Understanding Smoke and Carbon Monoxide Detectors | Ask This Old House

In this video, This Old House master electrician Heath Eastman teaches host Kevin O'Connor everything he needs to know about these fire safety devices.

Master electrician Heath Eastman shows Kevin O'Connor everything he needs to know about smoke and carbon monoxide detectors. The two discuss the different types of smoke detectors available and how they operate and interconnect. Then, Heath shows Kevin the difference between a smoke detector and a carbon monoxide detector, highlighting the importance of having both in a home.

Two Types of Smoke Detectors
There are two types of smoke detectors: ionization and photoelectric. These devices will activate in the presence of smoke, but they both specialize in a particular type of combustion. Photoelectric detectors will detect smoldering fires sooner than ionization detectors, whereas ionization detectors will react sooner to flames.

Each box (and the back of each device) will indicate which type of detector it is.

Location Matters
When choosing a smoke detector, the area in the home makes a difference. For example, a steamy shower can set off an ionization detector in the hallway outside the bathroom. A photoelectric detector is a better choice for that area. The same would apply to kitchens or anywhere moisture may be present. However, mixing and matching photoelectric and ionization detectors throughout the home is fine.

Ask the AHJ

Manufacturer Date
It's important to know how old a smoke detector is, so manufacturers label the back of each device with a manufacture date. Detectors older than 10 years should be replaced, regardless of whether they still appear to be working.

Battery vs. Hardwired
Battery-operated smoke detectors are easy to install and can go anywhere the homeowner needs one. However, these devices typically alert individually, which means an activation for one device will not trigger the others.

Conversely, many local codes now require hardwired detectors. These detectors plug into the home's fire alarm system, providing constant power while still having a battery backup. When these devices alert, they will typically activate all of the smoke detectors in the home. However, each device needs an alarm wire run to the device's location, making these models much more difficult to install.

Carbon Monoxide Detectors are Just as Important
Carbon monoxide is odorless and tasteless, meaning that a carbon monoxide detector may be the only way to know there's an issue. Carbon monoxide detectors come in several plug-in, battery-operated, and hardwired varieties. They even come in combination units, featuring both carbon monoxide and smoke detection from one device.

Combination models will give voice alerts to explain whether the issue is smoke or carbon monoxide-related.

Detector Location Best Practices
Always check with the AHJ, but there are some general rules and best practices when choosing detector locations:
* Place smoke detectors in the bedrooms
* Install a smoke and carbon monoxide detector just outside of the bedrooms in the hallway
* Install a smoke and carbon monoxide on every level, including the basement, first floor, second floor, and attic

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Understanding Smoke and Carbon Monoxide Detectors | Ask This Old House

[Dredpath]

This is very important. As a fire alarm and life safety technician, many people don’t know the difference and it’s something that people should know. A lot of people thinks it’s an annoying thing on the wall/ceiling, but it’s job is to save lives.

[Sparky-wwre]

Understanding the difference between the two technologies used in smoke alarms can mean the difference between life and death. Ionization alarms responds better to fast flaming fire producing little smoke, such as a cooking oil fire quickly spreading to towels, cabinets and so on. Photoelectric alarms respond better to slow smoldering fire producing heavy smoke such as caused by a lit cigarette dropped on a sofa, which also happens to be one of the most dangerous type of home fires due to the toxic fumes released. Dual sensor smoke alarms that combine both technologies in one unit offers maximum safety and should installed even if the authority having jurisdiction doesn't require them.

[jefflebowski]

A note on 2020 NEC code: all new work requires hardwired smoke detectors to be on a AFCI circuit breaker. GFCI protection is not required though.

[TheRChannel]

Technically smoke alarms not smoke detectors, at least by code definition. Smoke detectors do not make any sound and are connected to a fire alarm panel.

Yes they both detect smoke, but the difference is important.

[isaacmorse]

CAUTION!! Smoke detectors go on the ceiling, Carbon Monoxide detectors go near the floor! Carbon monoxide is heavier and will start lower so the detectors should be below the level of your head when sleeping.
Separate devices, separate locations, hard wired is preferable but combo detectors are the dumbest thing.
Just my opinion.

[billz]

good review. Note that in many areas only sealed batteries are now code, and 10 year old units must be replaced.
Also if you could include this in a future presentation and the intergration of a "system sensor" and how they are intergrated and uses.

[bettymaverick]

Per experts in the field you should always purchase a Combo photoelectric and ionization. The others are a bad idea. After learning from real experts I removed my old detectors and installed the new Combo type. Always purchase a separate CO2 detector. You can read the many reasons why. Please check with real experts. Many places require hard wired ones but again there are problems with them also. Please read about from true experts. They do catch fire at times. Please do more research do not rely on this article alone.

[dmitry]

I've always heard that you should mount CO2 alarms near the floor. NOT on the ceiling like a smoke alarm.

[lailas.]

"I keep getting indicators that I'm at the end of my life." Omg cracked me up!!! 🤣🤣

[itran]

Do you have a link of combo smoke and carbon monoxide alarm? I want to buy one but your video doesn’t show more details.

[grantmeyer]

Always wondered if smoke rises and Carbon monoxide sinks, how does does the system detect both?

[garyg]

Watch on YouTube :
What type of smoke alarm is best for your home - CBS Minnesota

[duaneulman]

Thank you good information vary helpful.

[Off-Grid]

Our CO alarm keeps going off lately. I think it's from heat in the attic coming down out of a vent. I'm also a little concerned the spray foam in the attic has something to do with it since there is a smell when it gets around 80 up there.

[spritemarie]

I have a question. If your smoke detectors are hard wired and test correctly work, do you still have to replace them? Mine were installed around 1983.

[bg]

I don't understand. If the photoelectric detector is triggered by smoke, I would expect it to be triggered by steam.

[jackfarrell]

Great information thank you for the video. I have a question, what is the best place for a CO detector, on ceiling, wall and if wall how high?

[mattparr]

Does the carbon monoxide detectors work with home fireplaces? I've got a wood burner, obviously the wood I get is untreated.

[russstrough]

Why don't you comment on how the regulations benefit the smoke detector manufacturers? They built the ones that failed then used their influence to write the regulations that require a 700 sq. ft. 2 bedroom cottage to have 5 smoke detectors and two CO2/smoke detectors hardwired and interconnected. Where's the proof that this over kill saves lives vs. lines the pockets of the corrupt manufacturers. Same deal with circuit breakers now costing $50 a piece.

[AAAA-lthq]

If different jurisdictions can't agree on proper fire safety, how is the consumer to make an informed decision?