How to Replace or Install Smoke and Carbon Monoxide Alarms (Phoenix Homeowner Guide)

Step-by-step guide to replacing or adding smoke and carbon monoxide alarms in a Phoenix home: where they are required, how far from fans, vents, kitchens and baths, picking the right listed alarm, swapping a hardwired unit safely, interconnecting and testing, with the code behind every step.

By Gabriel · Licensed Phoenix, Arizona General Contractor ROC 367058 (R-62) · 2026-11-04

Smoke alarms and carbon monoxide (CO) alarms are some of the cheapest safety equipment in a house, and also some of the most neglected. I walk into Phoenix homes all the time where an alarm was pulled off the ceiling because it chirped, where the only alarm is in the hallway with none in the bedrooms, or where the unit on the ceiling is older than the kids sleeping under it.

Replacing an alarm is a job many homeowners can do. But there are rules about where alarms go, what kind you can use, and how they are powered, and the rules changed a bit in Phoenix with the 2024 code. Below is the order I follow. Every step says why it matters and which rule is behind it: the 2024 International Residential Code (IRC) Sections R310 (smoke alarms) and R311 (CO alarms) with the Phoenix amendments, NFPA 72 (2022), the fire alarm code the IRC points to, the 2023 National Electrical Code (NEC) for the wiring, and the manufacturer's instructions (I quote one Kidde hardwired-alarm user's guide as an example; always follow the guide that comes with your alarm). If you are also changing outlets, see my guide on how to replace an electrical outlet.

Tools and parts

• New smoke alarms and CO alarms (or combination alarms) listed to UL 217 and/or UL 2034
• Same-brand replacement for hardwired units (check the adapter plug)
• Non-contact voltage tester
• Step ladder
• Screwdriver and drill/driver
• Wire nuts or the connectors supplied with the alarm
• Tape measure
• Pencil
• Permanent marker for writing the install date

Step-by-step

1. Map where alarms are required in your home

   Walk the house with a floor plan or a sheet of paper. Mark every bedroom, the hallway or area just outside each group of bedrooms, and each story. Then mark where you need CO alarms: if the house has any fuel-burning appliance (gas furnace, gas water heater, gas range, gas fireplace or dryer) or an attached garage with a door into the house, you need a CO alarm outside each sleeping area near the bedrooms, and inside any bedroom that has a fuel-burning appliance in it or in its attached bathroom.

   Why: A smoke alarm only helps if someone hears it in time. The code puts one inside every bedroom because most fatal home fires happen at night while people sleep with doors closed, and one outside the bedrooms so a fire in the rest of the house is caught before it reaches them. Many Phoenix homes are all-electric, but an attached garage with a car in it is still a CO source, which is why the garage is on the list.

   Code: 2024 IRC R310.3 (smoke alarm locations: each sleeping room, outside each sleeping area, each additional story) · 2024 IRC R311.2.1 (CO alarms where there is a fuel-burning appliance or an attached garage that opens into the dwelling) · 2024 IRC R311.3 (CO alarm locations)

2. Check the spacing rules before you pick the spot

   For each location, measure. Keep a smoke alarm at least 10 feet from a cooking appliance if you can (the IRC allows 6 feet when there is no other way to cover the area). Keep it at least 3 feet horizontally from the door of a bathroom with a tub or shower. Keep it at least 36 inches from the tips of ceiling fan blades and from the A/C supply registers, and out of their airflow. On a tray ceiling, mount on the highest part or within 12 inches down from the high point. If you mount on a wall, the top of the alarm must be within 12 inches of the ceiling.

   Why: Steam from a shower and cooking smoke are the two biggest causes of nuisance alarms, and nuisance alarms are why people pull alarms down. Moving air is the other problem: in Phoenix the A/C runs most of the year, and a supply register or a ceiling fan can blow smoke away from the sensor so it alarms late.

   Code: 2024 IRC R310.3 item 4 (3 ft from bathroom door with tub or shower) · 2024 IRC R310.3.1 (10 ft from a permanently installed cooking appliance; 6 ft where necessary) · NFPA 72 (2022) 29.11.3.4 (36 in from ceiling-fan blade tips and forced-air supply registers; tray ceilings) · NFPA 72 (2022) 29.11.3.3 (wall-mounted alarms within 12 in of the ceiling)

3. Watch for hot ceilings, garages and attics

   Do not put a smoke alarm in the garage or an unfinished attic. If a ceiling sits right under a hot attic and is poorly insulated, or a wall is an outside wall that gets very hot, mount the alarm on an inside wall instead, within 12 inches of the ceiling. Check the temperature range printed in the alarm's instructions; the Kidde guide I quote says not to install where it is colder than 40°F or hotter than 100°F.

   Why: Smoke alarms are tested to work in a temperature range. A Phoenix garage or attic in July is far above 100°F, and a very hot ceiling surface can create a layer of hot air that keeps smoke from reaching the sensor quickly. A garage also has car exhaust and dust that cause false alarms.

   Code: NFPA 72 (2022) 29.11.3.4 (not in unfinished attics or garages, or spaces below 40°F or above 100°F; inside wall where ceilings or walls are hot or cold) · Kidde i12040-series hardwired smoke alarm user's guide (locations to avoid)

4. Pick the right type of alarm

   Buy smoke alarms listed to UL 217 and CO alarms listed to UL 2034. A combination smoke/CO alarm carries both listings and can count for both, which is handy outside the bedrooms. If the alarm has to go between 6 and 20 feet from the cooktop, buy one that is listed for resistance to common cooking nuisance alarms (alarms built to the 8th edition of UL 217, which became required for new alarms in 2023, are marked for this). For a hardwired spot, buy a hardwired alarm with battery backup.

   Why: The listing means the alarm was tested by a lab to respond to real fires or real CO levels. The newer UL 217 test includes a cooking test, so an alarm near the kitchen is less likely to go off every time you sear a steak, while still detecting real fires.

   Code: 2024 IRC R310.1.1 (smoke alarms listed to UL 217; combination alarms UL 217 and UL 2034) · 2024 IRC R311.1.1 (CO alarms listed to UL 2034) · 2024 IRC R310.5 and R311.4 (combination alarms permitted) · NFPA 72 (2022) 29.11.3.4 (6 to 20 ft from cooking: alarms listed for resistance to common nuisance sources)

5. Check the date on the alarm you have

   Twist the old alarm off its base and read the manufacture date on the back. Replace any smoke alarm that is 10 years old or older, any CO alarm that has reached its end-of-life signal or the replacement date printed by its maker, and any alarm that fails the test button. Write the install date on the new alarm with a marker.

   Why: Smoke sensors and CO sensors wear out. The test button only checks the horn and the circuit, not how sensitive the sensor still is, so the code sets a replacement age. CO sensors often wear out sooner than 10 years; check your model's date.

   Code: NFPA 72 (2022) 14.4.5 (replace smoke alarms no later than 10 years from the date of manufacture; replace CO alarms at end-of-life signal or the manufacturer's replacement date; combination alarms at whichever comes first)

6. Match the power source: hardwired stays hardwired

   If the old alarm is wired to the house (it has a plug or wires behind it), replace it with a hardwired alarm with battery backup. Do not replace a hardwired alarm with a battery-only one. When you add alarms in new spots, battery-only or listed wireless alarms are allowed in Phoenix only when you are not opening walls or ceilings and there is no attic, crawl space or basement that could be used to run wires. Most Phoenix single-story homes have an attic, so ask an electrician whether new hardwired alarms can be fished from above.

   Why: Batteries die and get borrowed for the TV remote. House power with a battery backup keeps the alarm working either way. That is why the code requires building wiring as the main power, with no switch that can turn it off other than the breaker, and why Phoenix tightened the battery-only exception.

   Code: 2024 IRC R310.6 and R311.6 (primary power from building wiring with battery backup; no disconnecting switch other than overcurrent protection) · Phoenix amendment to R310.6 and R311.6 Exception 2 (battery power only where finishes are not removed, unless an attic, crawl space or basement could provide access for hardwiring) · 2024 IRC R310.2.2 and R311.2.2 (alterations and repairs that require a permit trigger alarms as for new construction)

7. 

8. Turn off the breaker and prove the wires are dead

   Find the breaker that feeds the alarms (it is often labeled 'smoke' or shares a bedroom or hallway lighting circuit). Turn it off. Twist the old alarm off its base, unplug the adapter, and hold a non-contact voltage tester against the black wire before you touch anything. Do not trust the chirp: the old alarm's backup battery can keep it beeping with the power off.

   Why: Hardwired alarms run on 120 volts. The manufacturer tells you to cut the power to the circuit before wiring, and the NEC requires listed equipment to be installed according to its instructions. If the breaker is not labeled, label it now so the next person does not have to guess.

   Code: Kidde i12040-series user's guide (turn off the main power to the circuit before wiring) · 2023 NEC 110.3(B) (install listed equipment per its listing and instructions) · 2023 NEC 408.4(A) (circuit directory)

9. Remove the old base and check the box and wiring

   Unscrew the old mounting plate. The splices and the wiring adapter must sit in an electrical box, not loose in a hole in the drywall. Check that the box is secure and the wires are not brittle or burned. Most hardwired alarms use a black (hot) wire, a white (neutral) wire and a red or yellow interconnect wire. Different brands use different plugs, so a same-brand replacement often plugs right into the old adapter. If you switch brands, you have to replace the adapter too.

   Why: Splices in a box are protected from damage and contained if they ever overheat. Matching the adapter avoids extra splices. And the alarm must be on a circuit that cannot be switched off with a wall switch or tripped by a GFCI, or it could be dead when you need it.

   Code: 2023 NEC 300.15 (box required at each splice and device point) · Kidde i12040-series user's guide (install on a listed junction box; 120 V non-switched circuit not protected by a ground fault interrupter) · 2024 IRC R310.6 (no disconnecting switch other than the overcurrent device)

10. Wire the new adapter tight and correct

    Connect the new adapter black to the house black (hot), white to white (neutral), and red to the red interconnect wire with the connectors supplied or listed wire nuts. Twist each connector until the wires turn together and tug each one. If the alarm will stand alone with no interconnect, leave the red wire capped and tucked away so it cannot touch anything. Fold the wires neatly into the box and screw on the new mounting plate.

    Why: A loose splice can arc, overheat or drop the alarm offline. The Kidde guide warns that improper connection can damage the alarm, stop it from working or create a shock hazard, and that the unused red wire must stay capped.

    Code: 2023 NEC 110.14(B) (splices joined with devices identified for the use) · 2023 NEC 110.3(B) (follow the manufacturer's instructions) · Kidde i12040-series user's guide (single-station: do not connect the red wire; keep it capped)

11. Interconnect so one alarm sets off all of them

    Every smoke and CO alarm in the house should be interconnected, so a fire in the living room sets off the alarm in the bedroom too. With hardwired alarms, that is the red wire, and all interconnected units should be on the same circuit. Mix brands only if the manual lists the other model as compatible. The Kidde guide I quote also follows the NFPA limit of 12 smoke alarms and 18 alarms total on one interconnect. Where running new wires is not possible, the code allows listed wireless interconnected alarms.

    Why: You will not hear an alarm at the other end of the house through a closed bedroom door, especially with the A/C running. Interconnection makes every alarm sound at once. Mixing brands that do not match can cause false alarms or failure to alarm.

    Code: 2024 IRC R310.4 (smoke alarm interconnection; listed wireless alarms permitted) · 2024 IRC R311.5 (CO alarm interconnection) · Kidde i12040-series user's guide (all units on one circuit; compatible models only; 12 smoke / 18 total limit)

12. Mount, power up and test every alarm

    Plug the adapter into the new alarm, install or activate the backup battery (some units have a pull tab), line the alarm up with the mounting plate and twist it on until it locks. Turn the breaker back on and confirm the green power light. Press and hold the test button on each alarm; on an interconnected system, every alarm should sound. Then test monthly, and replace any alarm that fails.

    Why: Many alarms will not seat on the base without a battery installed, which is a reminder built in by the maker. The test proves power, the horn and the interconnect all work before you put the ladder away. NFPA 72 asks for monthly testing at home, and some manufacturers ask for weekly.

    Code: 2024 IRC R310.1.2 and R311.1.2 (install per listing and manufacturer's instructions) · NFPA 72 (2022) 14.4.5 (test monthly per the manufacturer; replace alarms that fail testing)

13. Keep them working, and check your security bars

    Replace batteries as the manufacturer says (non-sealed batteries at least once a year; sealed 10-year alarms are replaced as a whole unit when they chirp low battery). Vacuum dust off the vents with a soft brush. If you have security bars or screens over bedroom windows, make sure they open from the inside without a key or tool, because Phoenix links that rule to having working smoke alarms.

    Why: A chirping alarm is the main reason people take alarms down. Dust and bugs inside the chamber cause false alarms. In a fire, a bedroom window is the second way out, and a locked bar defeats it.

    Code: NFPA 72 (2022) 14.4.5 (batteries replaced per the manufacturer) · Phoenix amendment to 2024 IRC R319.4.4 (bars, grilles and screens over emergency escape openings releasable from inside; smoke alarms per R310)

When to stop and call a licensed pro

• There is no hardwired power where an alarm is required and you need new wiring run from the attic. New or extended circuits in bedrooms, hallways and living areas need AFCI protection under the 2023 NEC 210.12(B) and 210.12(E), and the work can need a permit.
• The wires in the box are brittle, scorched, aluminum, or there is no box at all.
• The breaker trips when you turn the alarms back on, or the interconnect sets off all the alarms for no reason after the swap.
• You are remodeling, adding a room or finishing space; under IRC R310.2.2 and R311.2.2 permitted work can require the whole house to get alarms as for new construction.
• A CO alarm has gone off for a real reason. Have the gas appliances checked by a licensed HVAC or gas contractor before you use them again.
• You manage a rental or a property with a monitored fire alarm system; the rules and equipment are different (IRC R310.7 and NFPA 72 for household fire alarm systems).

Common questions