Put a carbon monoxide alarm outside each sleeping area, near the bedrooms, and on every level of the home. Put one inside any bedroom that has a fuel-burning appliance in it or its bathroom. Keep alarms at least 10 feet from gas appliances and out of humid bathrooms, direct sun, and drafts from fans or vents.
What does the Phoenix code require for CO alarms?
The code requires CO alarms outside each separate sleeping area in homes with a fuel-fired appliance or an attached garage. Phoenix enforces the 2024 International Residential Code, adopted by the city effective August 1, 2025. Section R311.3 of the Phoenix Residential Code says carbon monoxide alarms "shall be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms," and "Where a fuel-burning appliance is located within a bedroom or its attached bathroom, a carbon monoxide alarm shall be installed within the bedroom."
The rest of the section sets when and how:
| Rule | What the code says |
|---|---|
| When they are required in new homes | The home has a fuel-fired appliance, an attached garage that opens into the home, or both |
| Remodels and additions | Permitted alterations, repairs, and additions trigger alarms as if the home were new, with exceptions |
| Exceptions to that trigger | Exterior work like roofing, siding, windows, doors, porches, and decks; plumbing work; mechanical work that is not fuel fired |
| Listing | CO alarms listed to UL 2034; combination smoke and CO alarms to UL 217 and UL 2034 |
| Combination alarms | Allowed in place of separate CO alarms |
| More than one alarm | Interconnected so one sets off all, by wire or listed wireless alarms |
| Power | Hardwired with battery backup, with limited battery-only exceptions |
Phoenix amended the power rule. Under the city's version, alarms added during a remodel can run on batteries only when the work does not remove interior wall or ceiling finishes, unless an attic, crawl space, or basement gives access for wiring. The city's stated reason: gut remodels should get hardwired alarms like a new home.
One detail surprises people. The remodel rule has three exceptions, and one is "Installation, alteration or repairs of plumbing systems." So a plumbing repair, or swapping an electric water heater, does not by itself trigger the CO alarm requirement. Another exception covers only mechanical work "that is not fuel fired," so permitted work on fuel-fired equipment, like a new gas furnace, does trigger it. A gas water heater sits between the two. It is plumbing, but it also burns fuel, and the code text does not say which exception wins. Ask the city when the permit is pulled. Either way, a home with a gas water heater meets the code's own test for new homes, a fuel-fired appliance, so put CO alarms in regardless.
Where do safety agencies say to put them?
The safety agencies go further than the code. The U.S. Consumer Product Safety Commission says to "Install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas." First Alert, which makes alarms, puts the NFPA's recommendation this way: "on every level of your home, inside every bedroom, and outside each sleeping area," plus "near attached garages in case a car is left running, and anywhere else the manufacturer recommends."
Putting it together for a typical Phoenix house:
- Single-story home: one alarm in the hallway outside the bedrooms, plus more if bedrooms are split across the house. First Alert's guidance adds one inside each bedroom.
- Two-story home: an alarm on each floor, including one outside the upstairs bedrooms.
- Attached garage: an alarm in the room or hallway where the garage door enters the house.
- Gas appliance in a bedroom or its bathroom: an alarm inside that bedroom. The code requires it.
### What about gas appliances outside the living space?
Garages, closets, and backyards hold a lot of gas equipment in Phoenix homes, and each one burns fuel. Southwest Gas explains that carbon monoxide is "produced by the incomplete burning of fuel," so any burner that runs poorly can make it. The alarm plan should account for where those burners are:
- Water heater or furnace in the garage: put an alarm near the door from the garage into the house, the spot First Alert recommends for a car left running.
- Furnace or water heater in an interior closet: the hallway alarm outside the bedrooms may be the nearest one. Keep alarms out of the closet itself.
- Pool heater: Pentair's MasterTemp manual says "Pool heaters must be used in conjunction with carbon monoxide detectors installed near the pool heater." Read the heater manual and the alarm manual together to pick the spot.
The code and the listing both point back to the alarm's instructions for mounting height and exact placement. Follow the manual that came in the box.
Where should you not put a CO alarm?
Keep alarms away from spots that cause false alarms or slow detection. First Alert lists four:
- Too close to a fuel-burning appliance. Its guidance says "at least 10 feet away."
- Humid areas like a bathroom.
- Direct sunlight.
- Strong air movement, "like near fans or vents."
First Alert says those spots "can create false alarms or prevent your carbon monoxide alarm from properly identifying dangerous levels of CO." So the alarm goes in the hallway outside the water heater closet, not on the closet wall next to the burner. Close enough to hear, far enough to read the room.
How often should you test and replace CO alarms?
Test them monthly and replace them at the end of their rated life. First Alert says to "test home smoke and carbon monoxide detectors at least once a month and to change the batteries every six months."
The sensor wears out. Kidde says its CO alarms have "a proven life of seven to ten years, depending on the model type," and that its alarms made since 2013 have a 10-year life, with a few exceptions. Check the manufacture date on the back of each alarm. Kidde also explains the end-of-life signal:
- A chirp every 30 seconds that a fresh battery does not stop.
- "ERR" or "END" on a digital display.
That chirp is the alarm saying it can no longer sense CO. Replace it that day.
What should you do when a CO alarm goes off?
Get everyone outside to fresh air right away, then call for help. The CPSC's guidance is short: "If you suspect CO poisoning, get outside to fresh air immediately, and then call 911." Southwest Gas treats it the same way: "Any suspected carbon monoxide exposure is an emergency."
Do not stay inside to find the source, and do not go back in until responders say it is safe. Once it is, have the gas appliances checked before they run again. A gas water heater that backdrafts or a furnace with a venting problem can be the source, and our page on whether a water heater can cause carbon monoxide covers the warning signs.
A CO alarm is your only warning, because the gas has no smell. A rotten egg smell is a gas leak or sewer gas, never CO itself, as our page on whether carbon monoxide smells like rotten eggs explains. Southwest Gas says it "does not inspect for the absence of carbon monoxide detectors," so checking yours is on you. It's also worth a few minutes to check your water heater for a safety recall, since recent gas water heater recalls have involved carbon monoxide. HQ Plumbing & Air services gas water heaters and gas lines across the Phoenix area, with 24/7 emergency service.
Hallway outside the bedrooms, every floor, away from the burner. Test it monthly.