In most cases - no. A regular gas water heater has a galvanized steel vent - called an Atmospheric Vent. A tankless water heater needs a sealed vent to the outdoors. We can solve this by using a flexible vent to go through your existing flue out the roof. Using the existing flue as a conduit. OR if you have a shared flue (Water heater and furnace tied together), we offer a flexible stainless steel vent that can be used with keeping the other appliance working perfectly.