnected, violating R315.3.
• Wireless CO alarm interconnection using devices from different manufacturers or product lines that do not share a listed wireless interconnection feature.
• Replaceable-battery CO alarms installed as the backup power source in new construction where IRC 2024 requires sealed 10-year batteries.
• Combination smoke/CO units placed to satisfy only the CO outside-sleeping-area requirement without a separate smoke alarm inside each sleeping room.
• CO alarms placed on a circuit with a disconnecting switch other than overcurrent protection, which interrupts the required primary building-wiring power.
• Missing CO alarm coverage on an upper story or in a finished basement because the installer treated the sleeping-area alarms as covering the entire dwelling.
• Expired CO alarms left in service past the manufacturer’s stated sensor service life, which is typically shorter than the device’s apparent functional life.