Smoke Alarms

A smoke alarm only does its job if it matches the fire it’s most likely to face. That’s the part most buyers miss. There are two sensing technologies, three power setups, and a placement rulebook that the building code takes seriously. Get those three right and the rest is detail.

Start with sensor type. Ionization sensors provide the earliest warning to fast flaming fires — the kind caused by paper, kitchen grease, and other fast-combusting materials. Photoelectric sensors reliably detect smoke from smoldering fires, like those from an unextinguished cigarette. Two different fires, two different sensors. For maximum protection, use both ionization and photoelectric sensing technologies. The clean way to do that in one unit is a dual sensor. Dual sensor alarms detect both smoldering and fast-flaming fires, with both sensors working together for complete coverage.

That matters more than usual in Montana. Wood stoves and pellet stoves are common heat sources here, and smoldering combustion is exactly what photoelectric and dual-sensor units catch best. Whatever you put near a stove or fireplace, keep it back — placement rules below.

Every model covered here puts out 85 dB at 10 feet, so loudness isn’t the differentiator. Sensor and power source are.

Power Source and the Feature Ladder

Three ways to power an alarm, each with trade-offs on install effort and maintenance.

Hardwired ties into your 120VAC wiring. Most hardwire detectors include a hardwire connecter plug that makes installation easy, and a back-up battery feature provides continuous detection during power outages. The SMI100-AC and SMI105-AC are the interconnect hardwire models — the latter pairs hardwiring with a 10-year battery backup. Useful for new builds and renovations where you’re already in the walls.

10-year sealed battery is the low-maintenance choice. These eliminate the need for battery replacements and late-night low battery chirps for the life of the device. No wiring, no annual battery swap. The SMI110 (ionization), SM310 (dual sensor), and SM210V (photoelectric with voice) all run on a sealed lithium battery good for a decade. In a cold-winter house where batteries drain faster and 2 a.m. chirps are nobody’s friend, that’s a real advantage.

Replaceable battery keeps it simple and cheap to maintain. The SMI100 runs on a single 9V alkaline. It installs with no wiring required, and the front access battery compartment makes for easy battery replacements with a low battery warning when it’s time to swap. Good for spots where running power isn’t worth the trouble.

First Alert sorts features into three tiers worth knowing before you mix units. Standard alarms contain basic features such as easy installation. Superior alarms add features like 10-year battery and voice alerts. Premium alarms carry the most — 10-year battery, voice alerts, and wireless interconnect.

A couple models earn their tier. The SM210V talks. It delivers voice and location alerts that tell you where the danger is to help you escape safely, with 11 pre-programmed home locations to choose from during setup. It’s also slim — half the depth of a standard alarm. The SM310 is the dual-sensor workhorse: ionization and photoelectric sensors together for protection from fast flaming and slow smoldering fires.

Combination units fold in carbon monoxide. Combination smoke and CO detectors provide 2-in-1 detection against both fire and CO in one device. One catch on timing: the same 10-year rule applies to CO alarms, but most CO alarms expire after 7 years. So a combo unit’s life is governed by the shorter clock.

Specs at a glance

Model	Sensor	Power	Tier	Notable
SMI100	Ionization	9V replaceable	Standard	Front-access battery, end-of-life warning
SMI110	Ionization	10-yr sealed	Superior	No battery swaps for life of alarm
SM310	Ionization + Photoelectric	10-yr sealed	Premium	Dual sensor, end-of-life warning
SM210V	Photoelectric	10-yr sealed	Premium	Voice/location alerts, slim profile, UL 217 10th Ed.
SMI100-AC	Ionization	Hardwired + battery backup	—	Interconnect capable
SMI105-AC	Ionization	Hardwired + 10-yr backup	—	Interconnect capable

All units carry a 10-year limited warranty, and the SM310, SMI110, and SMI100 all signal an end-of-life warning when it’s time to replace the detector.

Western Building Center carries a range of First Alert units, including battery-operated models (code 501165, 513699), a hardwired smoke alarm with battery backup (8495111), the 10-year photoelectric slim-profile two-pack (546324), and combination smoke and CO alarms with slim-profile designs (8494999, 8495061).

Placement and How Many You Need

This is where most installs go sideways. The quantity rule is straightforward: install smoke alarms on every level of your home, including the basement, plus one in each bedroom and outside all sleeping areas. Basements count — and since Montana homes routinely have them, that’s an alarm a lot of people forget.

A two-story home with three bedrooms typically needs at least five alarms, and large homes may require more based on layout and code requirements.

Now the spacing, which keeps nuisance trips down and detection up:

• At least 20 ft. from appliances such as furnaces and ovens, which produce combustion particles. This is the rule that matters around wood stoves and combustion heaters.
• At least 10 ft. from high-humidity areas such as bathrooms with showers and laundry rooms.
• At least 3 ft. away from heating and AC vents, which can disrupt smoke alarms.

Avoid corners and ceiling fans, and never paint over an alarm.

On interconnect: when you want every alarm in the house to sound at once, compatibility is the whole game. For interconnected systems, alarms must be compatible — ideally from the same brand and series — to communicate properly. Mixing alarms from different manufacturers may prevent them from triggering together, even if the wiring is the same. Check the product manual for compatible models before you buy a mixed batch.

Frequently Asked Questions

How often do smoke alarms need to be replaced? First Alert smoke alarms should be replaced at least every 10 years. Check the manufacturer date on the back of the alarm — if it’s 10 years old or older, replace the entire unit even if it still seems to work, because the smoke sensors lose sensitivity over time.

Ionization or photoelectric — which should I buy? Depends on the fire risk. Ionization alarms are generally more sensitive to fast flaming fires and photoelectric alarms are generally more sensitive to smoldering fires. For maximum protection, use both technologies — either by running both types or installing a dual-sensor unit.

Can I mix battery and hardwired alarms in the same house? Yes, but understand the limit. Battery-operated and hardwired alarms can coexist in the same home, but they won’t function as a connected system unless specifically set up and designed to do so. Smoke-only and combination smoke & CO units can also be used together based on room placement.

Do hardwired alarms still work in a power outage? The ones with backup do. Most hardwire detectors include a hardwire connecter plug, and a back-up battery feature provides continuous detection during power outages. Worth having where winter storms knock out power.

How do I test an alarm, and how often? Test it regularly to make sure it’s working properly, and if it doesn’t test correctly, replace the unit immediately. Every model here has a single test/silence button on the front. Check the product manual for specifics on how and when to test.

Why do CO alarms expire sooner than smoke alarms? Smoke alarms get replaced every 10 years, but most CO alarms expire after 7 years. On a combination unit, plan replacement around the shorter CO timeline.

A note on standards and code: some newer units, like the SM210V, are built to conform to the UL 217 10th Edition Standard. Always confirm requirements with your local building authority and NFPA guidance before finalizing a layout — local code can call for more than the minimum.