top of page

Double Wall B-Vent System | Water Heater Pros | Sacramento, Ca

Double Wall B-Vent System

gas water heater vent pipe.jpeg

All water heaters that burn natural gas or liquid propane (LP) gas require a venting system. The process of burning of the gas is called combustion and it creates heat, exhaust gases (including highly poisonous carbon monoxide), and moisture. The water heater's ventilation system removes these byproducts from the home, making it a critical safety feature. In most cases, the type of ventilation system depends on the type of water heater.Type "B" double-wall vent pipe allows you to install it within 1 inch of combustible materials, such as wood framing and wall materials. The air space between the inner and outer walls of the vent's double wall construction insulates the pipe, preventing the hot inner wall from coming in contact with combustibles. Replacing your gas vent pipe may prevent carbon monoxide poisoning or even a fire.

Water Heater Venting Basics

All water heater venting systems use a vent duct or pipe—also called a chimney or flue—to bring exhaust gases from the water heater to the outdoors. The duct may be metal or plastic, depending on the type of vent system. Water heater ducts may lead directly outdoors, or they may tie into a larger vent duct that also serves a gas or propane furnace or boiler in the home. This is called a common vent configuration. In many cases, the exhaust from the larger appliance heats up the common vent, improving the flow of the water heater vent.

While properly installed common venting systems are completely effective, there is the potential for backdrafting problems if they are not installed correctly. For this reason, common venting configurations are no longer allowed in some code jurisdictions, where direct venting or power venting of water heaters is now mandatory. 

In addition to venting, gas and propane water heaters need an air supply for combustion. This may come from the atmospheric air in the house, or it can come through a vent pipe that pulls air from the outdoors.


Proper Venting Prevents Backdrafting

The most common problem associated with water heater venting is a condition called backdrafting, in which exhaust gases from the water heater fail to exit the home via the vent and instead end up in the house. Backdrafting can have many causes, but it is most commonly due to poor vent design or installation and/or an imbalance of air volume in the home. The latter is often a result of ventilation fans, such as bathroom or kitchen vent fans, that pull air out of the house and create a vacuum effect that draws exhaust gas downward and into the home from the water heater vent.

Some water heater venting systems remove the possibility of backdrafting with fan-assisted ventilation or direct venting technology.









Atmospheric Venting

Standard water heaters—the most common type—often use a method known as atmospheric venting. The vent consists of a vertical or upward-sloping vent duct that typically ties into a common vent. The system works solely through natural convection—based on the principle that hot air rises. Hot exhaust from the water heater naturally rises up through the vent and into the air outdoors, creating a draw that promotes this upward airflow. The power of the draw increases as the vent duct heats up.

Atmospheric vent systems work well (and without electricity) if they are properly designed and the home does not have backdraft issues. Poorly designed vents commonly have insufficient draw and/or are highly susceptible to backdrafting.


Power Venting

Water heaters with power venting come with an electric blower fan (often very quiet) installed on top of the water heater and can have either vertical or horizontal vent ducts. Since the vent does not rely on the buoyancy of hot air, it can be run horizontally to the outside of the house. The blower cools the air so the vent can be run in PVC pipe (rather than metal, as is required with atmospheric venting) and is easy to assemble. The water heater must have a nearby electrical outlet for powering the fan.


Direct-Vent Water Heaters

With a direct-vent system, air for combustion is drawn from a vent pipe that runs through an exterior wall or the roof. Exhaust gases are vented outdoors through a separate vent duct or through a separate chamber of the same pipe (this requires a double-wall vent duct). Direct-vent systems essentially "breathe" outdoor air, so they are not subject to the effects of backdrafting in the home. They also reduce the risk of accidental fires caused by flammable vapors around the water heater.










Water Heaters for Mobile Homes

Water heaters in mobile homes are similar to those used in standard homes, but they must be designed for this application. Manufacturers often will not warrant a water heater is installed in a mobile home unless it is specifically approved for that use. Standard atmospheric water heaters in mobile homes often require an outside access panel. If a water heater is located inside of a mobile home without outside access, it is most likely a sealed-combustion unit with direct venting.

double wall b vent_edited.png
Double Wall B-Vent _edited.png

Double-Wall Construction


Double-wall metal construction provides protection for health and safety, as well as benefits of economy, ease of installation, efficiency, and protection from property damage. Designers of vent systems should take particular note of the fact that vents constructed of brick, cement, asbestos, and mass-insulated metal require lengthy warm-up periods, whereas double-wall metal vents reach operating temperatures almost instantly for rapid removal of the flue gases.

The all-metal, double-wall gas vent is listed as Type B and is approved for residential and commercial Category 1 water heaters.  UL-approved Type B gas vent products form a continuous passageway from an approved gas appliance to the termination of the vent above the roof of the structure.


Energy Down The Drain

Type B gas vent uses a double wall of metal pipe for insulation and safety. The inner pipe of reflecting aluminum heats quickly, containing the warm flue gases inside. The airspace between the inner and the outer pipes provides insulation against heat loss to keep the warmer flue gases on the rise. The outer pipe of corrosion-resistant galvanized steel ensures strength, safety, and durability.

If the flue gases are kept rising at the proper rate, the structure is kept free from pollution, and condensation damage is eliminated. Although a large portion of the flue gas is composed of water vapor,  type B gas vents carry it out of the structure before it can condense.
One of the fire safety features of double-wall Type B gas vent is that its outer wall, insulated by the airspace beneath it, requires only 1” clearance to combustible material in the structure.



bottom of page