Rooftop HVAC units (RTUs) are a popular choice among building engineers and managers. Although there are a wide variety of models available, most rooftop HVAC systems have a few things in common: they’re compact, conveniently placed, and reasonably simple to maintain. Not to mention, one single piece of equipment has the ability to heat, cool, filter, and ventilate a building’s air.​Together with these common virtues, rooftop HVAC units are also prized for their versatility. Their design and location allow specifying engineers to tailor each building’s system according to its needs, including ventilation and noise control.

Of course, the measure of any HVAC system’s success is how much energy it can process. Units fall into several size categories—small, large, and very large. If you’re an engineer considering including a rooftop HVAC system in your building, knowing what these categories mean and how it could impact your HVAC design is critical.

​Capacities of Commercial Rooftop HVAC Units

​While the Air-Conditioning, Heating and Refrigeration Institute measures performance and efficiency in British Thermal Units/hour (Btu/h), most manufacturers measure using “tons,” a reference to when actual tons of ice were used to cool facilities. (For reference, 1 ton of ice is roughly equivalent to 12,000 Btu/h. of cooling) So, a unit rated at 24,000 Btu/h, or 2 tons, could remove roughly 2 tons of heated air from the building every hour.​Most rooftop HVAC units fall into three categories:

1. Small: Less than 65,000 Btu/h (roughly 5 tons)
2. Large: From 65,000 Btu/h to 250,000 Btu/h (roughly 5 tons to 20 tons)
3. Very Large: From 250,000 Btu/h and up or (roughly greater than 20 tons)

​Many HVAC manufacturers have models that fall outside of the 5‒20-ton range. However, most industry and federal regulations are based on the sizes listed above.

​Other Critical Features to Consider in Rooftop Units

​After determining the size of your rooftop unit, you’ll have a few more choices to make.

1. Condenser design (how to reject heat from the building): Your options include air-cooled, water-cooled or, in the case of very large rooftop units, evaporative cooling.
2. Heat pump inclusion: Water-source, geo-thermal or air-source heat pumps can be included in most large and very large rooftop units.
3. Heating source: Most rooftop units are very flexible with their ability to incorporate heating sources (steam, hot water, electric, and indirect-fired gas or reversing heat pumps).
4. Construction and life expectancy: There are many different options and materials of construction for RTUs.  Specifiers need to understand the tradeoffs between first cost and life expectancy. Single wall versus double wall construction has a major impact on cost.
5. Energy recovery heat exchangers: Managing humidity is crucial for both fresh air ventilation and maximizing efficiency when there are large differentials in indoor and outdoor temperatures.
6. Ability to treat outside air (OA): Specifiers need to understand what percentage of outside air the RTU will treat and make a product selection accordingly.  RTUs required to treat more than 30% OA relative to total supply air (SA) need special compressor and evaporator systems as well as dehumidification capabilities.

Modernizing Metrics

As technology advanced, so too did the methods commercial rooftop HVAC manufacturers used to measure their equipment’s efficiency. For decades, HVAC equipment efficiency was measured using energy efficiency ratios, or EERs, which only track a system operating at 100% capacity. Most systems only operate at such a high capacity a few days a year—if that. EERs also didn’t take climate into account, which decreased the accuracy of EERs as a measurement. To address this inaccuracy, the seasonal EER, or SEER, was introduced. 

Seasonal EERs rate units based on their performance across an entire season, providing a more balanced overview of a system’s efficiency.

However, this still did not address the problem of accurately measuring a large rooftop HVAC unit. Today, rooftop units are evaluated using integrated EERs, which calculate the system’s efficiency at different load levels and rate it accordingly.

Making The Right Call: Choosing A Rooftop Unit

​If you’re replacing a large commercial building’s rooftop HVAC system, you’ll need to keep several important points in mind. Although you might be tempted to simply choose a system with the same specifications as the older model, we’d advise against it. 

Instead, consider your building now—has its purpose changed since the last unit was installed? Do you need to bring in more OA for your occupants?  For example, think of a warehouse that is now being converted into commercial office space. Chances are the current rooftop HVAC system wasn’t installed with the building’s new occupants in mind, and you’ll need a model that can handle a higher level of demand (comfort cooling and heating, more OA, and quieter operation)​Regardless, be cautious. Rooftop HVAC unit installation is tricky: if your unit is too small, your building won’t be adequately heated or cooled. Even the installation process can prove problematic—without proper insulation and ductwork design, you might expose your occupants to excessive and unnecessary noise from the system.

​Find the Right Fit for Your Building with Windy City Representatives

Windy City Representatives works with leading manufacturers of commercial rooftop HVAC units. We understand how important it is to take multiple considerations like budget, existing systems, and ventilation needs into account. Call Windy City Representatives today at 630-590-6933 to discuss your options.