The most common gas water heater is technically called an “atmospheric vent” type. Because warm air rises, the hot gases created by combustion are pulled by natural convection up a vertical metal flue through the roof to the outside. The air around the base of the water heater is pulled into the burner chamber for combustion by the same convection flow. It’s a tried-and-true, simple system, that does not require electric power or moving parts.

   Direct vent water heaters, like the one shown above, do not utilize the air in the room around the water heater for combustion. Instead, a large flexible pipe runs from the water heater through an adjacent wall to the exterior. Centered inside the large pipe is a smaller pipe that carries out the hot air created by the burning gas. The donut shape area surrounding the small inner pipe is used for bringing outside air into the burner chamber for combustion. An alternate design uses two flexible pipes, with the supply air pipe being completely separate.

   The direct vent design solves two problems that are inherent in an atmospheric vent water heater:

1. Water heater manufacturers use a gap between the top of the water heater flue and the vent connector pipe above it in a regular water heater to pull additional air into the upward flow of combustion air to help it exit more efficiently. There is a tapered cone-shape above the gap, called a “draft hood.” If the draft hood gets knocked out of alignment with the opening at the center of the top of the water heater or there is negative air pressure created by an adjacent appliance such as a furnace or clothes dryer, combustion gases may flow out of the draft hood instead of fresh air being pulled into it.
       This is called “backdrafting” and, under certain circumstances, can allow carbon monoxide to be emitted into the area around the water heater. If the water heater is located, in or adjacent to, a living area it can result in death of the home’s occupants.
   
   
   
2. For every cubic foot of air that is drawn into the base of an atmospheric vent water heater for combustion, and then flows out the vent above the roof, another cubic foot of outside air is pulled into the room by the negative air pressure. This means a lot of air is being sucked out of—and into—the home, which is called “air infiltration” in building science. It’s not a good thing. 

    A direct vent water heater draws all its air for combustion from the outside and returns the combustion gases to the exterior, in essentially a sealed system. There is no chance of backdrafting and no negative air pressure created inside the home. Also, because they vent thru adjacent exterior wall, they are typically easier to install.

   The only downside is that direct vent water heaters are significantly more expensive, running up to twice as much as a regular water heater. There are also power direct vent models, which use a fan to exhaust combustion gases. This overcomes the limitation of a regular direct vent water heater, which must be located adjacent to an exterior wall. You can read about them at our blog post “What is the difference between a regular water heater and a power vent water heater?"

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code? 

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• Why is the water heater older than the house? 

• Does a water heater need a shut-off valve? 

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Does a tankless water heater require a pressure relief valve?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• Does an electric water heater require a disconnect? 

• Is a catch pan and drain piping required for a replacement water heater?

• What is the difference between a single element and dual element electric water heater? 

• What is an FVIR water heater?

• What is a heat pump water heater?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is backdrafting at a gas water heater? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.