IRC R315: Carbon Monoxide Alarms

Carbon monoxide is a colorless, odorless gas that comes from anything that burns fuel: a furnace, a gas water heater, a fireplace, or a car idling in the garage. You cannot smell it, and at high levels it kills before you wake up. IRC Section R315 decides which homes must have CO alarms and where those alarms go. The rule is narrower than the smoke-alarm rule, because not every house produces carbon monoxide.

When R315 requires CO alarms

A home needs carbon monoxide alarms if either of two conditions is true. Here is the trigger:

For new construction, an approved carbon monoxide alarm shall be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms in dwelling units within which fuel-fired appliances are installed and in dwelling units that have attached garages.

In plain terms: if the house has a fuel-fired appliance or fireplace, or an attached garage that opens into the living space, CO alarms are required. An all-electric house with no gas appliances and no attached garage is the rare case that may not need them.

Where the alarms go

The placement rule mirrors the smoke alarm logic. CO alarms go outside each separate sleeping area, near the bedrooms, so the alarm sits between a sleeping person and the source of the gas. There is one addition: if a fuel-burning appliance sits inside a bedroom or its attached bathroom, a CO alarm has to be installed in that bedroom. The alarms must be listed to UL 2034, and combination smoke/CO units must meet both UL 2034 and UL 217.

What it means in practice

For example, a custom home in Apache Junction with a gas furnace, a gas tankless water heater, and an attached three-car garage clearly triggers R315. The builder places a CO alarm in the hallway outside each bedroom cluster. Many builders use combination smoke-and-CO alarms to satisfy both IRC R314 smoke alarms and R315 with one hardwired, interconnected device, which keeps the ceilings cleaner and the wiring simpler.

The Arizona angle is the attached garage. The Valley's hot months mean cars sit in closed garages, and an attached garage that communicates with the house is enough on its own to require CO alarms even in an otherwise all-electric home.

How it connects

R315 is the second half of the alarm pair. It sits right next to IRC R314 smoke alarms in the code and is usually wired together with it, and it complements the garage fire separation in IRC R302 fire-resistant construction .

Edition note

The trigger conditions (fuel-fired appliance or attached garage) and the placement near sleeping areas are consistent across the 2018, 2021, and 2024 IRC. The 2024 edition tightens some power-source and listing details, but the "when required" and "where" rules above hold. Confirm your city's adopted edition on the Arizona building code editions by city page.

Source

Read the section on the official ICC site: 2021 IRC R315 on codes.iccsafe.org . The I-Codes are copyrighted, so only the short trigger clause is quoted here.