Expert Lee Kaiser covers THE BASICS OF DUCT SMOKE detection.
WHAT ARE DUCT DETECTORS?
Duct detectors are specialized housings installed on pieces of ductwork or air handling units that measure or sense that there is smoke in that air stream coming back to that unit. We use that signal from the smoke detector to shut down the air handler when it sees enough smoke in the air stream. We shut down air handlers so they don't spread smoke throughout the space.
The code calls for duct detectors so we don't use the air handling system to spread smoke throughout the space and then slow down the egress of the building. Duct detectors are a major headache for false detection in buildings. It's a fairly poor way to detect a fire and it causes more harm than good, but nonetheless, it's part of the code and we have to do it.
SMOKE ENTRY TEST
These duct detectors need to be tested on an annual basis by performing what is called a smoke entry test. When a spot smoke detector gets installed inside of a housing. The housing goes around the detector and one of two sensing tubes. We install the duct detector on the outside of a duct and from the housing Those two tubes then go into the duct. Those tubes are usually a thin wall conduit, like an EMT conduit. The conduits have holes drilled in them so air can penetrate into the pipes. As it flows across the holes it will cause a slight differential pressure and bring some of that air into the detector housing and then back out.
If we see smoke in that detector it's because we brought some of that air that's going through the duct into that detector housing. So then we need to test those detectors. Are they functional if smoked? During a smoke test, a sensitivity test would be also required, and we also verify air flow through the housing.
So, how do you do that? While the detector is running, you need to measure if there is any differential pressure between those two tubes. One option to test that is through using a gauge or a digital mono-meter which is what our technicians use. They use a Dwyer digital mono-meters and they hook up the tubes through two different holes and measure. If they see a differential pressure then they know that there's airflow through there. They don't have to hit a specific value, they just see if there's differential pressure. We have to have that while the air handle is running. It won't work if the air handler's not moving air.
HOW TO CHEAT DUCT DETECTOR
There are some cheats with this too. A lot of time duct detectors get installed in locations that are hard to access. With the duct detectors currently out there, less than 50% get tested correctly because of access issues. They're hard to get to but there's a solution for that. If you have some interest in fixing your problems with getting to the duct detectors, there are products that can be used remotely from the duct and still sense smoke in the duct.
Another cheat is using a remote test switch. It's not a code required thing. You don't have to have it there but it's an option. Any duct detector that's concealed needs to have a remote indicator lamp. A test switch has this remote indicator lamp that would light up red if the detector is activated. That would help during investigation of a false alarm, but the remote test switch is to test the interface between the duct detector and the shutoff relay to the air handling unit. We can test it that way, but it doesn't replace the smoke entry test, or the sensitivity test to make sure that the duct detector actually works. It just tests the interface between the two, although a lot of people use that as their actual test but I would say it doesn't meet the intent of the code.