To handle commercial-duty demands, Demand Duo™ combines heat exchanger technologies from 199 BTU, Energy Star®-certified C199 condensing commercial tankless water heaters with durable 119 gal tank. Hybrid water heating system, built for longevity and to conserve energy, features direct vent, common venting options, and single-unit/common vent room-air capability. Latter can use existing vertical vent penetration to eliminate need to core additional holes in walls/roofs.