Short answer
Smoke detectors sense combustion particles and carbon monoxide detectors sense an odorless gas, so most homes need both. Under common code requirements, smoke alarms go in bedrooms, outside sleeping areas, and on every level. Chirps signal maintenance, not fire. Smoke alarms are replaced no later than ten years after manufacture, and monitoring adds a response when nobody is home to hear them.
In this article
What do smoke and carbon monoxide detectors detect?
Smoke detectors sense particles from combustion, using either a photoelectric light sensor or an ionization chamber. Heat detectors respond to temperature instead of smoke. Carbon monoxide detectors measure an odorless, invisible gas produced by fuel-burning appliances, vehicles, and generators. Most homes need both smoke and CO protection because each device watches for a different danger.
The two smoke sensing technologies respond differently to different fires. Photoelectric sensors tend to react sooner to slow, smoldering fires, while ionization sensors tend to react sooner to fast, flaming ones; dual-sensor units combine both. The tradeoffs are covered in our comparison of photoelectric and ionization smoke detectors. Heat detectors fill a narrower role in spaces like garages and attics where smoke alarms misfire, explained in our guide to heat detectors versus smoke detectors.
Combination smoke and CO units exist, and they save a ceiling location, but they still contain two separate sensors with their own alarm patterns and their own service life. Knowing which sensor is speaking matters when the device sounds.
Where should smoke and CO detectors go in a home?
Common code requirements place smoke alarms inside every bedroom, outside each sleeping area, and on every level, including the basement. CO alarms typically go outside sleeping areas and on each level with living space or fuel-burning equipment. Exact positions depend on ceiling shape, kitchens, bathrooms, and applicable code, so the manufacturer's instructions and the local fire official decide the details.
Placement errors are the quiet failure in home fire protection: a detector that works but sits in a dead air pocket, next to a bathroom door, or too close to the stove. Our guides to smoke detector placement and where carbon monoxide detectors should go walk through the mechanics room by room. The total depends on bedrooms, levels, and hallway layout, which is the subject of how many smoke detectors a home needs.
Placement also interacts with wiring. When one alarm sounds in the basement, a person asleep upstairs may not hear it. Linking alarms so that one triggers all of them is explained in our article on interconnected smoke detectors.
Why do detectors beep or go off when there is no emergency?
A single chirp at regular intervals usually means a low battery, an end-of-life warning, or a fault, not smoke or gas. A full, continuous alarm without a fire is a nuisance alarm, most often caused by cooking smoke, shower steam, or dust. The two problems have different causes and different fixes, so identify the sound first.
Chirps are the device asking for maintenance. On smoke alarms, the causes and the order to check them are covered in why a smoke detector keeps beeping. Carbon monoxide alarms use different patterns, and one of them is an emergency signal rather than a maintenance one; our guide to what CO detector beeps mean explains how to tell them apart.
Full alarms with no fire are a separate problem, usually solved by moving the device or changing the sensor type, not by removing the battery. The common triggers are listed in our article on smoke detector false alarms. A household that disables an alarm to stop the noise is left with no protection in that room, which is a worse outcome than the noise it was meant to stop.
How long do smoke and CO detectors last, and how are they maintained?
Smoke alarms are generally replaced ten years after their manufacture date, which is printed on the back of the unit. Carbon monoxide alarms have a service life set by the manufacturer, often shorter. In between, alarms should be tested monthly, kept free of dust, and given fresh batteries unless they use a sealed long-life battery.
Sensors age. Dust collects in the sensing chamber and electronic components drift, so an old detector can be slower to respond and more prone to chirps. The date on the label, not whether the light still blinks, determines replacement. When one alarm in a set is replaced, it is worth checking the dates on the others, since alarms installed together tend to age out together; our guide to how often to replace smoke detectors explains how to read it.
Buildings with a fire alarm control panel, especially commercial ones, carry formal requirements as well. NFPA 72 sets the framework for inspecting and testing fire alarm systems, and the local fire official enforces it. What that inspection involves and who keeps the records is covered in our article on fire alarm inspections.
What is the difference between a standalone detector and a monitored one?
A standalone detector sounds an alarm inside the home and nothing else, so it depends on someone being there to hear it and act. A monitored detector is connected to an alarm panel that sends the signal to a central station, which can request the fire department and notify contacts according to the procedure agreed for the property.
Standalone alarms are effective at waking people who are home, especially when they are interconnected. They do nothing for an empty house, a pet left alone, or an occupant who cannot respond. Monitoring changes who learns about the event, not how the sensor detects it. The costs and limits of each approach are compared in monitored versus unmonitored fire alarms.
A panel-based system can also show trouble conditions, such as a weak battery or a device that has stopped communicating, so a problem does not depend on someone hearing a chirp. How detection, notification, and response work for a specific property is explained on our page about professional fire alarm monitoring.