If gas-fired water heater is more of a good option for you, your unit should come with a venting type that is most convenient for your location and situation. If the water heating unit is installed in a confined space, proper combustion and ventilation must be observed. All air for combustion and all exhausts from combustion should be directed through a ducting, from and to outside the building.
In installing the vent system for your gas water heaters, air surrounding your building should be free from volatile, combustible materials as well as halogenated hydrocarbons such as air in beauty shops, dry cleaning establishments, and photo processing labs. Halogenated hydrocarbons may be safe to breathe but they released corrosive elements when it passes through a gas flame. Failure of the heater due to operation in a corrosive atmosphere voids the water heater warranty. If you have proper ventilation with the outside for the gas and air, power vent or a direct vent would be a favorable option. Power vent is somehow similar to direct vent but it uses a powered fan or blower.
Direct vent water heater may be in a tankless version or a traditional tank type models. It is ideal for spaces with difficult angles or for those who wish to convert from electric to a gas heater. Direct venting pulls the fresh combustion air from outdoors and expels all the products of combustion outside through an adjacent wall. Installing this type of water heating machine is one of the easiest ways to save on your hot water heating costs, and to increase the efficiency of your home. And since direct venting system creates an air tight combustion chamber, it effectively prevents backdrafting, which occurs when the combustion gases can no longer go up and out flues and chimneys because outdoor air is already flowing down.