Building owners challenged with space constraints and variable loading due to changing occupancy.
After 12 years, the existing domestic hot water (DHW) system was not meeting the demands of the tenants at The Caroline, a rental building located in New York’s Flatiron District on 23rd St. between 5th and 6th Avenues. The 20-story building features 433 apartments with premium kitchen and bath amenities, a 50-ft. sky-lit swimming pool, on-site laundry, fitness center, and central climate control. Built in 2000, the building used a central hot-water delivery system using three commercial direct-fired water heaters.
The building owners were looking to upgrade their DHW system but were challenged with space constraints for new equipment and variable loading due to changing occupancy. Adding additional storage capacity was not an option as the mechanical room was already filled to the maximum. They opted to install 17 Bosch (Londonderry, NH) C 1210 ES condensing gas tankless water heaters cascaded in parallel for a tank-loading application. Two zones were required for DHW. The lower zone has 11 C 1210 units and five storage tanks, and the upper zone has six C 1210 units and three storage tanks (each storage tank has 240 gal. capacity).
Tankless water heaters are commonly used in residential on-demand applications. However tankless water heaters can be applied in a commercial tank-loading application providing comfort even with variable demand. An advantage of using the Bosch tankless units was that they fit through a standard 30-in. doorway, and they function with the lower gas pressure available in the building.
As many as 24 Bosch C 1210 gas tankless water heaters can be linked together to achieve higher flow rates than with single units—to 290 gpm. The 17-unit cascade used in The Caroline can deliver 205 gpm.
Commercial tank-loading design allows complete isolation of the system for maintenance and gives the ability to bypass certain system components in case of failure. Some design considerations for tank loading include:
• Ensure flow through each water heater is between 3.5 to 5.0 gpm.
• Isolation valves or boiler drains should be installed to facilitate descaling in applications with hard water.
• Whenever possible, plumb the system or configure the tank to draw cold supply water into the water heater during hot water use.
Unlike traditional non-condensing water heaters, these condensing units capture latent heat from condensation of the exhaust gas and use it to preheat the incoming cold water, making the water heaters 10% more efficient than traditional tankless water heaters, according to the manufacturer.
The building owners are pleased with the performance, energy savings, and flexibility of the water heaters, as well as the fact that not a single day of hot water supply was lost during installation.