Smells and Bells: Rooftop Unit Heat Exchangers

By Adam J. Hansen

When we enter our fire service, we must take an oath. We stand before the chief, mayor, or other dignitary, hold up our right hand, and swear to protect the citizens of our communities to the best of our abilities. This oath comes with great authority and great responsibility. When we pull up to a dwelling that is well involved with fire, we have a strong understanding of incident priorities and the proper steps needed to be taken to protect life and property. After the fire is knocked down, primary and secondary searches are performed, and a myriad of other tasks are completed, we feel fairly confident we have successfully fulfilled our mission. The incident priorities seem straightforward. But are the actions firefighters must remember to perform at an incident always this simple?


When arriving on the scene of a commercial occupancy, often we are presented with “nothing showing” and are forced into investigation mode. As highlighted in our previous article, firefighters often enter these structures to find an odor of something burning and/or a slight haze near the ceiling. Many times, rooftop units (RTU) are the culprit of a great majority of these smells-and-bells-type calls. Worn-out belts are often the cause, and although they are consistently activating the 911 system, seldom present us with a true emergency. Yes, it is vitally essential to ensure the building is not on fire, locate the affected unit, investigate and safely shut the RTU down. That said, more often than not worn out belts do not create a true immediately dangerous to life and health (IDLH) environment.

But what if there was an underlying emergency? What if we unknowingly assumed every one of these smells-and-bells-type calls was nothing more than an issue with a belt? What if we recalled an incident and returned to quarters, all the while leaving a faulty RTU running creating an IDLH environment within the building? Is this fulfilling our mission of protecting the citizens and visitors of our communities with which we were sworn to protect? It’s often no fault of our own, but so many of us were simply never provided the education to completely and effectively investigate these types of incidents, specifically RTUs.


When RTUs produce heat for a building, the heat from the burner is pulled by an inducer motor up into the heat exchanger. As fresh air meant for the building is pushed by the blower/motor, it passes over the heat exchanger tubes and is heated. This heated air is then pushed by the blower/motor throughout the ductwork of the building, heating the individual spaces the distributor vents service. It would be great if the only product to come from the burners was heat, but this far from the case.

Think about firefighting and the movies. We all know TV shows get this wrong. We don’t get a working fire and are remarkably able to see clear as day down the hallway. Along with fire come the products of combustion: smoke, carbon dioxide, carbon monoxide (CO), cyanide, etc. The same holds true for RTUs. When heat is being produced by the burners, along with the heat comes the unwanted byproducts of combustion including carbon monoxide. Instead of these unwanted byproducts entering the air destined for the inhabited compartments of a building, the heat exchanger creates for a single, safe, and airtight vessel for gases produced by the burners to be discharged safely back out into the atmosphere.


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