Just bought a new hot water tank. The installation tech said the house is too tight and we are gtting a backdraft of exhaust from chimney. Is there an after market system that could be used to eliminate this backdraft?

Did you ask the installer this question? Try this http://www.nextag.com/water-heater-exhaust/products-html If you home is really that tight a new fan will cause more back draft down that chimney. You may need to look into an outside air intake for the heater.