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Fire Alarm -- Descriptionwall mounted ac units price Duct Detectors and the RTU (Roof Top Air Handling Unit)air conditioning outside unit partsI have an alarm on my fire alarm panel saying something about RTU, but it won't reset," so goes the cry of the brick and mortar retail store manager, "I can't get any heat out of my furnace, either."air handling unit standThe display on the fire alarm annunciator says "Supervisory, RTU 1," but that's really kind of cryptic. Not very many people outside the fire alarm industry know what that means. If there's no fire, problems with the Roof Top Air Handling Unit (RTU) are actually building maintenance issues. Smaller retail stores, though, don't have staff or a proper means of taking care of building maintenance.

It's up to the store manager to figure out what to do or who to call. In all the time the store manager was managing the store, no one explained: what an RTU is what a duct detector is what these have to do with the furnace Often, the store manager didn't know there is an RTU. If the fire alarm panel is showing a problem, it's the fire alarm vendor that gets the call. Roof Top Unit (RTU) or Air Handling Unit (AHU) A roof top air handling unit provides Heating, Ventilation, and Air Conditioning (HVAC) to the area below. For RTUs over a certain size, duct smoke detectors are associated with the RTUs. Duct smoke detectors are like area smoke detectors, except they sample the air passing by inside the air ducts. The duct smoke detectors are mounted either inside the RTU, or in the space below the roof line just underneath the RTU. To prevent the spread of smoke by the ventilation system, as its main purpose, the duct smoke detector shuts down the Roof Top Air Handling Unit.

Either through direct wiring or through the fire alarm panel, the duct detector interrupts an electrical link inside the RTU, which in turn shuts down the RTU. Latching Duct Smoke Detector When they detect smoke, the duct detectors latch into alarm: they don't reset on their own. To reset, power to the detector is turned off and on. If the duct detector receives its power from the fire alarm panel, the fire alarm panel can reset the duct detector. However, when the duct detector gets its power from the RTU, unless there's a remote test switch or an accessible reset button on the detector itself, the only way to reset is to cut the power to the RTU. This is not intuitive, and this has to be shown to most people before they understand it. For those of us servicing fire alarm systems, the simple solution is to reset the duct detector from the roof top air handling unit, say it now works, and walk away. A little bit harder is explaining the whole process to a store manager:

That a supervisory is not an alarm that calls the fire department That a supervisory is telling the owner of the building that something besides the fire alarm system needs attention That there are smoke detectors in the air ducts That when these duct smoke detectors go into alarm, they shut down the air handling unit on the roof That they latch in alarm once they detect smoke That the red light on the duct detector shows which one is in alarm That the duct detectors are powered by the roof top air handling unit That the only way to reset the duct detector (if there's no test switch) is to power off and on to the roof top air handling unit Reason for the Alarm And then after that, explain the duct detector doesn't just go into alarm by itself; there is a reason. If something caused the detector to go into alarm once, the same thing will probably happen again and cause the detector to go into alarm again. Things that commonly set a duct detector into alarm (in no particular order):

The duct detector is dirty There really is smoke The filters of the Air Handling Unit (AHU) are dirty The duct detector is faulty There is dust in the ducts The room that the air comes from is being cleaned/painted/sanded Smoke or dust is being blown across the roof from another building Fire Alarm System Ownership An understanding by the store manager of what the duct detector is, and what can set it into alarm, will help because: The store manager will now have some ownership in dealing with the problem The store manager will now provide eyes and ears for when the fire alarm tech isn't on site to investigate (especially when the cause of the supervisory isn't obvious) The store manager will better deal with other duct detector and RTU problems as they come up Back to the Problem with the Supervisory on RTU 1 In this case, this supervisory was only happening in the fall, when the RTUs were turned on for the first time since spring.

This happened each year, and the store manager actually knew about the stirring up of the dust that collected in the ductwork during the summer. The trouble is the previous fire alarm technician hadn't explained anything to her; he just reset the duct detector and walked away. She wanted to know what a supervisory meant and what an RTU was. She was grateful to be shown the red light on the duct detector, and to be taken out on the roof to see the power switch for the Roof Top Air Handling Unit.Describing How It WorksTechnican's Corner HomeResidential Fire AlarmGeneral ElectronicsFire Alarm DescriptionsFire Alarm InstallationFire Alarm MaintenanceFire Alarm TestingFire Suppression SystemsTriviaNuts and Volts Magazine "Insulation Tester"Writer's HomeAboutPrivacyContact Short CircuitFree SubscriptionI'll Send You theTwice-MonthlyFire AlarmNewsletter Articles Fire Alarm Systems -- What are Class A Loops?In case a wire breaks, to keep more devices working, Class A uses a second path from the fire alarm panel;

a redundant wire loop goes around the broken wire. Fire Alarm Systems -- What is a Ground Fault?Ground Faults are one of the simplest problems in a fire alarm system, an yet they're one of the hardest problems to find. Electronics -- What is a Flyback Diode?Manufacturers the world over spend good money installing these diodes, they must think they're important. What could be so important about a little diode? Electicity - - Conventional Flow or Electron Flow?Electrical Engineers and Electronic Technicians each think electricity flows the opposite direction. Why don't they agree? Fire Alarm Systems -- What is a Waterflow Switch?It doesn't turn on the water. It doesn't turn off the water. A waterflow just detects that water is flowing. Fire Alarm Systems -- Wire Loop Supervision of the Fire Alarm System Helps Insure the System Will WorkBecause the fire alarm system is a Life Safety System (people's lives depend on the fire alarm system working in an emergency), using supervision, the panel makes sure the field devices are always connected to the loop Fire Alarm Systems -- What is an RTU (Roof Top Unit) and How is it Affected by Duct Detectors?

I have an alarm on my fire alarm panel saying something about RTU, but it won't reset. I can't get any heat out of my furnace, either. End-of-Line ResistorBecause both a thick and a thin wire are under the same screw plate, when screwed down on the wires the plate cannot press against both wires evenly. One wire is always looser, and the loose wire shows up later as a trouble on the panel. Fire Alarm Systems -- Wiring a Door HolderThe door holder looks innocent, and shouldn't be able to damage anything, so how does a door holder damage the Fire Alarm Control Relay that shuts off the current? Fire Alarm Systems -- What Should I Do When the Fire Alarm Sounds?When the Alarm Sounds -- I would never suggest turning off the fire alarms, or in any way delay calling the fire department -- So says Brad Sveum, fire marshal ... Addressable Supervision of the Signaling Line Circuit (SLC)Supervision of Addressable Fire Alarm Systems is done with Polling of the Devices. How does Polling of the Devices confirm that the wiring is intact?