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What is the difference between a regular water heater and a direct vent water heater?
Friday, August 17, 2018
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:
- 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.
- 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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