Yes, you can combine a traditional tank with a tankless water heater. The Demand Duo is a hybrid solution combining the best features of both the tankless water heater and a storage tank. With the Demand Duo 2 or 2V you can get up to 398,00 Btu delivering up to 551 gallons in the first hour. Depending upon your specific application you may require more tankless units connecting to a storage tank so we invite you to consult with Rinnai’s Applications Engineering Team at 800-621-9419 or engineering@rinnai.us to determine the best solution for your project. We are here to help.