Google discontinued Nest Protect in March 2024. First Alert SC5 mounts on the existing Nest Protect baseplate and is Google's officially listed replacement (Google Store). 5-minute swap per alarm. If you bought Nest Protects in early 2015 (1st gen), they're at the 10-year end-of-life mandate now — replace this season.
The life-safety hierarchy — how an SFFD Fire Safety Director picks alarms
- Detection (right sensor type in the right room)
- Notification (loud enough, low-frequency enough, visual + tactile if needed)
- Interconnection (one trips, they all trip — the difference between waking up and not)
- Survivability (sealed 10-year lithium = no dead-battery chirp at 3 a.m., no skipped replacement)
- Code compliance (NFPA 72, UL 217, UL 2034 — and your state's 10-year mandate)
Smart features (Wi-Fi, app alerts, voice prompts) layer on top of this hierarchy. They never replace it.
Photoelectric vs ionization vs dual-sensor
NIST full-scale fire test data: photoelectric beats ionization by ~30 minutes on smoldering fires — which are the ones that kill people in their sleep. Ionization beats photoelectric on fast-flaming fires (typically grease/kitchen). Dual-sensor (both in one unit) beats either alone. For residential primary, photoelectric is the right default.
The 5 smart smoke + CO combo alarms worth buying in 2026 (ranked)
1. Best smart combo overall — First Alert SC5 (Nest Protect replacement)
Photoelectric smoke + electrochemical CO. Six 3V CR123A lithium cells (~10-yr life). Voice + app alerts via First Alert Wi-Fi app. Mounts on the Nest Protect baseplate. Meets UL 217 (latest) + UL 2034. This is what I'd put in my own house in 2026.
| Spec | Value |
|---|---|
| Sensor | Photoelectric smoke + electrochemical CO |
| Power | 6× CR123A lithium (~10 yr) |
| Connectivity | Wi-Fi 2.4 GHz, First Alert app |
| Interconnect | Wireless to other SC5 + Onelink |
| Cert | UL 217, UL 2034, ETL |
| Price | $80-$100 |
- First Alert SC5 on Amazon — $80-$100
2. Best hardwired combo — Kidde i12010SCO
If you already have hardwired alarms with the orange interconnect wire, swap them for these one-for-one when they hit 10 years. Wired interconnect means every alarm trips together — pair with a photoelectric one down the hall for the smoldering coverage.
| Spec | Value |
|---|---|
| Sensor | Ionization smoke + electrochemical CO |
| Power | 120V AC + sealed 10-yr Li backup |
| Connectivity | None (hardwired interconnect) |
| Interconnect | Wired (Kidde i-series) |
| Cert | UL 217, UL 2034 |
| Price | $45-$60 |
- Kidde i12010SCO on Amazon — $45-$60
3. Best 10-year sealed battery combo — First Alert SCO5CN / SCO7CN
Sealed lithium, no battery doors, no replacements. Voice alert calls the hazard ("Fire" vs "Warning, Carbon Monoxide"). Photoelectric + electrochemical. Just runs for 10 years; replace the whole unit. The only legal battery-only combo to install or sell in California since 2014.
| Spec | Value |
|---|---|
| Sensor | Photoelectric smoke + electrochemical CO |
| Power | Sealed 10-yr lithium (non-replaceable) |
| Connectivity | None |
| Interconnect | None |
| Cert | UL 217, UL 2034 — meets CA SB-745 |
| Price | $35-$45 |
- First Alert SCO5CN on Amazon — $35-$45
4. Best wireless interconnect network — Kidde KN-COSM-B-RF
The B-RF variant adds wireless interconnect — when one trips, every alarm in the house trips, without running wires through your finished walls. If your house was built before 2000, your alarms almost certainly aren't interconnected — meaning the alarm in the basement won't wake you in the upstairs bedroom. This Kidde RF mesh fixes that without an electrician.
| Spec | Value |
|---|---|
| Sensor | Ionization smoke + electrochemical CO |
| Power | 2× AA + 10-yr unit life |
| Connectivity | RF wireless interconnect |
| Interconnect | Wireless to other Kidde RF |
| Cert | UL 217, UL 2034 |
| Price | $55-$75 |
- Kidde KN-COSM-B-RF on Amazon — $55-$75
5. Best for hearing-impaired / deep sleepers — Lifetone HLAC151
This isn't a smoke detector — it's a bedside listener that picks up the T3 horn pattern from any UL-217 alarm in earshot, then triggers a 520 Hz low-frequency alarm + flashing FIRE display + bed shaker. 520 Hz is the frequency NFPA research found wakes hearing-impaired, alcohol-impaired, and deep sleepers when the standard 3 kHz horn doesn't. If anyone in your house is over 65, hearing-impaired, on sleep meds, or a heavy sleeper — this is the missing piece.
| Spec | Value |
|---|---|
| Sensor | Listens for T3 smoke alarm pattern |
| Power | Plug-in + battery backup |
| Connectivity | Acoustic — no app, no wiring |
| Interconnect | Acoustic (works with any UL-217 alarm) |
| Cert | NFPA 72-compliant low-freq alerting |
| Price | $180-$220 |
- Lifetone HLAC151 on Amazon — $180-$220
NFPA 72 placement table
| Location | Smoke required? | CO required? | Notes |
|---|---|---|---|
| Inside each bedroom | Yes | Yes (near sleeping) | Combo alarm ideal |
| Hallway outside bedrooms | Yes (≤10 ft from doors) | Yes | Within 10 ft of every sleeping area |
| Every floor including basement | Yes | Yes | One per level minimum |
| Kitchen | No (close-by, not inside) | No | ≥10 ft from cooking appliances |
| Within 15 ft of fuel-burning appliance | No | Yes | Furnace room, water heater closet |
| Attached garage entry (inside house) | No | Yes | CO from vehicles |
| Garage itself | No | No | Temp/humidity swings = false alarms |
| Bathroom | No | No | Steam = false alarms |
The 10-year sealed mandate (the section nobody writes)
California SB-745 (effective July 1, 2014) was the first state law requiring every battery-only smoke alarm sold in the state to have a sealed, non-removable, non-replaceable 10-year lithium battery. Since 2015, California-sold alarms must also print the manufacture date and have a hush button.
Other states followed: New York, Oregon, Maryland, Iowa, and Vermont have similar sealed-battery requirements. Several more require sealed-battery alarms in rental properties specifically.
The rule nobody follows: NFPA says replace the entire unit every 10 years from the manufacture date — not the install date. The sensor degrades. CO cells age out even faster. Most US homes have at least one alarm older than the family pet.
What this means in 2026:
- If your alarms have replaceable 9V batteries, they're old. Replace the units.
- If you bought Nest Protects in early 2015 (1st gen), they're at end-of-life this year.
- If you bought Nest Protects in 2024 right before discontinuation, you have until ~2034 — but Google won't support the app forever. Plan the swap.
- The First Alert SC5 mounts on the existing Nest Protect baseplate, so the swap is 5 minutes per alarm.
FAQ
How often do you replace smoke alarms?
Every 10 years from the manufacture date (not install date). CO sensors actually go bad faster (7-10 yrs). Write the install date on the cover with a Sharpie.
Smoke alarm vs combo smoke/CO — do I need both?
Combo covers both. Don't put CO directly above fuel-burning appliance (15 ft away per UL). Standalone CO alarms still make sense in basements and near attached garages.
Hardwired or battery — which is better?
Hardwired with sealed lithium backup = gold standard. Battery-only with sealed 10-yr lithium is the legal default if not pre-wired. Don't install replaceable-battery alarms in 2026.
Photoelectric or ionization?
For residential, photoelectric — NIST data shows it's about 30 minutes faster on smoldering fires. Dual-sensor in one unit is better than either alone.
Where do I install them?
Inside every bedroom + within 10 ft of each sleeping area + every floor including basement. ≥3 ft from supply vents, ≥10 ft from cooking appliances. Not in kitchen, bathroom, or garage.
The bottom line
If you have Nest Protects, swap them for First Alert SC5s as they expire — same baseplate, same Google Home integration, current generation. If you have hardwired alarms, replace any over 10 years old with Kidde i12010SCO. If you have battery-only alarms with replaceable 9Vs, throw them out and install First Alert SCO5CNs. Older house, no interconnect: Kidde KN-COSM-B-RF wireless mesh. If anyone in the house is hard of hearing or sleeps heavy, add a Lifetone HLAC151 bedside. Write the date on every alarm with a Sharpie.
Affiliate disclosure: Building Talks may earn a small commission from qualifying purchases. Pricing subject to change.
Editorial standards: Cited authorities include NFPA 72 National Fire Alarm and Signaling Code, UL 217 (Smoke Alarms) + UL 2034 (CO Alarms), NIST full-scale fire test data on photoelectric vs ionization, CDC carbon monoxide poisoning data, USFA placement guidelines, California SB-745. Reviewed by Al, Building Doctor — SFFD Fire Safety Director (2001), IUOE Local 39 Stationary Engineer.