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Choosing between First Alert SMI100, SMI100-AC and SM310

By the Plain Prepper editors. Updated 2026-10-17.

We research products from makers' specs, manuals and safety agencies; we do not run hands-on tests. Links to Amazon may earn us a commission at no cost to you. How this works.

Quick answer

Choose SM310 for a standalone alarm combining ionization and photoelectric smoke sensors with a sealed battery. Choose SMI100-AC when a licensed electrician confirms it fits a compatible hardwired alarm system, or SMI100 for a standalone alarm with a replaceable battery where permitted. SMI100 and SMI100-AC use ionization sensing; neither alone supplies both smoke sensing technologies. None of these three is a carbon monoxide alarm. Match the home requirements before ordering.

Choose for the installed system first

Begin with the kind of system your home requires. A standalone battery alarm and a hardwired interconnected alarm do different jobs. Replacing a hardwired unit with a standalone model merely because it is easier to buy can remove an intended system function. Check the existing model, the installation requirements and local rules before deciding which purchase fits.

First Alert's SMI100-AC manual requires wiring and installation by a licensed electrician. Have the electrician confirm the power arrangement, interconnection and exact compatible models. This guide does not provide wiring steps or claim that every First Alert product uses the same connector, mounting bracket or communication system.

Compare power, sensors and interconnection

Model	Smoke sensor	System
SM310	Dual	Standalone
SMI100-AC	Ionization	Wired interconnect
SMI100	Ionization	Standalone

SM310 combines the two smoke sensing technologies in a standalone sealed battery unit. SMI100-AC supplies a hardwired interconnection option with backup power, subject to its manual. SMI100 has replaceable battery operation and no interconnection. Each is an exact documented model; the letters and suffixes determine features that a general smoke alarm title cannot establish.

A combination smoke and carbon monoxide alarm is another product type. The three picks here are smoke alarms. Do not interpret dual sensor on SM310 as meaning smoke plus carbon monoxide: its dual sensors are ionization and photoelectric smoke sensing. Use the separate carbon monoxide detector guide when that is the missing function.

Keep both smoke sensing technologies in the plan

CPSC explains that ionization alarms tend to respond faster to flaming fires and photoelectric alarms to smoldering fires. Its staff recommends both technologies on every level, or dual sensor alarms combining them. SM310 meets the comparison's dual smoke sensing role. SMI100 and SMI100-AC provide ionization sensing and should not be described as covering both technologies by themselves.

CPSC also advises against ionization alarms in or near the kitchen because of cooking nuisance alarms. Do not turn a product's nuisance reduction marketing into permission to ignore location guidance. Choose the model, sensing plan and placement under the official advice, the full manual and applicable requirements. We make no claim that these models are equally fast in every kind of fire.

Check placement and household alert needs

CPSC recommends alarms on every level, inside bedrooms and outside sleeping areas, and discusses the benefits of interconnection. A single replacement alarm is only part of that plan. The fact that SM310 provides two smoke sensing technologies does not make its lack of interconnection irrelevant where an interconnected system is required.

The selected manuals warn that these units do not alert hearing impaired residents and direct consideration of special alerting equipment. A louder sounding description or a sealed battery is not a substitute for that equipment. Choose the household alerting arrangement with appropriate guidance. Keep a practiced escape plan alongside working alarms; buying a new unit does not complete that preparation.

A sealed battery still requires maintenance

First Alert's manuals direct checking these alarms weekly. Follow the exact delivered manual and replace an alarm that does not respond correctly. A sealed battery removes routine battery replacement for that design; it does not remove the need to check operation, respond to an end-of-life indication or keep the alarm unobstructed.

Do not remove a battery or disconnect power to suppress an unwanted alarm. First Alert warns that doing so removes protection. Read how the exact unit distinguishes smoke, a low battery indication, a fault and end of life. Keep the manual accessible. Do not assume that all chirping means the same thing or use another model's instructions to decide whether an alarm can stay in service.

Confirm the suffix and warranty before ordering

Match SMI100, SMI100-AC or SM310 on the listing and packaging. Multipacks can appear under the same product family, so check quantity as well. SMI100 is not the AC model, and SM310 is not a similarly named combination smoke and carbon monoxide alarm. Check the actual seller and New condition before ordering an electrical safety product.

The selected First Alert manuals describe a ten year limited warranty for material and workmanship defects, with proof of purchase and exclusions. It is not a promise of coverage for misuse, improper voltage or every included battery, and the warranty period does not replace the alarm's use and replacement directions. Retain the receipt and check the manufacturing and replacement information on the delivered alarm.

For broader household preparedness, the winter emergency kit guide covers a different planning task. Keep alert maintenance part of the home routine instead of treating any kit or new detector as a replacement for a complete fire safety plan.

What to look for when you buy

Questions people ask

Does dual sensor mean smoke and carbon monoxide?

Not on SM310. Its two smoke sensors are ionization and photoelectric. It does not replace a carbon monoxide alarm.

Can SM310 join a hardwired First Alert alarm network?

No. Its manual calls it a single-station unit that cannot link to other alarms. Choose a compatible interconnect model under the installation requirements.

Can I assume a replacement First Alert uses the old connector?

No. Have the electrician check the exact model compatibility and installation requirements. A matching brand or similar appearance is not enough.

How often should these alarms be checked?

The selected First Alert manuals direct checking operation weekly. Follow the delivered model manual, and replace a unit that does not respond correctly.

Sources

• SM310 dual sensor smoke alarm (firstalert.com), First Alert
• SM310 user manual and limited warranty (digitalassets.resideo.com), First Alert / Resideo
• SMI100-AC hardwire interconnect smoke alarm (firstalert.com), First Alert
• SMI100-AC user manual and limited warranty (digitalassets.resideo.com), First Alert / Resideo
• SMI100 battery-operated smoke alarm (firstalert.com), First Alert
• SMI100 user manual and limited warranty (digitalassets.resideo.com), First Alert / Resideo
• Smoke Alarms: Why, Where, and Which? (cpsc.gov), U.S. Consumer Product Safety Commission

Sources checked 2026-10-17.