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The 101s: Rooftop Units

We’ve all seen those huge metal boxes on top of retail stores or warehouses. An RTU, or Roof Top Unit, is one of the most simple and common pieces of commercial equipment around.

While the name suggests it, RTUs don’t necessarily have to be installed on a rooftop, though the location does offer several benefits. First, being on the roof saves space around the building and uses a space that would otherwise go unused. This also makes it easier to add to the system if needed. Being elevated, the RTU stays cleaner, with less exposure to things like dust, dirt, or grass clippings. The unit is less accessible, so less chance of theft, tampering or accidental damage. Also, outside air is readily available, without the need to duct it in. And lastly, when cooling a space, the system doesn’t have to work as hard since cooler air naturally flows downward.

So, how does a rooftop unit work?

The primary purpose of an RTU is to cool air and provide that air to the building ductwork for distribution. It does this by cycling a refrigerant with a very low boiling point through its components. RTUs are also known as “packaged units” because they contain these components in a single cabinet – things like a condenser, compressor, evaporator coil and fan.

Here’s how it works. A couple fans kick on: one to begin moving air through the condenser and the other to begin moving air through the building’s ductwork. Air enters the RTU and runs through filters to trap contaminants. This incoming air is likely a mix of outside air and return air, which has already moved through the space. This mix helps maintain indoor air quality. The air then passes through an evaporator coil.

The evaporator coil has cold liquid refrigerant running through it. As the air moves through the coil, the coil and refrigerant absorb the heat in the air and the air becomes cooler. The fan then blows the newly conditioned air into the duct system, where it is distributed throughout the zone.

The heated refrigerant (which is vapor at this point) then flows into another component within the packaged unit: the compressor. Here, it is compressed to a higher pressure and heated. Outside, this vapor then moves through another component: the condenser coil, where a condenser fan blows outdoor air through the coil and back out the unit. This outside air cools the refrigerant down to a liquid, making it ready to have its pressure significantly reduced, thus cooling it and making it ready again for the evaporator coil. This process runs continuously until the space reaches the desired temperature.

Ever hear of a single-stage, 2-stage or 3-stage rooftop unit? This is typically referring to the number of settings which the compressor can run – off, low, medium or high. The more stages, the more control one has of the refrigerant, temperature, and air volume moving through the unit, increasing the energy efficiency of the RTU.

In northern climates, RTU systems will typically include a heating element to move warm air through the ducts when needed. RTUs can also include an Economizer, which pulls in outside air when the temperature and humidity are right, allowing the building to cool efficiently without the expense of running the compressor.

So, what are WE doing talking about RTUs? At KMC, we manufacturer a number of products that directly control rooftop units, whether our customizable FlexStat room controller, our application specific RTU version of our AppStat room controller, or our KMC Conquest Unitary controller. In short, we’ve got the products you need to get the job done in nearly any application.