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Gas furnace venting
Venting means the method we use to remove the products of combustion when we burn natural gas.
When natural gas burns, the products of combustion are carbon dioxide, water and heat.
When we extract all the heat we can, the rest of the gasses must be removed from the structure.
Because the gasses are hot and hot gas rises, we use an upward sloping pipe to conduct the gasses out the roof.
Masonry chimneys were the first method of removing these gasses. The masonry would not burn, be damaged by the heat or transfer the heat to combustible materials such as wood or paper.
To conduct the gasses to the chimney, we use a single wall galvanized metal pipe that the American Gas Association has designated as "C" vent.
When natural gas burns, the products of combustion are carbon dioxide, water and heat.
When we extract all the heat we can, the rest of the gasses must be removed from the structure.
Because the gasses are hot and hot gas rises, we use an upward sloping pipe to conduct the gasses out the roof.
Masonry chimneys were the first method of removing these gasses. The masonry would not burn, be damaged by the heat or transfer the heat to combustible materials such as wood or paper.
To conduct the gasses to the chimney, we use a single wall galvanized metal pipe that the American Gas Association has designated as "C" vent.
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On the left is a "C" vent fitting. It is generally 26 gauge wall thickness for the smaller sizes, galvanized, and is single wall.
This pipe is used on older gas furnaces such as the natural draft one above. The good things about this pipe is that it is cheap and easy to install. The bad things are it can burn you because of its high temperature and it must be at least 6" from combustibles. This pipe is generally used to connect the furnace to the vent or chimney, but not as the vent itself as it cannot be used in an unconditioned space. When installed, this pipe in the smaller sizes must be secured with 3 screws placed approximately equidistant around the circumference of the pipe. |
On the left is a length of "B" vent. This pipe uses a double wall construction. The inside wall is aluminum and the outside wall is galvanized steel.
This pipe, because of its double wall construction, can be installed with a 1" clearance to combustibles. It has a twist lock to connect pieces together and must not be installed with screws. This pipe is often times used as a vent to the outside. |
In the middle 1980s a few gas furnaces started using inducers to exhaust the vent gasses.
These furnaces were higher efficiency than the older natural draft furnaces. The vent temperature of these furnaces is much lower than natural draft. That meant the vent temperature sometimes was low enough to condense water from the gasses. When single wall "C" vent was used to connect the furnace to the vent, the resulting rust corroded out the vent connecter. So these higher efficiency furnaces must use "B" vent for the entire venting system. More coming |
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More coming