A power vent heater uses a blower fan for exhaust gases whereas a direct vent has a natural draft exhaust. A direct vent heater draws combustion air from the outside while a power vent pulls air from inside the house.
All oil-fired or gas water heaters need a source of combustion air and a way to vent the exhaust gases. They can draw combustion air from either inside or outside the house.
A direct vent water heater draws combustion air from the outside through a horizontal vent pipe and returns the exhaust gases back to the outside via another pipe. It has a sealed combustion chamber and relies on natural air flow for circulation.
A power vent heater draws combustion air from inside the house in the space around the heater. A blower fan forces the exhaust gases out through a flue, which can be vertical or horizontal.
Both power vent and direct vent heaters prevent backdrafting of poisonous flue gases into the house. The direct vent heater has a sealed intake and exhaust system and the power vent uses a blower fan to force exhaust gases out of the house.
Because a direct vent heater uses natural air flow, it can be located a maximum of only four feet from an adjacent outside wall. A power vent heater can have a flue length of up to 40 feet since it has an electric powered exhaust fan.