If you own or manage a commercial building of any size, at some point the question comes up: which system belongs in my building?
Maybe you are replacing aging equipment and a contractor is recommending something different than what you have. Maybe you are taking over a building and trying to understand what you inherited. Maybe you are evaluating a new construction project and the mechanical drawings show equipment you are not familiar with. Either way, the decision carries a lot of weight. Get the wrong system for your building and you pay for it in energy costs, maintenance complexity, and capital spend for the entire life of the equipment.
Here is how to think through it clearly.
RTUs: The Default for a Reason
A rooftop unit (RTU) is a self-contained system that lives on the roof. It handles heating, cooling, and ventilation in a single package. Outside air comes in, gets conditioned, and distributes through the building via ductwork. The refrigeration cycle, the air handling, the heat exchanger, the economizer, all of it is in one box.
RTUs are the dominant system in low-rise commercial buildings across the country because they are cost-effective to install, straightforward to service, and well-suited to buildings that can be divided into zones with each zone served by a dedicated unit. A strip mall with six tenants might have six RTUs. A single-tenant office building of 20,000 square feet might have two or three. The practical ceiling for RTUs is around 25 to 30 tons of cooling capacity per unit. Above that, the economics and logistics of rooftop installation start to favor other approaches.
How Chiller Systems Work Differently
A chiller system works from a completely different principle. Instead of conditioning air directly and pushing it through ducts, a chiller cools water down to around 44 degrees Fahrenheit. That chilled water circulates through the building via insulated pipes to air handling units, which blow air across coils carrying the chilled water. The cooled air distributes from there.
The heat the chiller absorbs has to go somewhere. In a water-cooled system, that heat transfers to condenser water that pumps to a cooling tower on the roof, where evaporation dumps the heat to the atmosphere. In an air-cooled system, the heat rejection happens directly to outside air through a condensing coil, similar to an RTU but at larger scale. Chiller systems are typically found in larger buildings: mid-rise and high-rise office, hospitals, universities, large hotels, and any facility where cooling loads exceed what rooftop units can practically handle.
The Size Question Is the Starting Point
Under roughly 150 to 200 tons of total cooling load, RTUs are almost always the more cost-effective choice. Lower upfront cost, less mechanical room space required, simpler installation, and easier for any qualified commercial contractor to service. Above 500 tons, chillers are almost universally the right answer. The efficiency and capacity advantages at scale are real and the long-term operating cost difference justifies the higher upfront cost.
The 200 to 500 ton middle range is where the decision requires real analysis. In that range, the comparison involves first cost, energy efficiency at your specific load profile, maintenance infrastructure, and the physical constraints of the building. A building with adequate roof space and clean zone layouts may still favor RTUs at 300 tons. A building with complex zoning, multiple floors, or load profiles that vary dramatically across zones may be better served by a chiller plant even at lower tonnage.
Efficiency at Scale
Water-cooled chiller systems are generally more energy-efficient than RTUs at equivalent capacity, particularly at partial load conditions, which is most of the time in a real building. A modern water-cooled centrifugal chiller can achieve efficiencies below 0.5 kilowatts per ton under optimal conditions. A commercial rooftop unit serving a comparable load typically operates in the range of 1.0 to 1.4 kilowatts per ton. That gap is real, though the total picture narrows once you account for the full chiller system including cooling tower fans and pumps.
The caveat is that chiller system efficiency includes the cooling tower fans, condenser water pumps, and chilled water pumps, all of which add to the total energy picture. When you account for the whole system, the efficiency advantage of water-cooled chillers is real but more modest than the chiller nameplate alone suggests. For a Michigan building, where heating loads are substantially larger than cooling loads, the efficiency math also involves how each system handles heating. RTUs with gas heat are a known quantity. Chiller plants pair with boilers for heating, which adds capital cost but allows independent optimization of each function.
What Happens When Something Breaks
RTU systems have a built-in redundancy advantage: if one unit fails, only its zone is affected. A building with four RTUs loses 25 percent of its cooling when one goes down. A building with a single chiller loses all of it. Chiller plants address this with multiple chillers and staging controls, so one chiller can carry partial load if another is out of service. But that redundancy comes at capital cost, and smaller chiller plant installations often carry less redundancy than the building actually needs.
From a maintenance standpoint, RTUs are simpler to service because the technology is widespread and parts are standardized. Chiller maintenance requires more specialized knowledge, particularly around the refrigeration circuit, cooling tower water treatment, and controls integration. The contractor relationship matters more with a chiller plant than with an RTU installation.
Which One Belongs in Your Building
Most commercial buildings in Southeast Michigan under 50,000 square feet are well-served by rooftop units. They are cost-effective, maintainable by any qualified commercial contractor, and appropriate for the load profiles typical of retail, restaurant, office, and light industrial applications.
Larger buildings, particularly multi-story office, medical, or mixed-use, warrant a real chiller evaluation. The efficiency and capacity advantages at scale are meaningful and the long-term operating cost difference can justify the higher first cost. The right answer for your building depends on your square footage, load profile, zone complexity, physical constraints, and budget. A contractor who gives you the answer before doing that analysis is giving you whatever they prefer to sell. We would rather do the analysis and tell you what actually fits.
maintenance plans, 24/7 service, and system replacement across Southeast Michigan. Call (248) 590-0010 or reach out at sales@vitalitymechanical.com.