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Can a Duct Detector That Dirty be Cleaned?

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The chamber of a duct detector having been taken out of the housing.
This duct smoke detector was in a hospital, three months after the wing was remodeled.
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By Douglas Krantz

Well, the contractor said the new air handling equipment wasn't even turned on until after the walls had been sanded and painted, but apparently the wait didn't help this duct detector.

Not only that, this was on a remodeled floor of a hospital -- a hospital is supposed to be clean.

Now, because I'm in the service division of the fire alarm company, and not a fire alarm system installer, I was the one privileged to deal with the dirty duct detector.

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Photoelectric Smoke Detection Principle

An air duct smoke detector is a regular photoelectric smoke detector, only set up to monitor the air in a plenum, or air duct.

The way most photoelectric smoke detectors work is based on reflectivity. A Light Emitting Diode (LED) shines light into a black chamber so its light will be absorbed by the inside black walls.

Hidden from the direct light is a photo-transistor. It's waiting to pick up any light reflections coming out of the same chamber.

When particles in the air get into this chamber, they reflect light. Some of this reflected light is picked up by the photo-transistor, and, taking the signal from the photo-transistor, the detector interprets this reflected light as smoke.

Besides real smoke particles, plaster dust, steam particles, cooking smoke, etc. reflect light. The photo-transistor is not really smart; it can't tell whether it was smoke or dust that reflected the light, it can only tell that light was reflected.

Dirty Duct Detectors

When dust particles stick to the sides of the chamber, the particles reflect light. Granted, one particle doesn't reflect much light, but added together, many particles do.

As more and more particles stick to the sides of the chamber, more and more light is reflected. This is not a good thing.

Intelligent Duct Detector

Nowadays, many air duct smoke detectors have 24 hour time averaging circuitry to determine if the light reflected inside the chamber suddenly increases (particles in the air, interpreted as smoke), or slowly increase over weeks or months (plaster or other dust sticking to the walls of the smoke chamber, interpreted as dirt).

This is part of what is meant as an intelligent detector.

When these duct detectors become dirty, rather than going into alarm, they're automatically taken off line by the fire alarm system. Then the fire alarm system goes into trouble, and you, the technician are called to fix it.

Cleaning Duct Smoke Detectors

The NFPA does talk about cleaning smoke detectors, including duct detectors. However, it doesn't talk about what the technician is really up against.

Canned air or vacuum cleaners are mentioned, but these methods really don't work.

Usually, just blowing canned air into the chamber will dislodge a little of the dust clinging to the walls of the chamber, leaving most of the chamber dirty.

A vacuum cleaner does even less; it can only stir the air inside the chamber a little bit, which doesn't do anything.

To clean the detectors properly, what's not mentioned by the NFPA is the detectors have to be taken apart to clean them.

Dirt in the Chamber

Do not use liquid, even rubbing alcohol or other chemical.

The liquid can creep into small cracks and get into the electronics of the detector. There is a possibility of causing damage and the whole detector has to be replaced.
Often, the chamber can be taken apart and cleaned.

Once the insides are accessible, the dust particles clinging to the sides of the chamber can be dislodged by rubbing a dry cloth on all surfaces of the chamber. You can watch your progress.

Make sure the dust is removed from the cracks and crevices. These surfaces are hard to get to, but any dust there can also be part of the reflections and part of the dirtiness.

Remember, this is a percentage cleaning. If you clean 1% of the surfaces, you reduce the dirtiness of the chamber by 1%. Cleaning 50% of the surfaces reduces the dirtiness by 50%. The more surfaces you clean inside the chamber; the cleaner the detector.

Once the dust particles are loose, canned air can be used to blow away the loose dust particles.

Duct Detector Dirty Memory

At this time, the duct detector can be put back together. However, the problem is the detector still remembers it was dirty.

After cleaning, before the duct detector quits saying to the panel "I'm dirty," it has to go through its time averaging of the reflections. You might have to wait up to 48 hours of the detector seeing a clean chamber before it says "I'm clean."

U L Rating

Some manufacturers make it easy to take the detectors apart and clean; other manufacturers make it difficult. Sometimes by taking it apart to clean, the UL rating of the detector may be ruined.

Basically, to find out, you have to read the instructions and know the detector.


Well, this was a hospital, and they wanted the system to be trouble free... right now.

I didn't have the time to wait for the detector to figure out it was no longer dirty; all I could do was replace the detector.

You can see how the fire alarm system works by ordering the book "Make It Work - Conventional Fire Alarms" by Douglas Krantz.

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