Standard atmospheric gas water heater: These are the most common water heaters found in many homes and mobile homes with outside access. Atmospheric venting works with the difference of temperature and pressure from inside and outside the home to move the exhaust gas up and out through the vertical flue pipe. A standard vent normally works great for this type of water heater, but can be susceptible to back drafting. Be aware of the signs of a gas leak and if you suspect any type of improper venting contact the gas company or call a plumber right away because this can cause damage or illness.
Mobile home atmospheric water heater: If you have a mobile home take care to purchase a mobile home water heater when you replace or install a new water heater. Manufactures will not warranty a water heater if it is installed in a mobile home and is not specifically approved for that use. Also, be sure that there is an outside access panel to get to the water heater if you are using a standard atmospheric gas water heater.
Direct vent water heater: Air for combustion is drawn from the vent which is either run from outside the wall or through the roof. Off gases are vented outside as well. Taking fresh air from outside can reduce the risk of accidental fires caused by flammable vapors around the water heater. Some units can only vent horizontally or up to so many inches before a power vent is required. Direct vent models will eliminate any problems caused by insufficient indoor ventilation. Some direct vent systems have a dual channel one pipe system meaning only one pipe needs to go through the wall or roof where fresh air is taken and off gases are vented. Other direct vent types take air from a pipe that is ran under the house and to the exterior and they vent the off gases through the roof.
Mobile Home sealed combustion or direct vent: If the water heater is located inside of a mobile home without outside access it is most likely a sealed combustion unit. When replacing this type of water heater you will want to replace it with a similar unit or there will be insufficient indoor ventilation and this will cause the water heater to fail. Mobile home sealed combustion water heaters vent through the roof like standard atmospheric gas water heaters but get fresh air from a bottom intake air vent that is run under the mobile home to the exterior.
Power vent water heater: Power vent models come with a blower (often very quiet ) installed on top of the water heater and can be used in vertical and horizontal application. Since the vent does not rely on the buoyancy of hot air it can be run horizontally to the outside of the house. The blower cools the air so the vent can be run in PVC pipe and is easy to assemble. This blower will require an outlet within 6 feet so if you are thinking about one of these models take that into consideration.