How to Vent a Furnace Through a Wall – Step-by-Step

You can vent a furnace in three main ways, through the chimney, through the roof, and through the wall. It all depends on the type of furnace and the property type to a lesser extent. However, recently, through-the-wall venting has become the most popular (and effective) furnace venting method.

Read on to learn more about through-the-wall venting. We cover common topics such as the pros and cons of through-the-wall venting, when to vent through the wall, and deciding between single and two-pipe sidewall venting. Ultimately, we also explain how to vent your furnace through the wall step-by-step.

Why Furnace Venting is Important

Let’s begin with the importance of furnace venting. Why do you need to vent your furnace? Is it necessary? The answer is yes. Furnace venting is crucial for the following reasons; 

  • It’s a legal requirement

The federal and local governments stipulate guidelines for gas furnace installation, with ventilation requirements clearly captured. Most of the regulations are based on the International Residential Code and Uniform Mechanical Code. Alternatively, you can check your building department or city’s website for codes specific to your area.  

  • It’s critical for occupant safety 

The gas combustion process generates toxic gases that pose a significant risk to humans, pets, and house plants. For instance, the standard heater generates tons of carbon dioxide. Carbon dioxide can cause drowsiness and difficulty breathing.

Exhaust gases also include carbon monoxide, sulfur dioxide, and nitrogen dioxide, which are even more dangerous. Removing these gases out of the house is critical for occupant health.

  • It’s important for appliance health

The gases listed above aren’t only bad for humans, animals, and plants but also the appliance. Take an example of carbon dioxide. When carbon dioxide and moisture (gas furnaces also produce lots of moisture) interact, they form carbonic acid. Carbon acid lowers the pH and ultimately corrode soft steel tube metals.

Then you can sulfur dioxide and nitrogen dioxide that form sulfuric acid and nitric acid, respectively, when exposed to moisture. Sulfuric and nitric acids are extremely corrosive.

  • It’s critical for heating efficiency

Finally, venting your gas furnace is also beneficial to the heating process. Remember that furnaces rely on oxygen-rich air from your home or outside the house for healthy combustion. That healthy blue flame is a direct result of sufficient airflow.

Unfortunately, poor or nonexistent ventilation can cause a backdraft of stale oxygen-starved exhaust gases into the burning chamber, causing incomplete combustion.

 Incomplete combustion is characterized by increased soot, yellow burner flames, and reduced heat output.

What is Through-the-Wall Venting?

Through-the-wall venting simply means removing furnace exhaust gases through the wall. Typically, you bore a hole in an exterior wall and connect the vent pipe from the furnace through it. That way, exhaust gases from the furnace can head out through the wall.

Can I vent My Furnace Through the Wall?

The short answer is – yes. However, it depends on the type of house and property type. Generally, through-the-wall venting is reserved for high-efficiency furnaces that generate denser exhaust gases. If you understand how high-efficiency furnaces work, you’ll be aware that they have two heat exchangers, with the second one dedicated to extracting heat from exhaust gases.

Thus, unlike traditional furnaces (low and mid-efficiency) that produce hot exhaust gases with a high buoyancy, exhaust gases from high-efficiency furnaces are cold and highly dense. Thus, it’s more practical to let the hot exhaust gases from traditional furnaces float out of the furnace via the chimney or roof and the denser exhaust gases from high-efficiency systems to “drain” out through the wall.

Single Pipe or Two Pipes? Which is Better?

This is a big question when venting a gas furnace. Do you install just one pipe and let the furnace draw combustion air from the room. Or is it better to have two pipes, one to remove exhaust gases and another to draw fresh air for combustion?

As you’d expect, most experts recommend two-pipe venting systems, an approach also known as direct venting. The following are a few reasons why and potential challenges to note if you decide to take this route.

Direct Venting Pros 

  • Get oxygen-rich air for the combustion process: The biggest advantage of direct venting is that you draw fresh, oxygen-rich air from outside the house for the combustion process. This typically results in richer flames and consequently higher heat output.
  • Avoid oxygen depletion inside the house: Indirect venting systems that draw combustion air from your home can “eat up” most of the oxygen in your rooms, exposing you and your family and guests to the risk of carbon monoxide poisoning.
  • Enjoy fresher, healthier air: Continually introducing fresh air from outside into your home is incredibly beneficial for your health. Instead of recycling stale air in your room throughout the day as indirect venting furnaces do, direct vent systems eliminate the stale air and pull fresh air from outside the house.

Direct Venting Challenges 

  • It means another hole in the wall: Another hole on your wall is not only unsightly but can also present fresh insulation challenges. So, you must take greater caution to keep your home warm in winter and cool in the summer.
  • Increased heating costs: Two-pipe venting means that your return air is way below room temperature, unlike single-pipe systems whose return air isn’t too far from room temperature. As a result, you may find that you have a slightly higher heating bill.
  • More “blocked vent” problems: Dealing with blockages in one vent pipe is already enough trouble for homeowners. Unfortunately, you’ll be dealing with one more pipe when you install the direct vent system. It can be a nightmare. 

How to Vent a Furnace Through the Wall Step-by-Step

Now, let’s discuss how to vent your furnace (direct venting) through the wall. The installation process details often vary depending on the furnace type and vent kit. However, we’ll try to make the process as general as possible.

Safety Precautions 

  • Wear gloves and safety glasses for protection.
  • Exercise extreme caution when using ladders.
  • Beware of electrical wiring in the wall.
  • Follow all the manufacturer’s instructions to the latter.

Planning the installation 

You need to keep a couple of things in mind during the installation;

  • Ensure you’ve got a horizontal venting kit: You cannot install a vertical kit through the wall or vice versa.
  • Ensure you have enough pipe length: Ideally, you want to measure the distance from the furnace to the installation location before ordering the kit.


  • Tape measure
  • Drill
  • 3/16-inch (4.5mm) diameter drill bits
  • Masonry drill bits
  • Chisel
  • Hammer
  • Reciprocating saw with appropriate blades
  • ¼ and 5/16-inch nut drivers
  • Head screwdriver

The Step-by-Step Installation Process 

Step 1: Determine the best location for the vent terminal

This can be the hardest part of the installation process. For one, you must consider existing plumbing and safety codes. Otherwise, your home will fail future inspections. Additionally, you must select a location that won’t compromise heating and related activities around the furnace.

So, we recommend that you choose a location based on the following considerations;

  • Make sure the terminal location minimizes the total length of the exhaust system and the number of elbows and piping.
  • Make sure the selected terminal location doesn’t expose your plants, shrubs, air conditioning, and other equipment outside the house to damage.
  • The selected location must not expose the venting system to strong winds, snow, airborne leaves, and debris that may block the vents.

Step 2: Locate the vent holes

Once you’ve identified the ideal termination location, the next step is to identify the location of the two pipes, i.e., exhaust pipe and air inlet pipe. Most installing kits come with a wall mount plate that allows you to mark the holes easily.

Step 3: Cut the vent holes

Size is critical here. So, make sure you double-check what the user manual says. However, typically, you need to drill two holes, each about four inches in diameter. Use the drill bit for speed and accuracy. Remember to drill from outside the house inward.

Step 4: Drill wall anchor holes

This time you need to drill four 3/16-inch-diameter holes. The holes will hold the wall anchor. Again, an electric drill makes the process so much simpler.

Step 5: Install the wall anchor

Insert the four screws anchors into the screw holes to mount the vent terminal base. Then, holding the base in position, insert the screws. However, don’t fully tighten the screws this time. Instead, just turn them a few rounds to hold the plate in place.

Step 6: Assemble the venting pipes

The assembly process varies from one vent kit to the next. However, in most cases, all you need to do is twist-lock lugs on male and female ends of the pipe sections. Insert the male end of the pipe into the female end until the locking lugs are covered.

Then, twist the female end clockwise to lock the sections together. You may or may not use screws to secure the connection as long as the screws do not penetrate the inner vent walls. Also, you may or may not need elbows.

Step 7: Install the vent pipe

Once you’ve assembled both pipes, go inside the house and install them. This should be a straightforward process if using a venting pipe that bends, such that you don’t need elbows.

However, it only takes a few extra minutes, even with elbows. All you need to do is connect the top part of each pipe to the wall connection and the bottom to the furnace vent openings.

Step 8: Seal and test

You’re almost done. The last part is to seal the gaps between the pipes and the wall using silicone. You can use the same silicon to seal the joint between the vent pipe to the vent cap. Do not use PVC cement for sealing. Once done, test the furnace for about 15 minutes to see if it works as intended.


  • How Do You Vent a High-Efficiency Furnace Through the Wall? High-efficiency furnace wall venting involves installing two separate pipes (exhaust and air inlet) horizontally through the wall to the furnace to vent exhaust gases and draw fresh air for combustion.
  • Can I Install a B-Vent Through the Wall? No. You cannot install B-vents or single wall vents horizontally through the wall.
  • Can I Vent My Gas Furnace Horizontally? Yes, you can vent your gas furnace horizontally. However, this only applies to high-efficiency gas furnaces. You can only install low and medium-efficiency furnaces vertically through the roof or via the chimney.


Venting is critical even for high-efficiency furnaces. Otherwise, you’d be badly exposed to dangerous combustion gases various health issues. Now you know how to vent a high-efficiency furnace horizontally through the wall.