Venting Gas Combustion Water Heaters


Water heaters often burn fuels (natural gas or propane) as a means to heat water. Burning fuels produce gaseous by-products that need to be exhausted out and away from the home. In this article, we will cover the basics of proper venting. We will also cover the three most common types of venting systems: standard atmospheric venting, power venting, and direct venting.

Standard Atmospheric Venting

Naturally, hot gasses rise. If we wish to force gas into any other direction, we will have to use energy. Atmospheric venting takes advantage of the air's tendency to rise and simply carries the gasses upwards and outside through the vent tubing. Atmospheric venting does not force gasses into a direction they do not want to go. Therefore, standard atmospheric venting does not require a fan or any other auxiliary equipment to support the venting, and it is the most common type of venting used.

The proper construction of a standard atmospheric vent is important for safety and for the proper operation of the unit. Here are a few critical things that we need to consider:

  • Materials: Exhaust gasses are hot and corrosive and can degrade the vent if not built with proper steel or stainless-steel exhaust piping
  • Appropriate hangers and fasteners.
  • Pipe size is also an important factor, and should comply with the manufacturer’s recommendations, as well as state and local codes, to ensure it can adequately handle the capacity of your appliance.
  • A vent cap will also be necessary where the piping exits the home to prevent water intrusion and backdrafts.

As we’ve discussed, vent ducts need to provide a path for exhaust gasses to rise upwards and away from the home. In order to provide an effective route, it’s important that the vent duct rise upward. If it’s feasible, the best ways to ensure a strong vent is to run the ductwork vertically through the house and through the roof. However, there is a possibility of the need for horizontal runs in your vent. Ductwork that is run horizontally must be installed at an upward pitch in order to remain effective. State and local codes will provide acceptable pitches for venting, but a good rule-of-thumb is a rise of at least 1/4” per foot, and the horizontal distance should not be more than 75 percent of the vertical distance. Turns and bends in the ductwork may also be required. Every turn in the ductwork will restrict the flow and should be kept to a minimal amount. Depending on duct size and capacity requirements, local codes should be referenced to determine the absolute maximum number of bends in the duct work.

Direct Venting and Power Venting

Direct venting is used in air-tight rooms where there may be insufficient air available to water heaters for combustion. Direct vents not only push exhaust gasses outside of the room, but also bring in fresh air required for combustion into the room. To accomplish this, Direct venting uses special coaxial piping with separate chambers for combustion air and exhaust. This type of venting can be run completely horizontally and directly outside through a wall, which provides flexibility of installation.

Another common type of venting system is a power vent. Power vents uses an electric blower to push exhaust gases through the venting. A power vent system does have some drawbacks, such as: the added noise of the exhaust blower running, the increased cost of a hot water heater equipped with a power vent, and the added consumption of energy to support running the fan. However, there are some conveniences afforded by this type of water heater. Perhaps the biggest advantage of this system is the flexibility of location. Since the exhaust is being moved by a fan, as opposed to natural convection, it isn’t necessary for the vent to be vertical. This allows the unit to essentially be installed anywhere, so long as power is available for the fan. The venting can be much longer and can run completely horizontally if needed, which is often much easier to install than a vertical vent.  Another advantage to these types of systems is the materials required for the venting. Since the blower cools the exhaust as it moves, PVC can be used for the construction, which is much more cheaper than traditional metal venting. If installing a direct or power vent horizontally, you should ensure the venting turns down towards the ground outside to prevent water intrusion.