On The Roof, Bigger HVAC Is Not Always Better


Roof-top HVAC units are popular for commercial and light industrial buildings because of their simplicity as complete, packaged units. But they also seem to be misunderstood.

The Consortium of Energy Efficiency has found in its studies that a minimum of 25 percent of rooftop HVAC units are oversized for their buildings. Government agencies warn of the same problem. An oversized rooftop system increases energy costs and equipment wear.

Rooftop HVAC systems provide excellent comfort for employees and customers. Their simpler installation, lower initial cost and lower operating costs make them a strong, economical choice. Where you can go wrong, however, is when estimating how big of a unit you need.

It’s a general rule of thumb that you need 3 tons of air conditioning for 1,000 square feet of office space. But following that rule of thumb is what can get builders and facility managers in trouble. The rule of thumb formula is only meant for general estimating early in the planning process. It will get you in the ball park, but you need to depend on engineers who are trained in HVAC systems to find a system with the right capacity that will fit your building size and conditions.

If you don’t, you’re likely to settle on the size of a system that gives you too much of a safety or comfort factor that would never be reached in your building. These factors allow for uncertainties in a building’s design, construction and eventual use, assuming a worst-case scenario simultaneously for all load components, including occupancy, lighting, weather, and more. An experienced HVAC engineer can apply the factors in a reasonable, real-world manner rather than hold to rigid and impractical margins.

When replacing a rooftop HVAC unit, building owners and managers often rely on the nameplate data affixed to the old unit.  Greater efficiencies in modern units, increased insulation, and additional energy savings improvements in the building over the years makes the nameplate data as obsolete as the old unit itself.

If your HVAC roof unit is oversized for your building, it is not simply a case of dialing it back. You incur more expense when installing and operating the unit. The compressor may cycle more frequently, leading to higher electrical costs and premature equipment failure.

Oversized fans in the unit, and the larger ducts to accommodate them, also increase energy use and cause greater variation in conditions for internal air, decreasing comfort experienced by the facility’s occupants. Oversized cooling systems may not work as well when dehumidifying the air. The interior air is cool but clammy.

A proper year-round maintenance program can also improve the rooftop HVAC unit’s efficiency. In cooling mode, a unit designed to operate at 1.3 kW per ton could see the efficiency drop to 1.6 kW per ton if it has a badly fouled coil. Regular cleaning with soap and water maintains the higher efficiency if done regularly.

How do you decide on the best size rooftop HVAC unit for your commercial or light industrial building? Call us at AbsolutAire. Whether you’re looking for rooftop units for new construction or a renovated building, or replacing your old units, our engineers can help you select the right size for your facility.