When Are Carbon Monoxide Alarms Required Under IRC 2018?

Quick Answer

IRC 2018 Section R315.1 requires carbon monoxide alarms in new dwellings that contain fuel-burning appliances or have an attached garage. If your home has a gas furnace, gas water heater, gas range, wood-burning fireplace, or an attached or built-in garage, a CO alarm is mandatory. The alarm must be placed outside each separate sleeping area in the immediate vicinity of the bedrooms.

What R315.1 Actually Requires

IRC 2018 Section R315.1 states that carbon monoxide alarms shall be installed in dwelling units that contain fuel-burning appliances or have attached garages. A fuel-burning appliance is any combustion appliance — gas or propane furnaces, boilers, water heaters, ranges, ovens, clothes dryers, fireplaces (gas or wood), and space heaters. An attached garage is any garage that shares a wall or ceiling with the dwelling, regardless of whether it has a door connecting directly to the living space.

Under R315.1, the trigger is the presence of a fuel-burning appliance OR an attached garage. A home with an electric-only mechanical system but an attached garage still requires CO alarms because vehicle exhaust from the garage is a carbon monoxide source. Similarly, a home on a slab with no garage but a gas furnace and water heater requires CO alarms due to the fuel-burning appliances.

Section R315.2 specifies placement: CO alarms must be installed outside each separate sleeping area in the immediate vicinity of the bedrooms. This is the hallway just outside the bedroom doors, not necessarily inside the bedrooms themselves. Where a dwelling has sleeping areas on multiple levels or in physically separate wings, a CO alarm is required in the immediate vicinity of each cluster of sleeping rooms.

Alarms must be listed to UL 2034 or CSA 6.19. Combination smoke/CO alarms that are listed to both UL 217 and UL 2034 satisfy both the smoke alarm requirement of R314 and the CO alarm requirement of R315 with a single device, provided the device is installed in a location that meets both sections' placement rules.

Why This Rule Exists

Carbon monoxide is odorless, colorless, and lethal at concentrations achievable within hours in a home with a malfunctioning combustion appliance or a running vehicle in an attached garage. The gas displaces oxygen in the bloodstream and causes confusion, unconsciousness, and death, often while occupants are asleep. Because CO symptoms mimic flu, many victims do not recognize the danger. The Consumer Product Safety Commission estimates that several hundred people die from accidental non-fire CO poisoning each year in the United States, with most deaths occurring in homes. An alarm at the sleeping area provides the earliest possible warning when occupants are most vulnerable and least able to detect the hazard themselves.

What the Inspector Checks at Rough and Final

At rough inspection the inspector verifies that rough-in wiring is present for hardwired CO alarms if the project uses hardwired units. Battery-only CO alarms are permitted in existing construction being renovated, but new construction typically requires hardwired units or combination smoke/CO units on the hardwired smoke circuit.

At final inspection the inspector confirms: (1) at least one CO alarm is installed outside each bedroom cluster; (2) the alarm is listed to UL 2034 or CSA 6.19 as required by R315.1; (3) a combination alarm satisfies both the smoke alarm requirement of R314 and the CO alarm requirement of R315 if only one device is present; (4) the alarm responds when tested. The inspector will also verify that the trigger conditions are met — looking for any fuel-burning appliance on the plans or a garage footprint that shares a wall with the house. If neither condition is present (all-electric home with detached garage), no CO alarm is required under IRC 2018.

What Contractors Need to Know

Carbon monoxide alarms have a finite service life — typically 5 to 7 years for the electrochemical sensing cell. Unlike smoke alarms, CO alarms will chirp or display an end-of-life signal when the sensor degrades. Homeowners who silence an end-of-life alarm without replacing it are left with a non-functional CO detector that passes visual inspection but provides no protection. Contractors completing CO alarm installations should advise clients of the expected replacement cycle and mark the installation date on the unit for future reference.

When installing a gas furnace, water heater, gas range, or fireplace in a new dwelling, CO alarm installation is part of the scope of work under IRC 2018 R315.1. Coordinate CO alarm placement during the electrical rough-in. If the project uses combination smoke/CO alarms, verify the unit is listed for both functions and plan the circuit to reach all required locations.

Pay attention to the garage trigger: an attached garage mandates CO alarms even when all other mechanical and appliances are electric. Review the site plan for garage attachment before submitting for permit. If a previously detached garage is being converted to an attached garage, CO alarms must be added. Run the CO alarm circuit on the same interconnected circuit as smoke alarms if combination units are used, so all alarms alert simultaneously.

When a fuel-burning appliance is added during a renovation to an existing home that currently has no CO alarms, the mechanical permit for the appliance triggers the requirement to install CO alarms. Contractors pulling a mechanical permit for a new gas furnace or water heater replacement in an existing home should confirm whether CO alarms are present and compliant before the final inspection. A failed final because CO alarms were not addressed is a common and preventable callback.

What Homeowners Get Wrong

Many homeowners believe CO alarms are only needed if they have gas appliances, overlooking the attached garage trigger. A home with an all-electric mechanical system and kitchen appliances still needs CO alarms if the garage is attached. Vehicle exhaust during the brief time it takes to start a car and back out can introduce dangerous CO concentrations into the living space through the door or shared wall, especially in cold weather when engines run rich.

Another common error is placing CO alarms near the floor. Unlike smoke, carbon monoxide is approximately the same density as air and distributes evenly throughout a room. Mounting height recommendations in UL 2034 call for alarms near sleeping areas, not at floor level. Homeowners who place CO detectors at baseboard height may be following outdated guidance.

Finally, homeowners frequently allow CO alarm batteries to expire without replacement. Because CO is invisible and odorless, a dead alarm provides no warning. Monthly testing and annual battery replacement are the minimum maintenance practices.

Homeowners who purchase an existing home should verify that CO alarms are present and within their service life. The manufacture date is printed on the back of the alarm. CO alarm sensing cells typically last 5 to 7 years, after which the unit will chirp or display an end-of-life indicator. An alarm that is past its rated service life cannot reliably detect CO at the concentrations required by UL 2034, even if it still sounds on the test-button press. Replacing expired alarms before move-in is inexpensive insurance, and verifying that any attached garage or fuel-burning appliance is covered is part of a responsible home purchase evaluation.

State and Local Amendments

IRC 2018 is the adopted code in Texas, Georgia, Virginia, North Carolina, South Carolina, Tennessee, Alabama, Mississippi, Kentucky, Missouri, and additional states. R315.1 carbon monoxide requirements are widely adopted, though some states have local statutes that differ. California and several northeastern states adopted CO alarm requirements earlier and through state law, sometimes with broader placement requirements than IRC 2018's bedroom-vicinity standard.

IRC 2021 expanded Section R315 to require CO alarms outside every sleeping area rather than only in the immediate vicinity of bedrooms in certain configurations, and clarified that a CO alarm is required whenever a fuel-burning appliance is installed during renovation even in existing construction. Jurisdictions adopting IRC 2021 also see updated listing references. Homeowners and contractors in IRC 2018 states should check for local amendments that may track the 2021 expansion.

When to Hire a Licensed Contractor

Installing or replacing a hardwired CO alarm is electrical work requiring a licensed electrician in most jurisdictions. If you are adding a gas appliance to an existing home that lacks CO alarms, the mechanical permit for the appliance often triggers a requirement to bring CO alarms up to current code. A licensed HVAC contractor pulling a mechanical permit will coordinate with the electrical contractor. Battery-only CO alarm replacement is a homeowner task, but verify that replacing a hardwired unit with a battery model does not violate the original permit approval.

Common Violations Found at Inspection

• No CO alarm installed despite an attached garage — the all-electric appliance argument does not exempt a home with an attached garage
• CO alarm placed inside the furnace room rather than outside the sleeping area as required by R315.2
• Alarm not listed to UL 2034 — generic detector or expired-listing unit installed
• Combination smoke/CO alarm installed but wired only to smoke circuit with CO feature not tested
• No CO alarm added when a gas appliance was installed as a renovation item in a previously all-electric home
• Missing CO alarm on a floor with a sleeping room when the fuel-burning appliance is on a lower level
• Battery-only alarm used in new construction without code allowance for that substitution