HVACWestern OhioDayton

Huber Heights Studebaker Building HVAC Replacement 2023

Seven aging rooftop units cooled the Studebaker building on R-22, a refrigerant that has been out of production since 2020, and every one of them was a two-deck multizone machine mixing heated and cooled air to hold 52 different zones. Prodigy replaced the lot — seven new multizone air handlers totaling roughly 161 tons and 58,100 CFM of supply air — and put a central chilled water plant behind them: a 180-ton air-cooled scroll chiller on a new pad at the corner of the building, two 375 GPM pumps on variable speed drives, expansion tank, air separator and a 30 percent propylene glycol loop routed through the building and up to all seven units. The new units landed on the existing curbs and connected to the existing ductwork and zone layout, which kept the interior of a 61,391 square foot building with 125 rooms almost entirely out of the project. Prodigy's own engineers designed the system, our crews installed it, and the rooftop work was staged into a summer break.

The building is served zone by zone rather than room by room, and the zone count is the reason the design went the way it did: 52 thermal zones across 33 classrooms, a 2,901 square foot library, a 1,825 square foot band rehearsal room, a music room and six practice rooms, a lecture room, offices and administration, with roughly 15,287 CFM of design outside air for 1,148 people. Every one of those zones had an existing duct run and an existing damper. Replacing the machines while keeping the distribution meant the district bought new equipment, not a new building.

Each new unit is an all-aluminum, double-wall cabinet with two inches of polyurethane foam, chilled water coils, two direct-drive supply fans on factory-mounted variable speed drives, stainless drain pans, seismic spring isolators, supply and return smoke detectors and factory BACnet controls, with gas heat retained. The controls were extended onto the district's existing building automation server rather than replaced, with a new plant controller sequencing chiller enable, lead-lag pumping, pump speed and chilled water setpoint against outdoor conditions, new graphics for every thermal zone, and sixteen hours of training for the district's own staff. The system was air and water balanced to NEBB standards before and after construction and independently commissioned.

The work was procured through the Aspire Works Purchasing Cooperative under a competitively bid cooperative unit price contract, and delivered with prevailing wage and certified payroll on a school calendar, with owner-coordinated shutdowns and two crane mobilizations.

The new chiller plant seen from the courtyard, with the Studebaker building and parking lot behind it

One of the original rooftop units on the Studebaker building before replacement

Technical Discussion

Buying the chiller for the hours it actually runs

Seven new rooftop multizone units at the Studebaker building draw chilled water from one new air-cooled chiller on a pad at the southeast corner. The chiller was the single largest piece of equipment on the project, and the number that chose it is not the one printed largest on the submittal.

Bar chart of the Studebaker chiller ratings: EER 9.7 at full load and IPLV 16.9 averaged across 100, 75, 50 and 25 percent load. — Figure 1 — The chiller was chosen on IPLV, the rating that describes a school's cooling year.

Two efficiency numbers

A chiller carries two ratings. EER is its efficiency at full load on a design day, and this machine's is 9.7. IPLV, the integrated part-load value, is a weighted average of its efficiency at 100, 75, 50 and 25 percent load, weighted toward the lower loads because that is where a building spends its year. This machine's IPLV is 16.9, well over half again its full-load figure. A school in Ohio sees design conditions on a handful of afternoons. Every other cooling hour is part load, and part load is where the two ratings diverge.

What makes a chiller good at part load

The 178-ton machine is built from six scroll compressors on two refrigerant circuits. At a quarter of the load it runs one or two compressors near their best efficiency instead of running one large compressor at a fraction of its capacity. The ten condenser fans are on variable speed drives, so on a 70-degree day the condensing pressure is allowed to fall and the compressors work against less head. The microchannel condenser coils reject heat with less refrigerant, 160 pounds of R-410A for the whole machine. None of those features improves the full-load number much. All of them improve the part-load number, which is the one the utility bill follows.

The loop it feeds

The chiller delivers 44-degree water and takes it back at 54, about 411 gallons a minute at design, through two 15-horsepower pumps on variable frequency drives. The loop carries 30 percent propylene glycol because it runs outdoors between the pad and the roof. On the roof, 58,100 cubic feet a minute of supply air across 52 zones is cooled by that water in the seven units' coils, and the units set on the existing curbs and connected to the existing ductwork, so the plant changed and the building's zoning did not.

Why chilled water at all

The alternative was seven packaged DX units, each with its own compressors and refrigerant charge on the roof. A central plant puts every compressor in one place a technician can reach, lets one machine serve the whole building's load curve, and leaves piping stubbed for the administrative wing to tie in later. Chilled water costs more on the first day and less on most of the days after it.

When a chiller quote arrives, find the IPLV. Two machines with the same tonnage and the same EER can be 30 percent apart on the number that describes the year.

Our Work