How Does a Furnace Vent Gas? Three Types of Venting and How They Work.

How Does a Furnace Vent Gas

πŸ”₯ How Does a Furnace Vent Gas?

Propane and natural gas furnaces, whether they are mid-efficiency or high-efficiency furnaces, will vent exhaust gases outside through a vent pipe (also called an exhaust flue) system.

There are three types of venting that we will discuss in this article; natural draft system, direct vent system, and power vent system.

The main difference between the three vent types is the temperature of the flue gases.

If you’re asking yourself why the exhaust gases need to be vented, it’s because the process of burning fossil fuels can create toxic gases like carbon monoxide.

πŸ”₯ How Older, Less Efficient Furnaces Vent Gas

Natural Draft Furnace Vent

CC Image by Bonnie Bogle via Flickr

If you have a furnace that has a metal exhaust pipe, also known as a B-vent pipe, which comes out the top of it, then your furnace is considered a standard efficiency furnace and is approximately 70% efficient.

These furnaces have a less efficient heat exchanger and, therefore have extremely hot combustion gases that naturally draft out the flue pipe to the outside.

The exhaust pipe is usually routed through the chimney and out through the roof.

On the negative side, natural draft vents are more conducive to backdrafts, which reduces efficiency and can be a safety concern.

What is B-Vent Pipe?

furnace b vent pipe

B-vent uses a double wall of metal pipe.

The inner pipe uses reflecting aluminum which heats quickly, with the purpose of helping the warm flue gases vent to the outside.

There is airspace between the inner and outer metal pipes which provides insulation against heat loss to keep the warmer flue gases naturally drafting.

The outer pipe is made of corrosion-resistant galvanized steel.

This ensures strength and durability.

Direct Vent

Old Furnace

If your metal exhaust vent is routed out of the house, it’s likely a direct vent and uses an exhaust fan.

This type of furnace has a slightly more efficient heat exchanger that is approximately 80% efficient; therefore, the combustion gases have a lower temperature and need help getting vented to the outside.

The flue pipe on these furnaces is only sheet metal and can be extremely hot to the touch.

Local codes are in place to ensure that the venting pipes don’t come in direct contact with combustible materials.

What is a Direct Vent Furnace?

A direct-vent furnace has a self-contained, sealed heat exchanger that warms inside air by the home’s air across the sealed heat exchanger.

The combustion byproducts are then vented outside using an induced exhaust motor, which means that they do not need to be attached to the chimney and can be installed on most exterior walls.

πŸ”₯ How High-Efficiency Furnaces Vent Flue Gases

Gas furnace combustion chamber in action

A high-efficiency furnace uses a primary and secondary heat exchanger to remove as much heat energy from the combustion process as possible.

Due to the lower temperature of the combustion gases, a metal exhaust pipe is no longer needed.

Therefore your high-efficiency furnace uses either a single-pipe or two-pipe vent system.

These propane or natural gas furnaces are considered to be at least 90% efficient.

One of the challenges with high-efficiency condensing furnaces is that the combustion gases end up condensing on the inside walls of the PVC pipe, drain back to the furnace, and then out the condensate drain line.

The combustion by-products have a high PH level and are considered acidic, so please make sure to drain them accordingly.

Single-Pipe vs. Two-Pipe

two pipe vented gas furnace

All propane and natural gas furnaces require sufficient carbon dioxide for the furnace to reach complete combustion.

If there isn’t enough, then combustion byproducts could include carbon monoxide, which could be poisonous to your family.

A single-pipe heating system uses combustion air from the space around the furnace.

The problem with this is that the furnace could use up all of the air in the room.

So, it is for this reason that local codes are in place to ensure sufficient air is available for single-pipe heating systems.

βœ‹ NOTE: All propane and natural gas furnaces have the capability to be installed with two-pipe furnace venting.

Many contractors will only install a single venting pipe system when a two-pipe system ensures that your gas furnace always has enough combustion air.

Ask your HVAC contractor what they do before accepting their bid.

For the safest HVAC system possible, request that your furnace’s exhaust have two pipes.

βœ… PRO TIP: Most regions require special high-temperature PVC pipes for use on high-efficiency furnace installations.

πŸ”₯ Blocked Or Damaged Exhaust Pipe

Furnace horizontal vent termination kit
Furnace horizontal vent termination kit fitted outside of a residential home

If the flue pipes are blocked or damaged in any way, the combustion process may create carbon monoxide. Other issues can include flue venting cracking or joints failing.

What can cause a vent pipe to get blocked?

Bugs, small animals, bushes, snow, or ice can easily block the furnace venting, which could cause any of the above issues to occur. Most furnaces these days have safety switches in place to turn the furnace off before any damage could arise.

πŸ›‘ What’s The Danger?

A blocked vent pipe might only make a furnace short cycle and nothing more. However, a blocked or cracked flue pipe also has the potential to circulate carbon monoxide into your home.


πŸ”₯ Final Thoughts on Furnace Venting Systems

Obviously, making a decision on what type of furnace to buy has more variables than just the venting system, but it’s definitely one to consider. Generally, I would say that a direct vent is the least expensive option.

It’s easier to install than a natural draft but isn’t as efficient as a system with one or two PVC flue pipes.

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If you found this article on furnace vent types helpful but you’re still looking for more information, I would recommend contacting a highly rated and experienced local HVAC company to ask for help.

Ensure they thoroughly explain their recommendations for furnace sizing and vent types.

Want to learn more about your home’s heating system? Feel free to check out our other furnace articles!

Jake Gibson

Jake Gibson

Jake Gibson is a force to be reckoned with in the HVAC industry. As the CEO of Phyxter, he has made it his mission to revolutionize how contractors work. With a background in HVAC, entrepreneurship, and technology, Jake brings innovation and expertise to this traditional industry.

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