Yes. Gas does burn clean, but you may still run into issues with the cooler gases that condense quicker. This allows for the formation of hydrochloric acid and the potential for a corrosive coating to build up in your flue. The biggest problems come when a customer buys a new appliance and hooks it into an older venting system. The requirements may be much different for the newer stove or furnace, creating a serious mismatch with the older flue system.