Eight is enough: Team meets CA residence hall loads and installation questions with tankless strategy
Beverly Watson can easily laugh about the idea today, now that her recent tankless water heater retrofit project has been successfully completed and is fully meeting her requirements. But the associate director of operations for California State University, Northridge (CSUN) admits to wondering last summer while the work proceeded whether eight - as in eight tankless water heaters - were truly enough for her particular application.
“Frankly, to accommodate the hot-water needs of each of our students, I thought we were going to need more than that number at each of the four residence halls we were upgrading. So I kept asking the lead engineer on the project: ‘Are you sure we don’t need another tankless water heater or two?’”
Completed in late August, the CSUN project entailed the complete removal and replacement of the original atmospheric boilers and storage equipment installed when the four student-apartment buildings were constructed roughly two decades ago. These rooftop systems provided not only domestic hot water to the residents of all four structures, but also space heating (through a fancoil in each apartment). Besides no longer being able to handle the hot-water loads in an energy-efficient manner, the old boilers could not meet the rigorous emission control standards of the South Coast Air Qualify Management District (SCAQMD).
In short, it was time for a new solution at the four buildings, each spanning three or four floors and housing 37 to 52 individual apartments for two to four students apiece. The solution, specified by James Valiensi, P.E., LEED® GA of P2S Engineering (Long Beach, CA), called for each apartment building to have its own multiple-unit tankless water heater system, furnished by Noritz America. These eight units per building would furnish domestic hot water only, with a new and separate heat pump system handling the space heating.
Along with Valiensi and Watson, two other individuals who did their homework in preparing for the CSUN project were Bryan Suttles, president of Suttles Plumbing and Mechanical Corporation, and his project manager, Dan Boulais, who quarterbacked the installation. The final rooftop assembly at each building consists of four pairs of Noritz NCC199 condensing tankless water heaters, mounted back-to-back in two rows onto a custom-made steel rack. Designed by the P2S Engineering team, including the Pasadena-based Wheeler & Gray Consulting Engineers, the racks were engineered to handle the required static loads and seismic forces in accordance with the California Building Code. Adds Valiensi: “Suttles Plumbing subsequently enhanced our design in the process of making it easier to build.”
The installer also wisely decided against erecting these rack structures from scratch on the four rooftops. Instead, the Suttles Plumbing team chose to prefabricate them at their shop in Chatsworth, located only a mile and a half from CSUN. “Our proximity to the job site made this approach ideal,” says Suttles.
“We built the racks, fixed the water heaters to them, loaded the racks onto flatbed trailers, and shipped them to the four job sites. Once there, the assemblies were hoisted by crane to the roof, where all we had to do was to fasten them onto the same rooftop platforms where the old boilers used to sit and connect them to the main piping. It all went very smoothly.”
Adds Boulais: “By avoiding the time and trouble of lugging all that material piecemeal up to the roof for assembly - water heaters, racking, piping, valves, and fittings - we were able to cut our overall labor time in half.”
CLEAR-CUT CHOICEThe CSUN project commenced in July 2009, when Beverly Watson commissioned P2S Engineering to do a feasibility study: Could the university simply replace the four existing boilers with up-to-date and more efficient models, thereby generating all the hot water the apartments needed while also fully complying with the SCAQMD restrictions?
“We came up with several options,” says Valiensi, “but the main alternative to a tankless solution was a boiler with a 200-gal storage tank. At eight pounds per gallon of water plus the structure to support it, the storage-tank idea just wasn’t cost-effective. We would have had to launch a complete structural study and upgrade of the building. Because of earthquake regulations, the engineering costs would have gotten really high.”
Suttles agrees: “Tankless was the only practical option. The university would’ve had to restructure that roof to support the weight of any boiler-and-tank system.”
To finesse the weight challenge, Watson and Valiensi did briefly consider mounting the storage tank at ground level alongside the building. But the university was not happy with the prospect of creating a shed to enclose the tank for aesthetic reasons, or with running pipe up the sides of the apartment exteriors or with building pipe chases on the inside. “Tankless quickly became the frontrunner,” says Watson. “We decided to go as sustainable as we could and save as much energy as possible with the new water heating system.”
‘COOKIE-CUTTER' STRATEGYInstallation of the four racked systems spanned approximately six weeks, from June until August of this year. Shop fabrication of each assembly required approximately five days and two or three plumbers. Transferring and anchoring the assembly to the roof was done in a half-day, with another two days to finish all of the pipe connections within each apartment.
The only snag in the CSUN installation occurred after the fact when it was discovered that the pressure balancing valves in the apartment showers were past their operational lifetimes and not properly compensating for the differential pressures on the hot- and cold-water systems. The result was a crossover of excess cold water to the hot-water side of the valve, triggering hot and cold bursts to the new, low-flow shower heads.
To address this problem, Suttles opted to install a larger recirculation pump on the hot-water system to balance the system pressures and prevent the cold-to-hot bleed-through. This issue should be considered on all retrofit projects involving new low-flow fittings with older shower valves, so that appropriate actions can be taken. ES