Buyer's Guide
What the UL 2034 standard actually requires, where to mount an alarm, and why the same warning that pairs with a generator matters year-round.
Reviewed August 2026. We re-check this guide quarterly.
A furnace pilot burns yellow instead of blue. A water heater's flue joint has loosened over the years. A generator sits fifteen feet from a bedroom window during a power outage. In each case, carbon monoxide is entering a space where people sleep, and every warning sign is invisible to the people breathing it. The Centers for Disease Control and Prevention describes carbon monoxide as a gas that "kills without warning," and notes that people who are sleeping can die from CO poisoning before they show symptoms at all.[2]
A carbon monoxide alarm is the only reliable warning a household will get. This guide walks through the standard that governs residential alarms, where to place them, how to tell a good sensor from a limited one, and how long they actually last. The reader who finishes this page will know what to look for on the box, on the wall, and on the calendar.
Before you buy
Alarm technology is largely commoditized; what differs household to household is how many alarms you need, where they go, and which features actually help. These five questions decide most of it.
How many levels does your home have, and how many sleeping areas?
The CPSC baseline is one alarm on each level and one in the hallway near the bedrooms in each separate sleeping area. Count levels including a finished basement, and count each grouping of bedrooms served by a common hallway as one sleeping area. That count is your minimum.
What fuel-burning appliances are in the house?
Gas or oil furnace, gas water heater, gas range, gas dryer, fireplace, wood stove, or an attached garage all raise the CO risk. An alarm should be placed to detect leaks from these sources without sitting so close it produces nuisance alarms from normal combustion.
Does anyone in the household sleep with a closed door?
A hallway alarm has to be loud enough to wake someone through a closed bedroom door and any fan or white-noise machine running inside. If that is a concern, add an alarm inside the bedroom, ideally interconnected with the hallway unit so both sound together.
What is the power situation?
Three options exist: battery-only, plug-in with battery backup, and hardwired with battery backup. The right choice depends on your outlets, whether you rent, and whether you want to remember battery replacements. Any of them can meet code, but they age differently.
Do you have anyone at elevated risk from low-level exposure?
The CDC notes that infants, older adults, and people with chronic heart disease, anemia, or breathing problems are more likely to be sickened by CO. A standard UL 2034 alarm is engineered around healthy adults; a household with a vulnerable member may want a digital-readout model that shows lower concentrations well before the alarm threshold.
Guided Finder
Answer a few quick questions about your household and we will point you to the right section of this guide.
The Gas
Carbon monoxide is a colorless, odorless, tasteless gas produced whenever a carbon-containing fuel burns without enough oxygen for complete combustion. Natural gas, propane, oil, kerosene, gasoline, wood, and charcoal all produce CO at some level. In a properly vented appliance the CO leaves with the exhaust; when venting fails, or when an engine runs in an enclosed space, CO accumulates in air people are breathing.[1]
Once inhaled, CO binds to hemoglobin in the blood roughly 200 times more readily than oxygen does. That binding forms carboxyhemoglobin and displaces the oxygen the body's tissues depend on. The CDC lists the most common early symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion, and notes that these are often described as "flu-like."[2]
The problem is that the flu comparison is also the trap. A household with a slow leak experiences headaches, tiredness, and nausea that ease when they leave the house and return when they come home, and CO is not the first hypothesis anyone tests. A working alarm is the difference between recognizing what is happening and mistaking it for a virus.
The Standard
Read this section, even if you skip the others
The certification on the box is not marketing. It defines exactly what the alarm will do at each carbon monoxide level, and what it will not do below a certain floor. Understanding the standard tells you why a professional-grade monitor beeps at 5 ppm while your kitchen alarm is silent at 25 ppm, and whether that difference matters for your household.
UL 2034 is the Standard for Safety for Single and Multiple Station Carbon Monoxide Alarms, published by Underwriters Laboratories. The CPSC states that it worked closely with UL to develop this standard, and recommends every home have a CO alarm that meets the requirements of the most recent UL 2034 standard, installed in the hallway near every separate sleeping area.[1]
The heart of the standard is a set of time-weighted response tests. An alarm meeting UL 2034 must sound within a specified window at each of three test concentrations, and must remain silent below a fourth. The CPSC's own conformance testing report on UL 2034 documents the sensitivity test values.[4]
| CO concentration | Required alarm behavior |
|---|---|
| Below 30 ppm | Alarm must NOT sound. This floor is designed to prevent nuisance alarms from ordinary background CO produced by traffic, cooking, or normal appliance use. |
| 70 ppm | Alarm must sound within 60 to 240 minutes. |
| 150 ppm | Alarm must sound within 10 to 50 minutes. |
| 400 ppm | Alarm must sound within 4 to 15 minutes. |
Two features of the standard shape everything that follows. First, the response is time-weighted rather than instantaneous. A UL 2034 alarm does not measure the current concentration and shout as soon as it sees CO; it measures dose over time, roughly modeling the buildup of carboxyhemoglobin in a healthy adult's blood. That choice reduces false alarms but means the reading on your alarm's display can climb into a range professional monitors would treat as a warning while the alarm itself stays silent.
Second, the standard reflects a design assumption of a healthy adult. Infants, older adults, people with heart or lung disease, and pregnant women can be affected by CO at levels below the UL 2034 alarm thresholds. That does not make the standard wrong for general use; it makes a digital-readout model, which shows the concentration before the alarm sounds, a more informative choice for households with an at-risk member.
Sensor Technology
Three sensor technologies have been used in residential CO alarms. Only one dominates the market today.
A small sealed cell contains an electrolyte and two electrodes. CO diffusing through a membrane reacts with the electrolyte and generates a small current proportional to the concentration. Fast, accurate, low power. Nearly every UL 2034 alarm sold today uses this sensor. Its limitation is that the electrolyte slowly dries and the sensor degrades over its service life; the alarm cannot be recalibrated or refreshed, which is why the whole unit is replaced rather than the sensor.
A chemical film that darkens when exposed to CO. An LED and photocell measure the color change, and the alarm sounds above threshold. The film recovers when CO clears, which lets the sensor "remember" long low-level exposures the same way blood does. Historically used by First Alert in some models. Less common now than electrochemical.
A heated tin-dioxide film whose electrical resistance changes in the presence of CO. Rugged and long-lived, but power-hungry (which limits battery life) and less selective for CO specifically. Rare in current household alarms.
For a household buyer, the practical result is simple: look for UL 2034 certification and do not worry about the sensor family. Nearly every alarm on the shelf uses electrochemical technology, and the UL standard tests behavior rather than mechanism. The one place sensor type matters is the digital-readout question. Electrochemical sensors respond quickly and accurately at low concentrations, which is why models that show the current PPM reading almost always use them.
Placement
Placement is where most households leave protection on the table. A working alarm mounted in the wrong place is not a working alarm.
The CPSC recommends one CO alarm on each level of the home and in the hallway outside the bedrooms in each separate sleeping area.[1] NFPA 72, the National Fire Alarm and Signaling Code, sets the same baseline in its household chapter and is the standard most state and municipal building codes reference. A household starting from zero should plan for at least one alarm per level plus one per sleeping area, then add more where the questions in the checklist above suggest they help.
Carbon monoxide has almost the same molecular weight as air, which means it mixes throughout a room rather than sinking or rising. The CPSC states that CO alarms may be installed into a plug-in receptacle or high on the wall because CO from any source will be well-mixed with the air in the house.[1] Ceiling, high wall, or outlet-height plug-in all work. Follow the specific mounting instructions in the alarm's manual, since manufacturers set their own preferences based on their sensor's optimal orientation.
An alarm mounted directly over a gas range or beside a furnace will produce nuisance alarms during normal operation and lose credibility fast. Manufacturers commonly recommend a minimum distance of 10 to 15 feet from any fuel-burning appliance or cooking surface. The specific number varies by model, so check the manual; the principle does not.
Inside a bathroom or within a few feet of one. Humidity and steam damage the sensor.
In direct sunlight, behind curtains or furniture, or where drapes could cover it. CPSC specifically warns to keep furniture and draperies from blocking alarms.[1]
Directly next to a supply vent or return, ceiling fan, or open window. Air currents can push CO past the sensor.
In an unheated attic, unheated garage, or crawl space. Temperature and humidity extremes outside the alarm's rated operating range cause failure.
In a dead-air corner where the wall meets the ceiling. Circulation is poor and detection is delayed.
A car left running in an attached garage is one of the most common sources of residential CO poisoning. If the home has an attached garage, place a CO alarm on the wall shared with the garage, at the level where people spend time. The CDC warns that CO can build up inside a car even with the garage door open, which is why a house alarm on the shared wall is a critical layer beyond simply not running the engine in an enclosed space.[2]
A CO alarm is a required part of any household plan that includes a generator. The CDC's guidance is direct: when using a generator, use a battery-powered or battery-backup CO detector in your home, and operate the generator outdoors, more than 20 feet from any window, door, or vent.[2] The battery-backup requirement matters: the generator is running because the power is out, and a plug-in alarm without battery backup is silent when it is most needed.
Power and Interconnection
Every UL 2034 alarm gets its power one of three ways. All three can meet code; the trade-offs are different.
| Type | Best fit | Trade-off |
|---|---|---|
| Battery-only | Renters, quick placement, no wiring changes | Battery changes on a schedule; dead battery is silent failure |
| Plug-in with battery backup | Rooms with an accessible outlet, digital-readout models | Uses a wall outlet; loose outlet or unplug renders it battery-only |
| Hardwired with battery backup | New construction, whole-home coverage, interconnected sets | Installation cost; requires an electrician for retrofit |
Many battery-only alarms sold since roughly 2015 include a sealed lithium battery rated to power the alarm for its full listed service life, typically 7 to 10 years. When the sensor reaches end of life, the whole alarm is discarded. Some jurisdictions now require sealed 10-year batteries in new installations. The advantage is that no one forgets to change the battery. The trade-off is that a dead unit cannot be revived, and you cannot swap in fresh batteries during a long outage if the alarm chirps low-battery unexpectedly.
In a large home, an alarm sounding in the basement may not wake someone on the second floor. Interconnected alarms solve this: when one triggers, they all sound. NFPA 72's household chapter (chapter 29) requires interconnection where multiple alarms are installed, unless a specific exception applies. Interconnection can be hardwired (traditional new-construction wiring between alarms) or wireless (radio-frequency handshake between compatible units, usually all from the same manufacturer and often the same model family).
For a household adding CO alarms to an existing home without opening walls, wireless-interconnect sets are the practical path. Buy them together, from one manufacturer, and follow the pairing instructions in the manual. Mixing brands or model families usually breaks interconnect even when the alarms themselves work fine as standalone units.
Beyond the Standard
A separate category of instruments exists beyond UL 2034 alarms: low-level CO monitors used by home performance technicians, HVAC contractors, and fire departments. They read continuously in single-digit PPM, alarm at concentrations well below 30 ppm, and are calibrated periodically. They are not UL 2034 devices and are not marketed as household life-safety alarms.
The distinction matters. UL 2034 alarms are designed around a specific risk model: prevent CO deaths in healthy adults by responding at doses that would otherwise cause serious harm, and avoid nuisance alarms that erode confidence in the alarm. That model has saved a great many lives and remains the appropriate baseline for a household. It also has a documented gap for vulnerable people, since low-level chronic exposure can produce symptoms in infants, older adults, and people with heart or lung disease at concentrations below the 30 ppm floor.
Two practical options exist for a household concerned about that gap:
A digital-readout UL 2034 alarm. The alarm still respects UL 2034 alarm thresholds, but a small display shows the current concentration in PPM. A reading of 15 or 25 ppm that appears repeatedly is a signal to have appliances inspected, well before the alarm ever sounds.
A low-level monitor as an add-on, not a replacement. Low-level monitors from manufacturers like Defender and Forensics Detectors are sold as diagnostic instruments. They are useful for a household with a vulnerable member, but they do not replace UL 2034 alarms and should be used alongside them, not instead.
For most households, a digital-readout UL 2034 alarm is the right upgrade over a basic on-off model. It costs a few dollars more and gives you the number the alarm is measuring, which turns a mystery beep into a piece of actionable information.
Sensor Life
The electrolyte inside an electrochemical sensor slowly dries and the sensor's response drifts. That drift is why every UL 2034 alarm has a service life set by the manufacturer, after which the alarm is no longer trusted to detect CO reliably even if it appears to work when tested.
The CDC recommends replacing a CO detector according to the manufacturer's instructions or every 5 years.[2] Manufacturers today typically specify 5 to 10 years depending on the model, with 10 years now common on units with a sealed 10-year battery.
Modern CO alarms include an end-of-life warning: a distinct chirp roughly every 30 seconds, or an ERR or END code on the digital display, that begins once the sensor's service life is reached. The signal continues even after a battery change; a fresh battery does not extend the sensor's life. When the end-of-life signal begins, the alarm must be replaced, not reset. Confusion between the low-battery chirp (which stops when the battery is replaced) and the end-of-life chirp (which does not) is one of the most common reasons households live with a chirping alarm rather than acting on it.
The manufactured date is printed on the back or side of every UL 2034 alarm. That date, not the install date, sets the replacement clock. An alarm installed a year after purchase is a year closer to end of life than the calendar suggests. When you install a new alarm, write the install date on the front with a permanent marker, and set a phone reminder for the replacement year.
What Goes Wrong
An alarm past its service life may still power on, still respond to a test button, and still show a green light. What it may not do is detect CO. The test button on many models only checks the horn and circuitry, not the sensor. The manufactured date is the honest indicator; if the date is more than 5 to 10 years old, the alarm is not protecting anyone.
A CO alarm chirping every 30 seconds is either signaling low battery (fixable with a fresh battery) or end of life (fixable only by replacing the alarm). Households sometimes remove the alarm from the wall to stop the sound and never reinstall it. The fire department's arrival at a house with a chirping alarm and unaware occupants is a real pattern documented in CPSC statistics.
An alarm behind a couch, above the stove, or beside a bathroom door will either miss a leak or fire so often at cooking and shower steam that the occupants disable it. The placement rules in the section above are cheap insurance against both failure modes.
A plug-in-only alarm is silent in a power outage, which is the moment a generator is running. A hardwired alarm without battery backup has the same problem. Every alarm in a household that plans to use a generator needs a battery, and that battery has to be fresh.
CPSC maintains a running list of recalled CO alarms. The recall list is at cpsc.gov/Recalls; check the model number on your alarms against it at least annually. A recalled alarm is by definition not protecting the household as intended.
When the Alarm Sounds
Never ignore a sounding CO alarm
CPSC guidance: never ignore an alarming carbon monoxide detector. Treat every full alarm as a real event until emergency responders confirm otherwise.[1]
The CPSC response protocol for a sounding CO alarm is three actions, in order:[1]
Do not stay indoors to search for the source, open windows for ventilation, or try to shut down appliances yourself before evacuating. Every extra minute of exposure adds to the dose.
Do not assume you can trust how you feel. CDC notes CO can render someone unconscious before they experience symptoms, and that people who are sleeping or under the influence of substances may die before symptoms appear.[2]
Do not re-enter until emergency responders say the level is safe. A silenced alarm is not the same as a clear house.
If anyone in the household is experiencing headache, dizziness, weakness, nausea, chest pain, or confusion, and there is any question about CO in the home, treat it as a CO event even if the alarm has not sounded. The CDC lists these symptoms as often mistaken for the flu, and notes they can be the first sign of a slow exposure that a UL 2034 alarm has not yet reached its response threshold on.[2] Get out, call 911, and let responders test the air.
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After You Buy
Owning a CO alarm well is a light maintenance load spread over years, and a few decisions that outlast any single alarm.
Test monthly. Press the test button on every alarm on the first of the month. Add it to whatever monthly cadence you already keep.
Change batteries on a schedule. For alarms with replaceable batteries, the CDC recommends checking batteries when you change the time on your clocks each spring and fall.[2] Replace with fresh cells rather than reusing the ones already installed.
Set a replacement reminder. When you install a new alarm, set a phone calendar reminder for the replacement year. Write the install date on the alarm with a permanent marker.
Service fuel-burning appliances annually. The CDC recommends having your heating system, water heater, and any other gas, oil, or coal-burning appliances serviced by a qualified technician every year.[2] The alarm is the last line of defense; a clean flue and tight venting is the first.
Rehearse the response. Everyone in the household should know the three steps: reset button, get out, call 911. Talk through it with children.
Smoke alarms and carbon monoxide detectors
Placement details, testing rhythm, and what to do when each type sounds. Sourced from NFPA 72.
Generator and backup power safety
The operational side of the CO threat: placement, distance, and exhaust discipline when the power is out.
Power outage action plan
Day-two thinking for a household through an extended outage. Where the CO alarm fits in the sequence.
Home fire preparedness
The companion detection story. Fire and CO are separate hazards, and combination alarms are a common household choice.
All Comparisons
Sources
The Weekly Readiness Brief covers new guides, product updates, and one thing to do this week.
Continue reading
Buyer's Guide
The other half of the CO safety pairing. Placement rules, PGMA G300 and UL 2201, and sizing your household.
Buyer's Guide
Battery-powered backup with no combustion and no CO. Safe indoors and a common pairing with a generator.
Shelter
Placement, testing rhythm, and combination-alarm decisions. Sourced from NFPA 72.
Energy
Where a CO alarm sits in the sequence of decisions during an extended outage.