Smoke and Carbon Monoxide Alarms: Where They Go, How They Are Wired, and When to Replace Them

What is the difference between a smoke alarm, a CO alarm and a combination unit?

A smoke alarm warns you about fire. A carbon monoxide (CO) alarm warns you about a poisonous gas that has no color or smell. A combination unit does both in one housing. You need protection from both dangers, and the right mix depends on your home.

Here is how the three compare.

Words matter here. NFPA points out that a “smoke detector” is a sensor wired into a building fire alarm system. A “smoke alarm” has the sensor and the horn in one unit. Most homes use smoke alarms. Learn more about the device in our glossary entry on the smoke alarm and the carbon monoxide detector.

Ionization or photoelectric?

NFPA says the two basic technologies behave differently. Ionization alarms respond faster to flaming fires. Photoelectric alarms respond faster to smoldering fires. NFPA also says photoelectric alarms are the best choice near kitchens and bathrooms, because they cause fewer nuisance alarms. Newer multi-criteria alarms state “helps reduce cooking nuisance alarms” on the package.

Whatever type you choose, look for a unit listed with a nationally recognized testing laboratory. The listing means it met a set of protection standards.

Where should smoke alarms be placed in a house?

Put a smoke alarm inside every bedroom, outside each sleeping area, and on every level of the home, including the basement. NFPA also says to mount alarms on the ceiling or high on a wall, and to keep them at least 10 feet from the stove.

Use this placement checklist when you walk your home.

1. Count the bedrooms. Each one needs an alarm inside it.
2. Find the hallway or open area outside the bedrooms. Add an alarm there.
3. Check each floor. Every level needs at least one, including a finished basement.
4. Look at the kitchen. Keep alarms at least 10 feet from the stove. Do not put them in kitchens or bathrooms.
5. Read the maker’s instructions for the exact spot. The package or the maker’s website gives details for each model.

NFPA says you may have as little as two minutes to get out of a home during a fire. That short window is why coverage in every sleeping area and on every level matters.

What if the layout is unusual?

Open stairwells, vaulted ceilings, lofts and additions all change the best spot. The maker’s instructions cover many of these cases. If you are unsure, an electrician can walk the house and mark locations before any work starts.

Where should carbon monoxide alarms go?

CPSC says to install CO alarms on every level of the home and outside sleeping areas. Battery-operated alarms or alarms with battery backup are the right choice, so they still work when the power is out. Interconnected CO alarms are best.

CO comes from burning fuel. In North Texas homes, common sources include gas furnaces, gas water heaters, gas ranges, fireplaces, and vehicles running in an attached garage. CPSC also warns that a portable generator can make as much CO as hundreds of cars. Run one outdoors only, at least 20 feet from the house, with the exhaust pointed away. Never run one in a garage, even with the door open.

CPSC lists the symptoms of CO poisoning: headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion. If you suspect poisoning, get outside into fresh air first, then call 911.

What does hardwired and interconnected mean?

Hardwired alarms connect to your home’s electrical wiring and usually have a battery backup. Interconnected alarms are linked so that when one sounds, they all sound. You can link alarms by wire or, on many models, wirelessly. NFPA says interconnected alarms are best, because you hear the warning wherever the problem starts.

The difference matters at 2 a.m. A single battery alarm in the basement may not wake a child upstairs. A linked system does.

NFPA states that only qualified electricians should install hardwired smoke alarms. The reason is simple. The alarm ties into a circuit in your walls and ceiling. A loose or wrong connection can fail silently. Our glossary covers hardwired smoke alarms and interconnected alarms in more detail.

If you are replacing old alarms, tell the electrician what you have now. Different brands often do not link to each other. Replacing the whole set with one brand is usually the cleaner fix.

How do I test smoke and CO alarms, and what do the sounds mean?

Press the test button on every alarm once a month. You should hear a loud, repeating beep. If you do not, change the battery or, if the unit is over 10 years old, replace the whole unit. Follow the maker’s instructions to clean each alarm.

Learn the sounds before you need them.

Alarm patterns vary by brand, so read the user guide for each model. NFPA also offers a “Learn the Sounds of Fire Safety” sheet to help the whole family recognize them.

Make testing a habit. Pick a day each month, such as the first Saturday. Test every alarm in the house in one pass. If a unit fails, fix it that day.

When should smoke and CO alarms be replaced?

NFPA says all smoke alarms must be replaced 10 years after the manufacturing date. The date is on the back or side of the unit. Replace the whole alarm, not only the battery. If a unit chirps after you install a fresh battery, it has reached the end of its life.

A sealed 10-year alarm and a yearly-battery alarm both give protection, according to NFPA. What matters is that you change batteries on time or swap the whole unit on schedule.

CO alarms have their own end-of-life dates. They are set by the maker and printed on the unit, so check the package or the user guide. Treat a CO alarm that is past its date as not working.

If your home is older and still has the original alarms, check the dates now. It is common to find units that went up with the house and have never been replaced.

Who should install or replace alarms?

You can swap a battery-only alarm yourself if you follow the maker’s instructions. Call a licensed electrician for anything that touches home wiring: new hardwired alarms, a replacement for a hardwired unit, adding alarms to a circuit, or linking a whole system. NFPA recommends qualified electricians for hardwired alarms.

Do not open ceiling boxes or handle wiring yourself. If an old hardwired alarm is loose, hot, discolored, or has any burnt smell, turn the alarm’s circuit off at the breaker panel only if you can do so safely, and call an electrician. Read more about the electrical side in our guide to electrical warning signs.

Daniel Electric lists smoke and CO detector installation as one of its services. The work on its page includes testing, repair, new alarms and replacement. Our smoke and CO detector installation page has the details.

What else should you check for fire safety at home?

Alarms are one layer. Add a few others.

• Make a home fire escape plan and practice it. NFPA says working alarms are part of that plan.
• Keep doors closed at night. NFPA notes a closed door may slow the spread of smoke, heat and fire.
• Check that people with hearing loss have the right alert. NFPA describes strobe lights and bed shakers that work with smoke alarms.
• Use a surge protector and a good GFCI on kitchen and bath outlets. Our home electrical safety checklist walks through each room.

Next step

Call Daniel Electric at (817) 481-8606 or use the quote form at /contact to ask about smoke and CO alarm installation or replacement. See the smoke and CO detector installation page for what the work includes.