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Fire Alarm -- Description

RTU or Roof Top Air Handling Unit
The RTU (Roof Top Air Handling Unit) provides Heating, Ventilation, and Air Conditioning (HVAC) to the space below. Air Duct Smoke Detectors (Duct Detectors) can be inside the RTU or in the air ducts below the RTU.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






What is an RTU (Roof Top Unit)?

By Douglas Krantz

"Help! I 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."

Yes, I know. The 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.

Need for Knowledge

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 Supervisory means
  • 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.

Reset Training

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.





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