Selecting the Gas Water Heater Venting Options

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Find out how to select the best water heater venting option for the safe, reliable and economical home water heating. Explore the difference between atmospheric, power-vent and direct-vent systems.


Every natural and propane gas water heater needs properly designed venting system so it can safely remove the flue gases - byproducts of combustion, outside and draw the fresh air in for the gas combustion. The venting system must be correctly sized to handle the total BTU input of the water heater.

The flue gases contain mainly the water vapor and carbon dioxide, which is acidic by the nature, so for the safe removal from the heater they are moved outside through the metal flue, chimney or side vent.

Depending on the location of the new gas water heater, different venting system should be considered, for the most economical, safe and reliable water heating. Venting flexibility is very important when selecting the right model, so buy a water heater that allows you a variety of installation options.

Types of the venting systems

There are three, the most used venting systems in residential water heating: atmospheric, direct-vent and power-vent.

Atmospheric venting

The most common type of gas water heaters is the atmospherically vented type that uses a natural way to move the flue gases from the unit out. Atmospherically vented heater uses a vertical metal pipe, sometimes connected to the chimney, where the hot flue gases rise naturally through the draft hood and flue pipe, and out in the atmosphere. The advantage of using this type of venting is the low cost of the unit, installation and operation.

Direct venting

Direct-vent water heaters are designed with the sealed combustion chamber, and they use the horizontal flue pipe directly through the external wall and out. They do require a zero tolerance for the proper and safe transfer of the flue gases, and they can use a separate vent pipe to draw the fresh air in. Due to its air-tight design, they are much more flexible in venting installation than the above atmospheric type and are quieter than heaters with the electric fans.

Power venting

Power-vent water heaters use a mechanical fan to move the byproducts of combustion outdoor. They are expensive but bring lots of flexibility during the installation. The venting system can be installed vertically, horizontally while the unit can be far away from the point of termination.

Since the byproducts of combustion are acidic and with the high temperature, both tank-type and tankless water heaters must use metal or stainless steel venting pipes, while condensing units, due to the lower temperature of the exhaust gases may use pipes made of different and cheaper material such as PVC, CPVC or ABS.

Venting layout

The water heaters venting system might consist of one or two pipes. Two-pipe system might be further divided on the concentric (pipe inside the pipe) or two separate pipes, and the vents can run horizontally, vertically or combined.

One-pipe venting water heater, such as the above atmospheric type, utilize one pipe for transferring the flue gases out while the air for combustion is brought through the combustion chamber.

There are also water heaters, such as tankless outdoor models, that do not require venting, as they are installed outside where they safely transfer the gases.

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Installation tips

Keep in mind that water heaters must be installed according to the manufacturer's instructions and local or national codes and regulations.

Gas water heaters are usually installed in the technical room, garage, closet, confined or unconfined space, and depends on the type provisions for combustion and ventilation air must be provided.

Water heater venting systems, which includes the vent pipe, flue baffle and draft hood, must be inspected and cleaned at least once a year.

Venting system is very important for the safe operation of the gas water heater as the incorrectly designed system can lead to incomplete gas combustion, problems and injuries, including death.

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