Matthew Steger WIN Elizabethtown

Smoke Detectors: What You Should Know

Most home owners only think about their smoke detectors when the TV weathermen suggests that they should change their batteries when the clocks change twice a year. Did you know that smoke detectors only have a useful design life of about 10 years? Most people assume that if it beeps when you hit the ‘test’ button, the smoke detector will reliably alarm if there was a fire. This assumption can have deadly consequences.

You’d also be surprised at the number of homes that I inspect that have no (zero!) smoke detectors installed. I run across this situation at least a few times per month. Just as many homes, from my experience, have smoke detectors that do not 'beep' when tested (indicating no power or a dead battery). I view the situation as potentially dangerous. A modern and powered $10 smoke detector is a cheap insurance policy to help get you and your family out of a burning home if the time ever comes.    
A few fairly common occurrences when inspecting homes is to find bedrooms (or sometimes the entire home) lacking smoke detectors and/or smoke detectors lacking batteries. Each is a safety hazard.

What does pressing the TEST button do?

It must be stressed that pressing a smoke detector’s test button and hearing a 'beep' only confirms that the smoke detector is powered; it does not confirm that the smoke detector will actually 'go off' if there is a fire. Most people are under the erroneous notion that as long as the smoke detector beeps when tested, that you are somehow automatically safe. To confirm that the smoke detector's actual sensor is functional (and would likely 'go off') in the presence of smoke, you would need to test the smoke detector with a source of smoke, for example. A smoke detector is only as good as its sensor. A new battery in an old smoke detector with a non-functional sensor provides no protection, but only gives a false sense of security.

The test button simply confirms that there is 120 Volt household power (if a hardwired unit) or that the battery is still good (if battery powered).

As part of the ASHI (American Society of Home Inspector) Standard of Practice, home inspectors are required to 'inspect' smoke detectors, however the standards don't say specifically how to inspect them. Some inspectors only report if smoke detectors are installed. Personally, I go a step further.. I report if they are present and I report (if the smoke detectors are physically accessible) if the units appear to be more than 10 years old. If they are older than 10 years old, I recommend they be replaced.

Replace smoke detectors at least every 10 years

Many smoke detector manufacturers started stamping their manufacture dates on smoke detectors in the late 1990s and early 2000s. Prior to that, most units, I have found, don’t have their manufacture dates listed. When did you last check your smoke detectors’ manufacturer dates? If you’re like most people, probably never. I often ask this to my clients during their home inspection and nearly 100% of the time, I get a ‘deer in the headlight’ look with a ‘I didn’t know I needed to’ response. If your smoke detectors have no dates stamped on their back, it is probably safe to say that they are more than 10 years old and are in need of replacement now. Smoke detectors should be replaced at least every 10 years. If your smoke detectors have no manufacture date stamped on them, they are likely 20+ years old and should be replaced. A 20 year old smoke detector may be powered (and make a beep when you press the test button), but may not 'go off' (due to an internal sensor that has gone bad due to age) if there was an actual fire. Why risk your family’s safety for a $10 device that's well beyond its design life?

Two Types Of Smoke Detectors

There are two types of smoke detectors: ionization and photoelectric. They each have a different purpose and way of detecting smoke or fire. The ionization type of smoke detector is installed in the vast majority of homes in the USA.

The ionization type (type "i") tends to be best for what is called ‘fast flame’ fires which is a less common type of house fire. This type of smoke detector tends to have more frequent nuisance alarms which may cause the home’s occupants to disable this type of smoke alarm, by removing the batteries. Now you have no alarm. Ionization smoke detectors tend to nuisance trip from burnt toast or taking a steamy shower. When a house fire occurs, the fire department or a fire marshall is often called in to investigate the cause. Studies have been released stating that many fire fatalities could have been prevented if the home had photoelectric smoke detectors. Similarly, some studies also indicate that some of these fatalities were linked to ionization smoke detectors not going off early enough to alert the home’s occupants to escape. Nearly 90% of all homes have only ionization type smoke detectors installed and many homes, I find amazing, have no smoke detectors installed. Also, a sobering thought... from a recent study, only 55% of ionizing smoke detectors failed to alarm in a timely manner in a smoldering fire. Ionization detectors have a hard time detecting smoldering fires which is the most common type of house fire.

The photoelectric type (type "p") tends to be better at detecting smoldering smokey fires. Most home fires tend to be the smoldering type and, according to statistics, approximately 2/3 of house fires happen when everyone is asleep. A smoldering fire is the type that most often starts small and takes time, possibly hours, to build, such as a cigarette setting a mattress on fire. A smoldering fire often has slow moving, but large dense particles which are often best detected by the photoelectric type of smoke detector.


Ionization type smoke detectors Ionization (type "i") smoke detector

Photoelectric type smoke detectors Photo-electric (type "p") smoke detector

NOTE: Some smoke detector manufacturers make both varieties (ionization and photo-electric) and from the outside of these unit, they often look identical other than a small "i" or "p" located somewhere on the unit. Look closely either at the packaging or at the front or rear of the smoke detector to determine ionization versus photo-electric.

There’s been a lot of talk lately within home inspection and building code circles about the potential issues with various smoke detector types. Some states and cities have started changing their codes or ordinances to now require photoelectric smoke detectors in homes and other buildings. To the best of my knowledge, none of the local municipalities in the Lancaster, Harrisburg, York, or Lebanon areas have done this yet.

Smoke detectors are required to meet Underwriters Laboratory’s (UL) 217 standard (also called "UL 217"). This standard sets the minimal criteria that must be met for testing smoke detectors. Tests have been done by various groups to examine the two smoke detector types and see how well and how quickly they each respond to various common fire scenarios. Some of the results have been quite shocking. In many of the most common house fire-type tests, the photoelectric smoke detectors went off 15-50 minutes BEFORE the ionization smoke detectors went off. In some tests, the ionization smoke detectors never went off at all. If you look on the back of your smoke detectors, you can tell if you have ionization or photoelectric units. Some units will actually say "ionization" or "photoelectric" on their rear label; others simply have an “i” ( for "ionization") or “p” (for "photoelectric") either in the model number or stamped somewhere on the back of the unit.

Combination type photoelectric and ionization smoke detectors do exist, however based upon testing, these units tended to actually work less reliably than the photoelectric type. Based upon independent testing, having a combination of multiple ionization and multiple photoelectric smoke detectors in your home appears to provide the best protection.

Modern building codes (for new construction) require smoke detectors in all bedrooms, the areas outside bedrooms (such as in hallways), and on all levels of the home (including the basement) and some areas also require attics. These modern standards also require that smoke detectors must be hardwired (powered by the home’s 120 Volt power supply) and interconnected (if one smoke detector goes off, they all go off). The standards for new construction also require hardwired smoke detectors must incorporate a battery backup. The current building code throughout PA (2015 International Residential Code) does not say, however, whether the required smoke detectors must be the photoelectric type or the ionization type.

Some local townships, cities, and boroughs (such as Manheim Township, Columbia Borough, and Lancaster City) have specific smoke detector ordinance requirements that do not depend on the home’s age. These local ordinances closely follow the current IRC code regarding required locations for smoke detectors within a home, but don't require hardwired units in most cases. To find the requirements for your area, contact your local township/city/borough code official. It is a good idea that if you’re a Realtor and will be listing or selling homes across a wide range of Lancaster County municipalities that you know what each township, borough, or city requires in terms of smoke detectors (and any other local requirements).

Or course, home inspectors don’t inspect to building code nor determine code compliance, but we use the IRC (International Residential Code) as a general reference for common building, electrical, and plumbing standards. Based upon the testing data that’s been done, I believe that a future code revision (the IRC is updated every 3 years) may actually spell out a photoelectric requirement at some point for the above-stated reasons. In the meantime, it is best to have both separate photoelectric and ionization type smoke detectors installed throughout your home that meet the location requirements stated above (in all bedrooms, in the hallway(s) outside the bedrooms, and on each level).

UPDATE - A new smoke detector standard (UL 217 9th edition) was officially published as of 2 Jan. 2020. It requires new smoke detectors manufactured after 30 June 2021 to be better at detecting smoldering fires, which is the type of fires most commonly found in homes, and better at preventing nuisance tripping, such as from cooking. The new standard will also make combination smoke/carbon monoxide (CO) alarms more reliable. More info here: https://savemoreonfirealarmparts.com/blog/2020/01/ul-217-268-new-smoke-detector-standards-extended-to-2021-but-does-this-matter/

A short video about smoke detectors:

Some additional info about smoke detectors from the NFPA:

Smoke alarms in US house fires

Smoke Alarm Standards

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© 2020 Matthew Steger

Matthew Steger, owner/inspector of WIN Home Inspection, is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at: 717-361-9467 or msteger@wini.com.

WIN Home Inspection has provided a wide array of home inspection services in the Lancaster, PA area since 2002. This article was authored by Matthew Steger, ACI - owner of WIN Home Inspection in Lancaster, PA. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.