Smoke detectors and carbon monoxide detectors look almost the same. They both mount on the ceiling or wall, both run on power or batteries, and both beep to get your attention. But they protect against two completely different threats, and one cannot do the other’s job. Understanding the difference is what keeps your family protected from both fire and carbon monoxide.
Here is how a smoke detector and a carbon monoxide detector differ, where each one belongs, and what Maryland law requires.
What a smoke detector does
A smoke detector senses the smoke particles produced by a fire and sounds an alarm to give you time to get out. There are two common sensing technologies, and they respond to different kinds of fire:
- Ionization sensors tend to respond faster to fast-moving, flaming fires.
- Photoelectric sensors tend to respond faster to slow, smoldering fires.
Because a home can face either kind of fire, many safety experts recommend dual-sensor alarms or a mix of both technologies so you are covered in either scenario. What a smoke detector does not do is sense carbon monoxide.
What a carbon monoxide detector does
A carbon monoxide (CO) detector senses carbon monoxide gas in the air. Carbon monoxide is colorless, odorless, and tasteless, which is why it is often called the silent killer. You cannot detect it with your senses, so a working alarm is the only reliable warning.
Carbon monoxide is produced by the incomplete combustion of fuel. Common sources in and around a home include gas furnaces, water heaters, gas ranges and ovens, fireplaces, an attached garage, and portable generators. A CO detector warns you before the gas reaches dangerous levels. It does not sense smoke or fire.
The key differences at a glance
| Feature | Smoke detector | Carbon monoxide detector |
|---|---|---|
| Detects | Smoke from fire | Carbon monoxide gas |
| Threat | Fire | CO poisoning |
| Common sources | Any fire in the home | Furnaces, water heaters, gas appliances, garages, generators |
| Can it detect the other threat? | No | No |
Why you need both
Because each device detects only its own threat, a smoke detector will not warn you about a carbon monoxide leak, and a CO detector will not warn you about a fire. Homes need both types of protection.
To make this simpler, combination smoke and carbon monoxide alarms are available and are permitted in Maryland. A single combination unit can cover both threats in one device, which reduces the number of alarms on your ceiling while keeping you protected.
Where to place each detector
Placement matters as much as having the devices at all.
Smoke detectors should generally be installed:
- Inside every bedroom
- Outside each separate sleeping area
- On every level of the home, including the basement
Keep them a reasonable distance from kitchens and bathrooms to reduce nuisance alarms from cooking and steam.
Carbon monoxide detectors should generally be installed:
- Outside each separate sleeping area
- On every level of the home, including the basement
They should be positioned according to the manufacturer’s instructions relative to fuel-burning appliances. Maryland guidance allows a CO alarm in a central location outside each sleeping area, or, with a qualifying centralized system, within 25 feet of a carbon monoxide-producing fixture.
What Maryland requires
Maryland has specific rules for both types of alarms, and local jurisdictions can be more stringent, so confirm the details for your county.
Smoke alarms: Under Maryland’s smoke alarm law that took effect January 1, 2018, battery-only alarms must be replaced with units that use a sealed 10-year battery and a hush feature. Hardwired alarms more than 10 years old must be replaced, and a hardwired alarm must be replaced with a hardwired alarm, which should be installed by a licensed electrician. Over time, all smoke alarms should be replaced once they reach 10 years from the date of manufacture.
Carbon monoxide alarms: Maryland requires CO alarms in certain dwellings, including those that rely on a fossil fuel for heat, ventilation, hot water, or clothes dryer operation, newly constructed dwellings permitted on or after January 1, 2008, and hotels, rooming houses, and rental dwelling units. Many jurisdictions also require CO alarms in existing homes with attached garages or fuel-fired appliances when permitted work is done. CO alarms should be listed to the UL 2034 standard.
Keeping your alarms working
Having the right devices is only half the job. To keep them effective:
- Test every alarm monthly using the test button.
- Replace smoke alarms at 10 years from the manufacture date.
- Replace carbon monoxide alarms on the manufacturer’s schedule, commonly every 5 to 7 years, though many newer units last up to 10 years. Check the date printed on the unit.
- Never disable an alarm or paint over it.
