Combustion Air

• The gas burners of a hot water heater need a source of combustion air. The air can come from either inside the house or be drawn from the outside. If the burner takes air from inside the house, it competes for this air with other appliances, such as air conditioning, which increases energy costs. A direct vent heater draws intake combustion air via a pipe that passes through an outside sidewall.
  
  

Exhaust

• The exhaust gases from combustion must be discharged to the outside. Depending on the location of the heater, the discharge could be vertically through the roof or horizontally out through an adjacent sidewall. In some cases, construction obstacles prevent the installation of a vertical flue pipe. A direct vent heater avoids this problem by expelling the exhaust gases horizontally through an outside wall.
  
  

Direct Vent

• The intake and exhaust piping of a direct vent heater is sealed to create a closed loop and an air-tight combustion chamber. This system does not have a power fan and relies on natural air flow for circulation. The piping system can be either two separate pipes, which would require two holes through the outside wall, or co-axial piping, where one pipe is inside the other, and only one hole needs to be cut through the wall.
  
  

Efficiency

• A house that has air-tight construction makes it difficult for the heater to draw sufficient combustion air for efficient operation. Exhaust fans in bathrooms and the kitchen pull air out of the house and create a slight vacuum. The water heater has to compete against these exhaust fans for its air. A direct vent heater overcomes this problem by drawing outside air, which doesn't have those restrictions.
  
  

Backdrafting

• When a water heater draws combustion air from inside the house, there is a danger from backdrafting where harmful gases, such as carbon monoxide, can escape into the area around the heater. The depressurization effect from exhaust fans and clothes dryers can pull combustion gases back into the house. A direct vent heater prevents this possibility because the combustion chamber in the heater is air-tight and the piping system is sealed.
  
  

Limitations

• Because the direct vent system depends on natural air currents to get adequate flow, a gas water heater can only be located a maximum of four feet from the sidewall. If this is not possible, it may be necessary to install a power fan in the exhaust system.