3 Different Types of Gas Furnace Vent Pipes Explained

A gas furnace and a filter replacement.

For many homes, a gas furnace is the preferred heating device. It has proven to be an efficient and reliable way of meeting household heating needs over the years. The vent pipe, or flue, is a vital component of the furnace that transmits emissions from the combustion process out of the building. This ensures that those in the building are not harmed by toxic gases and other byproducts of combustion. The flue is also a safeguard against fire. If the waste gases come into contact with other combustibles, a fire can easily break out. Below are three types of venting for gas furnaces.

1. Natural Vent

It operates on natural air convection and is also known as the Type B vent. Type B vents are a standard design that most modern gas furnaces use. It draws air from within the house for combustion. Gases produced during combustion are very hot and as such, easily rise through the flue. The air is then vented outside through a B-vent pipe which is set vertically to expel combustion air through the roof. Natural vent pipes can be installed fairly easily, and costs are fairly modest, making them a popular choice.

However, back-drafting is more common with natural vent pipes than other varieties. The indoor air can easily pull combustion gases back indoors. This detracts from the efficiency of the B-vent piping. The tendency to back-draft also raises questions over furnace safety where such pipes are used. Back-drafting can affect the health of household occupants through inhalation of toxic emissions if it remains a consistent problem.

2. Direct Vent

This type is an ideal alternative to natural vent systems. This piping system incorporates two pipes of different sizes in one unit. The outer pipe draws air for combustion from outside, and the smaller pipe occupies the inner position and is used to vent the exhaust fumes. Piping can be set horizontally to allow venting through a sidewall, or vertical installation can also be undertaken to allow venting through the roof. Direct vent systems have versatility to their installation that natural vents do not. Also, no chimney is needed regardless of whether horizontal or vertical installation is used. Installation of direct venting is fairly easy, but more costly than the B-vent type.

Less environmental pollution occurs with direct venting since they extract most of the combustion heat and expel it outside. They also do not spoil the quality of indoor air unlike the B-vent type, so safety ratings are higher for direct vent furnaces than natural vents.

3. Sidewall Power Vent

This is also referred to as an induced draft fan. A fan is placed at the end of an exhaust pipe that runs through a sidewall which draws out combustion gases and expels them outside. The fan works in conjunction with the furnace, usually turning on before the furnace starts. Sidewall vent systems are especially great for replacing old chimneys, and it is one of the more affordable options to consider when installing a venting system for your gas furnace. Sidewall power vents also rate highly in efficiency and safety.