Douglas Krantz - Technical Writer - Describing How It Works
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What is an RTU?

RTU stands for Roof Top Unit. It's an air handler located on top of the roof. Often these air handlers have duct smoke detectors, and the fire alarm panel might say something like "Smoke RTU #3". Sometimes the air duct smoke detectors are in the ductwork below the air handlers.

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.


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: 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:

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):

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:

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.
Having serviced fire alarm systems for nearly 20, Douglas Krantz has compiled his knowledge of the causes of Ground Faults and how to reliably detect them into the book Make It Work - Hunting Ground Faults. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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