Recent technological advancements in HVAC manufacturing have made dedicated outdoor air systems (DOAS) easier to specify and install for contractors.

When rooftop units (RTU) from the 1980s and 1990s approached the end of their lifecycles, HVAC contractors are faced with retrofits that require upgraded outdoor air compliance with the American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.1.

The easy way out is a drop-in RTU replacement with the same brand, roof curb and possibly more capacity to handle additional outdoor air, even though it probably doesn’t fit the end-user’s current IAQ needs. From the contractor’s perspective, using the same curb makes the job easy and avoids labor-intensive modifications and potential liabilities of roof penetrations.

A drop-in RTU with no ability to dehumidify and heat outdoor air, burdens the building owner with significantly more energy costs to condition it. Instead, a conscientious contractor might suggest adding a DOAS to help pre-condition the outdoor air, save energy and economically downsize the RTU’s refrigeration section because the DOAS is now handling the latent load.

Unfortunately, specifying a decidedly different RTU capacity, brand and adding a separate DOAS surpasses most contractors’ in-house engineering abilities. However, manufacturers are now offering contractors RTU replacements with integral DOAS capabilities all in the same packaged unit. Any assistance can be provided by manufacturers’ application sales teams. Best of all, contractors skip the roofing work, because this new breed of combination RTU/DOAS are designed to fit the existing curbs (without expensive curb transitions) of the two most commonly specified RTU brands. All the contractor needs to provide to the RTU/DOAS manufacturer is the original RTU’s brand and model number. Eliminating new curbs or curb transitions streamlines the process. A curb transition, in particular, adds costs, complicates the process, slows down the installation by a few hours and carries the risk of not fitting properly.

Curb transitions can raise a unit more than 24 inches high. This is problematic, because the height complicates service access, adds to wind loading and unaesthetically increases equipment visibility from spoil ground level. Furthermore, curb extensions will create more sheet metal connection labor/materials to connect down flow designs as well as horizontal supply/return air discharge units. No duct modifications are needed for RTU/DOAS units designed to fit existing curbs of top RTU brands.

Besides energy savings and compatibility, these easily-applied RTU/DOAS provide a more comfortable and heathier environment, because there’s little chance the space has the same tenant needs as when the original RTU was specified 20 years ago. For example, a veterinarian office, a nail salon and even an apparel store will require additional outdoor air to offset inherent volatile organic compound (VOC) contaminants and CO₂. Actually any indoor space, especially those that are renovated, will require upgraded outdoor air to offset paint, carpet formaldehydes, furniture adhesives and a host of other VOCs that off-gas continuously for years.

Today’s technological advancements combined with manufacturers’ application sales team support can help contractors easily navigate through the specification and installation process of providing increased outdoor air to their building owner customers.