Decision focus: identify the two hazards

This page explains what smoke alarms and carbon-monoxide alarms detect and why one cannot replace the other. Budget and maintenance planning are covered further down this page.

Do not choose between two different hazards

A smoke alarm warns about smoke associated with fire. A carbon-monoxide alarm warns about carbon monoxide, an invisible and odorless gas that can be produced by fuel-burning equipment and other combustion sources. One detection type cannot stand in for the other.

Start by listing every existing alarm, including its location, model, power source, interconnection, detection type, and replacement information. Treat a missing, disabled, damaged, or end-of-life device as a coverage gap. Do not assume a device detects both hazards because it has several indicator lights or alarm patterns; verify the label and manual.

Carbon monoxide is not reliably detected by smell or sight, so the absence of visible smoke, symptoms, or an obvious fuel appliance is not proof that a CO alarm is unnecessary. An unsupported claim that one alarm is quieter or easier to own is not a basis for leaving the other hazard uncovered. Read the signal meanings for the exact model, because a warning, a low-battery signal, and an end-of-life signal may use different patterns. When an alarm sounds, follow its emergency instructions and move to safety; for a CO alarm, do not wait until people feel ill or search for the source first, and call emergency services from a safe location and follow their direction about re-entry.

Build two placement plans

Use NFPA smoke-alarm placement guidance for smoke protection and CPSC carbon-monoxide guidance for CO protection. Then apply the exact device instructions and current local requirements.

CPSC’s fact sheet says to put battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas. It says interconnected CO alarms are best, and it does not recommend CO alarms in kitchens or above fuel-burning appliances.

A combination alarm can detect both named hazards, but only at a position allowed for that model and suitable for both plans. One combination device does not automatically provide complete home coverage. Preserve required hardwiring and interconnection when replacing an existing alarm, and check the manufacturer’s compatibility list before mixing models or generations.

Decide what work is required

Battery-powered replacement may be straightforward when the replacement is permitted for the location. New wiring, altered interconnection, an unsuitable electrical box, or uncertainty about existing protection calls for a qualified electrician or alarm professional. Never disable an alarm to solve unwanted alerts; use the maker’s troubleshooting and placement instructions.

Two battery examples show the difference on the label. First Alert lists the CO400 as a CO-only alarm with an electrochemical sensor and a 9V battery; it does not detect smoke. The SC2-B01 is a combination alarm with a photoelectric smoke sensor, an electrochemical CO sensor and two AA batteries. Neither is a drop-in replacement for a hardwired, interconnected alarm. The First Alert CO400 review covers that model’s limits.

For a combination line with battery and hardwired versions, the First Alert SC5 review explains what to verify before treating one device as a compatible replacement.

Budget and maintenance for complete coverage

The meaningful cost comparison is the price of complete, compatible coverage, not one smoke-only shelf price against one CO-only price. Inventory each alarm’s location, detection type, model, power source, interconnection and replacement date, then count what is missing, disabled, damaged, incompatible or at end of life. The cost drivers are the number and type of locations for each hazard, battery, sealed-battery or hardwired power with backup, exact compatible models for interconnection, boxes, wiring and qualified labor, and ongoing testing, cleaning and replacement. Avoid estimating device count from floor area alone. Do not assume an all-electric home makes CO protection irrelevant, or that every failed alarm chirps before it stops protecting the home. Shared branding does not prove that two alarm generations can interconnect. Keep a record of tests, cleaning, batteries, faults and replacement dates, and do not paint an alarm or assume silence proves it works. Include accessible alerting where an occupant cannot reliably hear a conventional alarm.

Sources and limits

This is installation-planning guidance, not a code determination or product test. Exact placement, power, interconnection, accessibility, and replacement requirements depend on the home, device instructions, and local rules.

Frequently Asked Questions

Can a smoke alarm detect carbon monoxide?

A smoke-only alarm cannot detect carbon monoxide. Use a listed combination smoke/CO alarm or separate devices that provide the required coverage for both hazards.

Can a CO alarm detect smoke?

A CO-only alarm cannot detect smoke. Its label and manual identify the hazard or hazards the device is designed to detect.

Where should the alarms go?

Use the current placement guidance, the exact product manual, and local requirements. A convenient shared location is not automatically suitable for both hazards.