How Does A Gas Furnace Vent? (Find Out Now!)


How Does a Furnace Vent Gas? Your Gas Furnace’s Ventilation Process

Gas appliances are very common. Additionally, there are plenty of gas furnaces out there in homes across America and the world. And if you know anything about the process of fuel-burning, you know that there are combustion gases such as carbon monoxide that get released throughout the process.

So, what happens to that gas? How does the furnace vent it? Some furnaces have flues that point up and through your roof. For high-efficiency furnaces, there is a second heat exchanger involved. Then, a fan blows the byproducts of the combustion out through a flue pipe, though this one goes out the side of your home instead of the roof.

How Does My Furnace Produce Combustion Gases?

When the furnace in your home turns on, there are a set of burners that light. The flames that are created get directed down into a metal chamber. This is your furnace’s heat exchanger. Inside, hot combustion gases (like the aforementioned carbon monoxide) are produced, heating up the chamber walls along the way.

The furnace will blow air over the outside of the heat exchanger walls to protect it from those gases. Combustion then builds up on the interior of the heat exchanger chamber.

In order to vent those gases building up in the heat exchanger, there is a venting pipe attached to the exchanger known as a flue. Depending on the efficiency of the furnace, those gases can get directed through one of a few places.

Venting Gases: Mid-efficiency Furnaces

Mid-efficiency furnaces are generally rated below 90 AFUE. In these systems, those combustion gases get vented out in a gaseous vapor. Hot vapor will naturally rise, meaning it needs to get vented out above the furnace.

So, the flue that is attached to your furnace’s heat exchanger points up, generally extending out through your roof. While the furnace runs, those combustion gases rise up. They go toward the flue pipe, traversing their way through and all the way outside. Depending on the style of home, the flue may even be attached to the upper portion of the chimney.

Venting Gases: High-Efficiency Furnaces

For furnaces that are high efficiency (above the 90 AFUE mark), those combustion gases are in a liquid form instead of a gaseous state. The reason for this is due to a second heat exchanger. That additional heat exchanger is the main difference between the two systems.

The second exchanger extracts more heat from those combustion gases, cooling them down so that they become a liquid. In high-efficiency systems, a fan will blow those combustion byproducts through a flue pipe that points out of the side of the house instead of the roof.

How to Ensure That Your Furnace Vents Gases Properly

Making sure that your furnace vents those combustion gases – particularly carbon monoxide – is essential. Carbon monoxide can be quite dangerous if it leaks into your home, leading to carbon monoxide poisoning or even explosion.

There are a couple of ways that you can ensure that the carbon monoxide is vented properly, keeping your home and family safe. You can perform them on your own if you have the experience and confidence, otherwise it is best to contact your local HVAC technician.

  1. Troubleshoot the issue. If you think that venting may be an issue, troubleshoot the issue yourself. Locate the flue and heat exchanger, inspecting them thoroughly. Look for things like cracks or clogs that could be hindering the venting performance of your system.
  2. Tune-up. Even if there is seemingly nothing wrong with your HVAC system, a yearly tune-up is a great idea. During most tune-ups, the technician will inspect for clogs or cracks in the flue pipes and any other common issues that can impact the venting process.

The Exhaust Flue is Essential to Your Furnace

One of the most important components of your furnace is the exhaust flue. It plays a pivotal role in venting those potentially dangerous gases out of your home. Here are a few things that you should know about your furnace’s exhaust flue.

1. They vent exhaust gases. As covered above, we know the role that they play. What you might not have known is that combusting natural gas generates carbon dioxide and water vapor. Because no furnace is completely efficient, some of the gas won’t combust.

Partial combustion results in a wide array of byproducts. The most notable is carbon monoxide, which is extremely dangerous and highly toxic. The flue is the direct conduit for that gas to move outdoors, safely away from your home.

2. High-efficiency furnaces use draft inducers. The draft inducer allows for quicker passage of those byproducts out through the exhaust flue. The inducer is a high-powered fan that is located close to the heat exchanger.

The inducer helps to get rid of any of those gases that might be lingering inside of the combustion chamber, therefore making room for more fresh air.

3. Natural draft. What you might not know is that most furnaces use what is known as natural draft. It means that the gases rise up through the exhaust flue on their own. The pressure differences are the biggest key to a natural draft.

The pressure difference is what pushes the gas out of the combustion changer and towards the exhaust flue. When the gases move through the heat exchanger, they continue into the flue, and eventually out of your home.

Signs of a Carbon Monoxide Leak

Carbon monoxide is a serious issue. If your furnace can’t disperse those dangerous gases, it can lead to carbon monoxide poisoning and even explosion. So, it helps to notice the signs that there is a carbon monoxide leak or buildup somewhere in your home.

  1. Dripping condensation. If you notice that there is a lot of condensation on windows near the furnace, it is a good indicator that there is a carbon monoxide leak somewhere. Moreover, it could be a sign that your humidifier is set too high.
  2. Brownish color. When inspecting your furnace, you may notice a brownish-yellow color that seems sooty in appearance.
  3. Stale air. If the air smells off, stuffy, or even stale, that can be a sign as well. It will be reminiscent of overheating or burning.
  4. Smoke or soot. Does your home have a chimney or fireplace? You may notice smoke, soot, fumes, or even back-draft in your house coming from those areas.
  5. No upward draft. Check the exhaust flue if you suspect issues. If there is no upward draft in the exhaust flue, that is a good indicator that either the flue isn’t working or there is a leak somewhere in the system.
  6. Pilot light blows out. Gas furnaces require a pilot light to stay lit. When the pilot light blows out all the time, it could be due to a carbon monoxide leak in your system.
  7. Unusual gas smell. Carbon monoxide itself is actually odorless. Still, it can be accompanied by the smell of exhaust. It is that smell which will tip you off to the presence of carbon monoxide in your home.

Can You Vent High-Efficiency Furnaces Through the Roof?

For the most part, high-efficiency furnaces tend to vent through an exterior wall in your home. But, depending on the way it was installed, your high-efficiency furnace can come with two PVC pipes that exit your home.

In some newly built homes, one vent goes through an exterior wall while the other goes through the roof. Should there be two pipes in your furnace, one will likely be for combustion air intake while the other disperses combustion gas byproducts away from the home.

Related Questions

Does a Gas Furnace Need a Chimney?

When we think of chimneys, we picture smoke rising out through them. So, it is only natural to think that a gas furnace may need the chimney to get rid of those toxic gases that it produces. Well, that isn’t necessarily the case.

In some older furnaces, the chimney can be used to disperse gases. However, medium- and high-efficiency furnaces generally have PVC piping meant to do that job. Depending on which one that you go with, the pipe will run out through the roof of your home or out of an exterior wall.

Should You Put a Carbon Monoxide Detector Near Your Furnace?

Ideally, there should be carbon monoxide detectors located in various spots throughout your home. Likewise, you should have smoke alarms in place as a fire prevention tactic as well. Having protective measures in place is never a bad thing.

If you have a furnace that is located in the basement, a carbon monoxide detector is a great idea. It is just an extra measure of protection to give you peace of mind and ensure that you and your family are safe.

Are Old Furnaces Dangerous?

Older, less-efficient furnaces present a major risk for carbon monoxide poisoning. The components within an old gas furnace can become worn down and inefficient over time. That can mean carbon monoxide leaks in various forms.

Since carbon monoxide is colorless and odorless, it can permeate your home without you being aware. Consider replacing your furnace if it is clearly old and showing signs of wearing.

Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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