Douglas Krantz - Technical Writer - Describing How It Works

Do Heat Detectors Have an Operational Life?

Most fire alarm conventional heat detectors are rather simple devices; when the temperature gets high enough, some low temperature solder melts, setting off the alarm. Once the solder is melted, these heat detectors can't be restored, they have to be replaced.

Connventional heat detectors like this one may be a fixed temperature or a rate of rise/fixed temperature.

Greetings Douglas,

Once again thank you for all your helpful advices to me.

I just have a question regarding the Edwards Heat detector product recall. Not so big issue here in Canada but what is puzzling or confusing is what they indicated also in their bulletin. It says "Any heat detectors that are 15 years or older must be replaced".

In US, do you religiously practiced replacing Heat detectors that are the same age or do they really have an operational life? Can you please give me your insight about the issue? Nothing is written until Edwards claimed it.

Thank you, GF

NFPA 72 Code

The National Fire Protection Association 72 (NFPA 72) does go into some testing procedures for the non-restorable, spot type heat detectors. This would be a world-wide requirement. The code numbers in the code book will be different according to the year of publication, but the information can be looked up in the Index, under heat detector testing. There the NFPA refers to a table showing the procedure to test the different types of heat detector.

The NFPA 72 Code Book, however, only shows a list of laws; the NFPA 72 Handbook provides a lot more information.

NFPA Code with a Plus

As far as the NFPA requirements go, my copy of the NFPA 72 Code has more information than most other copies of the NFPA 72 Code. My copy shows insights into what the writers of the code were thinking when they wrote the laws shown in the Code.

The difference between my copy and most other copies is that I have the Handbook. I learned years ago that to fully understand the NFPA Code, a person needs the "National Fire Alarm Handbook NFPA 72". The Handbook is available from the NFPA alongside the regular Codebook. The Handbook just costs more, so most people don't spend the extra money. I, however, think the extra money is well worth it.

The NFPA 72 Code Book that most people have is just a list of laws, without explanations of why the laws are there. On the other hand, the Handbook, available from the NFPA has a plus. Not only does it list the laws, but the appendixes to the laws are in with the laws, and the reasons for the many of the laws are included in with the laws. That way, when there is a law, the Handbook shows the reason or an explanation for the law. The Handbook even has a lot of diagrams and pictures scattered throughout. It really does help when trying to understand the Code.

Heat Detector Construction

The heat detectors shown in the recall include the fixed temperature heat detectors, and the fixed temperature/rate-of-rise heat detectors. The recall appears to be related to the fixed temperature part of both types of heat detector.

On the ring around the detector is lettering that says either rate-of-rise and a fixed temperature, or just a fixed temperature. Whether there is a rate-of-rise part included or not, the two types of heat detectors look the same on the outside.

With the heat detectors, for the rate-of-rise part to activate, an air chamber is warmed up by the ambient room temperature. If the temperature of the ambient air around the heat detector is rising fast enough (more than 15 F, or more that about 9 C, in one minute), the air expands. When the air expands, a diaphragm inside the detector pushes contacts together, causing an alarm. Another way of looking at the alarm is that the heat detector assumes that if the temperature is rising fast enough, there must be a fire.

The rate-of-rise part is not the cause of the recall, though. It's the fixed part. The fixed part is activated by melting a ring of low-temperature solder, which releases a spring-loaded button. In the case of the recall, the solder isn't melting at a low enough temperature, so they're being recalled.

Replacing the Heat Detectors is how they are Tested

In the fixed temperature heat detector, the problem with testing the low temperature solder is that once the solder is melted during the temperature-test, the detector has to be replaced.

Yes, the solder is metal, but the internal spring places the metal under continuous stress. The temperature that the metal melts at needs to be tested because there's at least a chance, once in a while and over years of time, that the crystal structure of the solder can change a little, causing the solder to have a slightly different melting point.

The solder ring is the issue. It's a spot type - non-restorable - heat detector. Yes, there is a rate-of-rise heat detector in the same enclosure as the fixed-temperature solder ring, but that is a separate heat detector. The NFPA also wants the low-temperature solder ring to be tested, or the detector regularly replaced.

15 Years

In the Handbook, the NFPA says that every 15 years the whole bunch of heat detectors in the building need to be replaced. The replacement is required because when the heat detector is tested, the fixed temperature solder ring can't be restored. But the NFPA also gives an alternate to replacing all of the heat detectors every 15 years.

Every 5 years, remove a small sample of detectors and send them to a third-party testing laboratory - one that regularly tests the heat detectors. For every 100 detectors, two detectors are to be removed and sent in for testing. More testing is required if they fail. Of course, they want new ones to replace the ones being tested.


Keeping track of which detectors have been tested will produce a lot of paperwork; which ones have been taken out and replaced have to be kept track of... forever.

Who's going to take care of keeping track of the replacements is an issue because the building owner rarely files this kind of information, but then which fire alarm company is servicing the system is often changed during a 15-year cycle. For most installations, the ability to keep track of the testing just isn't there. Edwards knows this, and so they're telling you to just replace the whole bunch of detectors every 15 years, and be done with it.

Keep in mind also, the NFPA gives testing authority to the manufacturers. If a manufacturer says to do something, like replace their heat detectors every 15 years, you can consider that the NFPA has authorized that requirement.

Is the Code Followed?

In regard to the replacement and testing of fixed heat detectors, I have not really noticed that the Code is followed. But then, the manufacturer (Edwards) is recalling these detectors. They are also now saying that the 15-year-old heat detectors have to be replaced, so the replacement issue has become more urgent.

Douglas Krantz
Life Safety
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