Case study: Theater HVAC issues resolved

A theater’s rooftop units were replaced to aid in dehumidification

By John Song, PE May 24, 2022
Courtesy: McGuire Engineers

Existing rooftop units were replaced for Goodman Theatre, located in downtown Chicago. The two main theater spaces were continually plagued with high humidity issues. Although they have never missed a show, there was cause for concern as the operation of the existing rooftop units became more and more questionable. Managing the building asset was integral to the success of the theater as an inoperable unit could prevent the theater from opening, preventing its primary form of business.

Figure 5: New rooftop units with fan arrays were installed at the Goodman Theatre. The new unit was smaller and was installed without modifying the existing structural steel. Courtesy: McGuire Engineers

Among many other concerns, the existing rooftop unit was incapable of dehumidification with the lack of a reheat downstream of the cooling coil when the theater was operating at minimum capacity. The rooftop units cooling coil would cycle on and off as the temperature of the space would be met. Without continuous cooling, the relative humidity would increase well over 60% and mold would be found on the seats when operating during occupied times of low load (rehearsal, setup, etc.).

The central supply fan was also known to fail in the past and the operating team would be required to use only the return/exhaust fan to move air throughout the system. As the units were already more than 20 years old, starting to show signs of cabinet failure and operating on a phased-out refrigerant (R-22) the theater was ready to replace their rooftop units with new. McGuire Engineers recommended new rooftop units with double wall, R-13 insulated cabinet construction and fan arrays with air blenders to deliver properly conditioned air to the theater spaces.

Fan arrays were specified to provide internal fan redundancy to operate close to 100% if one fan in the array failed. In coordination with the acoustics consultant, sound attenuation requirements were able to be reduced compared to the original attenuator schedule and requirements from more than 20 years ago as the new fan system was much quieter and resulted in less vibrational concerns.

A central chilled water plant provides chilled water to the rooftop units, so a hot gas reheat coil for dehumidification control was not used. Instead, a silicon-controlled rectifier electric heater was used downstream of the cooling coil to reheat the air to meet space temperature and humidity requirements.

Author Bio: John Song is a mechanical engineer and project manager at McGuire Engineers with 10 years of experience in the HVAC industry.