- Top Pick: Kidde 21007584 I4618 Firex Hardwire Ionization Smoke Detector with Battery Backupwhite
- Best Value: Kidde 21007588 Hardwired Interconnectable 120-Volt Smoke Alarm with Battery Backup, 4-Pack
- Premium Choice: Kidde Hardwire Smoke Detector with 9V Battery Backup and Front Load Battery Door | Model I4618AC
Anyone searching for Firex smoke detectors is usually standing on a ladder with a dead or expired alarm in hand, trying to match a hardwired unit already screwed to the ceiling. Firex is the hardwired family long sold under the Kidde name, and the decision is rarely about brand discovery. It is about whether a new alarm will snap onto the existing mounting plate, plug into the existing wiring harness, and keep talking to the other alarms in the house.
This guide is for homeowners and landlords replacing older 120-volt alarms, and for renovators who want interconnected protection that still has a backup battery. Seven listings made the cut: one that names Firex outright, and six sibling Kidde hardwired models from the same family. A wiring adapter, a plain battery alarm from another brand, and a third-party hardwired six-pack were left out because they are not Firex-style replacements. All judgments rest on the listed titles in 2026, so confirm model numbers against the unit you are removing.
Quick Picks
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| Product | Best for | Key spec from the title |
|---|---|---|
| Kidde Firex i4618 (21007584) | Like-for-like swap | Hardwire ionization, battery backup, white |
| Kidde 21007588 4-Pack | Whole-floor replacement | 120-volt, interconnectable, 4 alarms |
| Kidde i4618AC | Easy battery swaps | 9V backup, front load battery door |
| Kidde 20SAR | AA battery households | AA backup, wired interconnect, 85 dB |
| Kidde SMACFEX (21033081) | Replacing an i4618AC | 120V hardwired, battery backup |
| Kidde 20SA10 | No battery changes | 10-year battery backup, interconnect |
| Kidde 30CUA10 | Smoke plus carbon monoxide | Hardwired, 10-year battery, CO detection |
How to Choose
Match the existing system
Hardwired alarms connect to household 120-volt wiring through a plug-in harness, which is why a like-for-like swap is so common, and all alarms on one interconnect line need to be compatible. Read the model number on the back of the old alarm, then look for the same or a listed replacement number in the title. Mixing unrelated brands on one interconnect run can cause silent failures, so stay within one family when you can. Also confirm that the replacement draws from the same circuit; alarms that share an interconnect wire need to sit on a common power source so the whole chain sounds together.
Battery backup type
Three styles appear here: a 9V backup, AA backup, and a sealed 10-year battery. Replaceable batteries are easy to source but need attention the moment the low-battery chirp begins, often at three in the morning. A 10-year sealed battery removes that chore, which suits ceilings that are awkward to reach. A front-load door lets you swap the cell without removing the alarm from its base. If the household includes someone who cannot climb a stepladder, the sealed option is the more considerate choice, since nobody has to touch the ceiling for a decade.
Sensor technology and features
Ionization sensing, named on the first listing, is usually associated with fast-flaming fires, while photoelectric designs are typically better at smoldering ones. Most listed titles do not state the sensor type, so read the package before buying if that matters to you. Look for LED status lights, an audible alarm level such as 85 dB, and interconnect capability. A combination smoke and carbon monoxide unit consolidates two jobs into one fixture. Remember that every alarm has a limited service life, usually printed on the back as a manufacture date, so an old unit should be replaced even if it still seems to work.
What to ignore and where corners get cut
Ignore generic feature stacking in lists of icons. Check instead for the replacement-model note, the voltage, and the battery type. Be cautious with off-brand packs that call themselves hardwired but arrive with a different plug or no compatible harness, which forces extra wiring. Also confirm the mounting plate matches, because a mismatch means drilling new holes or buying an adapter. Alarms should also sit on each level and outside every sleeping area, so count how many you need by listing bedrooms, hallways, and each floor, since packs only help if the number lines up.
The 7 Best Options
Kidde Firex i4618 (21007584)
The only title here that says Firex, this is a white hardwire ionization alarm with battery backup. It is the cleanest match for a homeowner replacing an older i4618 on the ceiling, because the name on the listing mirrors the name printed on the old alarm. Buyers who just want the original arrangement back usually find this the least surprising option.
- Strength: Firex name and i4618 model number make matching simple
- Strength: hardwired with battery backup for outages
- Trade-off: ionization sensing only, which may not suit buyers who want photoelectric technology
Skip it if you want a combination alarm or prefer a sealed battery, or if a kitchen-adjacent hallway tends to trigger nuisance alarms and you would rather have a different sensor type.
Kidde 21007588 Hardwired 4-Pack
A four-pack of hardwired, interconnectable 120-volt alarms with battery backup. It is aimed at someone replacing every alarm on a floor in one go, such as a landlord refreshing an apartment or a family updating an older house before a sale. The pack format keeps the set uniform.
- Strength: four alarms per order for a consistent set
- Strength: interconnectable across the house
- Trade-off: more units than a single-room replacement needs
Skip it if only one alarm has expired, since buying a pack leaves spares sitting in a closet.
Kidde i4618AC
This hardwire alarm uses a 9V battery backup with a front load battery door. The model number matches the common i4618AC replacement, which makes it handy for anyone who has already identified that code on the old unit. Because the battery door sits on the front, upkeep stays simple.
- Strength: front load door makes backup battery changes quick
- Strength: 9V cells are easy to find
- Trade-off: the battery still needs periodic attention
Skip it if you want to forget battery maintenance entirely.
Kidde 20SAR
A hardwired unit with AA battery backup, wired interconnect, LED status lights, and an 85 dB alarm. It is sold as a single pack for homes and apartments, which suits a one-off swap in a bedroom or hallway. The AA format also matches the batteries many households already keep in a drawer.
- Strength: AA batteries are widely available
- Strength: status lights help confirm power
- Trade-off: the title does not list the sensor type
Skip it if your harness expects a 9V-style alarm, since this model changes the battery format, or if you want the sensor type spelled out on the listing before you commit.
Kidde SMACFEX (21033081)
Listed as a 120V hardwired alarm with battery backup and as an i4618AC replacement. It is a straightforward swap for owners of that older model, and the replacement note in the title takes the guesswork out of matching. Homeowners who simply want the old alarm gone and a working one in its place will find it direct.
- Strength: explicit replacement language in the title
- Strength: standard 120V hardwired design
- Trade-off: the listing offers few extra features beyond hardwired plus backup
Skip it if you want a longer-life battery or a combination alarm, since this listing keeps things simple. Buyers who rely on the replacement-number match should still compare the plug and the base plate before discarding the packaging.
Kidde 20SA10
This one pairs hardwiring with a 10-year battery backup, so it avoids routine battery replacement and low battery chirps. LED status lights and wired interconnect are listed. It is a natural pick for a ceiling that is high or awkward, where the idea of changing a battery each year is unappealing.
- Strength: no battery replacement for the life of the sealed cell
- Strength: interconnect capability for multi-alarm homes
- Trade-off: the whole alarm must be replaced when the battery period ends
Skip it if you prefer user-replaceable batteries, because a failed sealed cell means swapping the entire alarm rather than just the battery.
Kidde 30CUA10
A hardwired alarm that detects both smoke and carbon monoxide, with a 10-year battery and interconnect capability. It is meant for rooms where you want both protections in one fixture, such as a hallway outside bedrooms. Consolidating the two sensors also frees wall or ceiling space.
- Strength: one device covers smoke and carbon monoxide
- Strength: sealed battery, so no replacements
- Trade-off: a combination alarm retires as a whole unit if either sensor fails
Skip it if you already have dedicated carbon monoxide alarms and want a smoke-only replacement.
Which One Fits Your Situation
You are matching an old Firex alarm
Begin with the Kidde Firex i4618 (21007584), since the model name matches what is probably on your ceiling. For an i4618AC unit, the SMACFEX 21033081 is the listed replacement.
You are replacing every alarm on a floor
The Kidde 21007588 four-pack keeps all units consistent and interconnectable. Landlords and families alike tend to prize uniformity. Fit them in one afternoon rather than mixing batch dates, so every alarm reaches the end of its service life at the same time.
You hate chasing low-battery chirps
The Kidde 20SA10 and 30CUA10 both carry sealed 10-year batteries. Choose the 30CUA10 if you also want carbon monoxide detection in that spot.
You like changing batteries yourself
The i4618AC gives you a front load door for a 9V cell, while the 20SAR uses AA batteries. Pick whichever battery size you already stock at home, and set a reminder to swap the cells on a fixed date each year.
Common Mistakes
- Skipping the model number check and buying a replacement that does not fit the existing plate.
- Mixing different brands on one interconnect run without confirming compatibility.
- Assuming the sensor type is the same across every listing when most titles do not state it.
- Forgetting to switch off power at the breaker before pulling the old unit down.
- Ordering a combination smoke and carbon monoxide alarm for a spot where local rules call for separate devices.
Related guides
- Battery Operated Smoke And Carbon Monoxide Detectors
- Combination Smoke And Carbon Monoxide Detectors
- Aio Liquid Coolers For High Performance Gaming
Frequently Asked Questions
Is Firex a different company from Kidde?
Firex is a hardwired alarm line sold under the Kidde name. That is why most of the listings here carry Kidde labels, while only the first one uses Firex in its title.
Do hardwired alarms still need batteries?
Yes, the listings here all include battery backup. The backup keeps the alarm working if household power drops out, and some models use a sealed 10-year cell instead of a replaceable one.
Can I install a hardwired alarm myself?
Many homeowners do, but the power must be off at the breaker and local electrical rules apply. Confirm the circuit is dead with a non-contact voltage tool before touching any wire. If wiring looks damaged or unfamiliar, bring in a licensed electrician.
Should I replace all alarms at once?
It is common practice to replace interconnected alarms together so they share a similar age and feature set. A four-pack makes that practical for a floor or a small home. Mixed ages make it harder to track when the next replacement is due.
