During the first morning meeting of the day, several supervisors suddenly jumped up and ran out with their pagers in hand. Five fully manned Fire Department New York (FDNY) fire trucks drove up to our high-rise building. The fire-alarm system indicated smoke at a top-floor air-handler duct smoke detector (DSD). No smoke was evident, so the problem counted as a false alarm. Unfortunately, FDNY charges about $500 each time it responds to a false alarm. The firefighters reset the system, and everything
The system has been operating for years, but it still gets out-of-the-blue false alarms. Actually, I should say, "out-of-the-fog" alarms. The weather that morning had suddenly turned cool, causing a low-level fog. The air handler was bringing in 100-percent outside air, but was not heating it because the building still was quite warm from previous hot weather.
An optical smoke detector also acts as a fog detector. With fog, the DSD manufacturer's 90-percent relative-humidity maximum inlet rating is exceeded, and the unit can alarm at any time. Some manufacturers put heaters in their DSDs to help them stay above the dew point. However, when the weather swings, there still can be trouble.
That building should have humidity control on the outside-air intake, but it does not yet fit the budget or the air-handling unit. However, a newer building nearby has full humidity control on its outside-air air-handling unit. Because of piping and duct restrictions, the humidifier could not be installed in the duct where indicated on the plans. After being relocated, it is a few feet upstream from the DSD "fog detector." This installation works most of the time. As long as the humidity stays in range, there is no problem.
Fortunately for the owner, that building's fire-alarm system is not required to dial directly to FDNY.
Experience has proved that high humidity in a duct can cause more than mold and water damage. It can bring fire-department responders and citations, too.
John Cole, PE
New York, N.Y.