Gas Furnace Venting Options (*Top 5 Choices*)

All furnaces produce combustion gases that must be vented out of the system to ensure a healthy furnace flame and efficient heating while preventing potential gas poisoning. As a result, nearly all furnaces have a venting mechanism to direct the exhaust gases out of the burner chamber. Unfortunately, choosing the best venting mechanism isn’t always a straightforward process.

 Although roof venting is the most popular approach, other venting mechanisms may prove even better in certain conditions. This guide seeks to help homeowners and furnace installers select the best venting mechanism under different circumstances.

Do Gas Furnaces Need to be Vented?

Yes, a furnace must be vented. It’s both a legal requirement and a safety and health obligation. If you’re wondering why, the answer is – toxic fumes. Furnaces produce toxic gases that are dangerous to you, your loved ones, the appliance itself, your home, and even the environment. For instance, a standard gas furnace generates significant carbon dioxide and moisture.

These two can combine within the furnace to produce toxic carbon acid that can quickly eat up the furnace’s internal components. Moisture is also harmful to the structural integrity of your home. Additionally, gas furnaces produce carbon monoxide, sulfur dioxide, and nitrogen dioxide, poisonous gases that kill hundreds of people every year.

Exhaust venting systems offer a means to remove these byproducts from your home safely.

Benefits of Venting a Gas Furnace

  • Health benefits

Gas furnace combustion byproducts, such as nitrogen dioxide and sulfur dioxide, can cause many health issues, including inflamed airways, reduced ling function, and increased asthma attacks. Venting protects you and your family from these dangers.

  • Safety benefits

Gas furnace combustion byproducts such as carbon monoxide can also cause suffocation and death of humans and pets. Additionally, exhaust gases are very hot, typically up to 400°F, thus can cause severe burns. Venting protects you from these hazards too.

  • Protect your furnace

When water mixes with carbon dioxide, sulfur dioxide, or nitrogen dioxide, it forms carbonic acid, sulfuric acid, and nitric acid, respectively. All three are corrosive chemicals that can quickly eat up internal components of the furnace. Venting protects the furnace from such damage.

What Are Some Ways of Venting a Gas Furnace?

Different technicians prefer different venting mechanisms. However, the five most common venting methods are as follows;

1. Venting Gas Furnace Through Chimney 

Let’s begin with one of the oldest ways to vent your gas furnace – through the chimney. According to Jade Learning, chimney venting was the most popular way to vent a gas furnace. Technicians used draft hoods, also known as draft diverters, that introduced dilution air to the furnace’s exhaust gases and prevented moisture issues in the chimney.

Unfortunately, today’s mid-and high-efficiency furnaces can destroy tile-lined chimneys because 80% and 90% efficient furnaces create a lot of condensation. However, this doesn’t mean it’s something you cannot try.

For instance, homeowners hellbent on venting their furnaces through the chimney can invest in stronger metal chimney liners that are unlikely to crack or corrode.

You can also tie the liner into your furnace exhaust pipe to reduce the risk of CO backdraft into your home. Alternatively, you may upgrade to a closed-combustion furnace (typically 95%+ AFUE). This especially makes sense if you have a very old furnace or a large external chimney that becomes extremely cold throughout the winter.

A high-efficiency furnace uses a PVC pipe to pull air through the chimney for combustion while simultaneously venting exhaust gases. This way, you don’t have to worry about the limitations of your chimney as the PVC pipes expel exhaust gases directly outside the house. Ultimately, though, chimney venting isn’t recommended for today’s furnaces.

 So, if you decide to install one, you need a combustion safety test to be absolutely sure that the combustion gases aren’t drafting back into your home. Also, it helps to check the venting system regularly to catch potential issues early.

For instance, white corrosion on the furnace vents is usually a sign of backdraft. Either the combustion gases stay inside the pipe for too long or are forced back.

Above all, make sure to invest in a carbon monoxide detector. CO backdraft is extremely dangerous. It can easily cause suffocation and death.

2. Venting Ga Furnace Through the Roof

Since moderate to highly-efficient furnaces are impractical to vent through the chimney, homeowners have two broad options: vent them through the roof or the sidewall. Through-the-roof venting is the preferred solution for moderately efficient furnaces with between 80% and 90% AFUE ratings.

Fortunately, venting a gas furnace through the roof isn’t too difficult. Begin with preliminary checks to ensure that the ductwork is in good shape. You must also ensure that the furnace is aligned with the ductwork and existing gas pipes. If everything is fine, hook up the heater to the ducts, taking care not to destroy the existing piping.

 We recommend first connecting the plenum and ensuring that the duct is tightly connected. Use a clamp if necessary. Once done, hook up the new furnace to the gas line and turn on the gas. At this point, you want to check for gas leaks. If everything’s good, install the gas furnace event. Here’s how to proceed;

  1. Purchase the venting pipes: Measure the distance between the flue discharge on the furnace and the chimney and purchase a sufficient length of galvanized metal sheet ducting and elbows.
  2. Drill a 4¼ -inch hole into the chimney: You need to pitch the hole about ¼ inch per foot. Make sure the hole is high enough for the pitch.  
  3. Connect the venting pipes: Connect the pieces by inserting one crimped end of one piece to an uncrimped end of the next piece. Then use metal screws to secure the connections and elbows to make turns and twists. Feel free to use hanger straps to support the venting.
  4. Attach the vent to the furnace: Use high silicon caulk to seal the joint around the vent and furnace.

3. Venting Gas Furnace Through the Wall

Newer, modern gas furnaces are best vented through the wall. It’s easier to vent through the sidewall. Moreover, sidewall venting is more practical for higher efficiency systems due to the lower temperature of the exhaust gases.

Whereas traditional furnaces can rely on the buoyancy effect of warmer gases to vent exhaust gases through the roof or chimney, today’s modern gas furnaces produce colder exhaust gases.

These furnaces typically deploy a second heat exchanger that extracts nearly all the heat from exhaust gases, leaving the exhaust gases cold and denser. Generally, a side wall vent comprises two pipes running through the exterior wall exterior adjacent to your furnace – an intake and exhaust pipe.

The intake pipe brings in fresh air from outside to facilitate the combustion process while the exhaust pipe vents the furnace’s byproducts, including cooled fumes and small amounts of condensate.

The fumes mainly comprise carbon dioxide and minor traces of carbon monoxide, nitrous oxides, sulfur oxides, and particulates. Unfortunately, installing sidewall venting systems is a tricky process that requires a professional’s input. For instance, the average high-efficiency furnace (50,000 BTUs) puts out the equivalent of 2.5 liters of moisture per hour.

 If that moisture isn’t properly vented, it can cause untold damage to the property and the public. For instance, attic moisture mold is a common challenge. Additionally, you may encounter icing issues during peak winter weather.

Worse still, poor venting can result in dangerous backdrafts that impact indoor air quality and cause health issues. That’s why most states require that all sidewall venting terminals are at least 12 inches below, 12 inches horizontally from, and 12 inches above any window, door, or gravity air inlet into the building. Remember that vent pipes must also be obstruction-free.

Authorities recommend installing the pipe at least 14 inches from any obstruction and three feet from an inside corner of an L-shaped structure.

Take note of the following additionally installation requirements too;

  • Don’t place sidewall vent terminations near openings or underneath any structure that may obstruct the removal of exhaust gases.
  • Ensure the vent pipe terminates in an open area where exhaust gases can dissipate widely and freely.
  • Ensure that penetrations through the wall enclosure are sealed to maintain the continuity of the air barrier.
  • Ensure to seal all soffit vents immediately above the vent, even if the vents are two floors above the flue vent.
  • Never vent exhaust gases over public walkways or any area where surface wetting by condensate or water vapor may cause nuisance or a hazard.

 4. Concentric Venting 

Concentric venting describes a coaxial “pipe in pipe” venting design where the exhaust gas and intake air vent through a shared pipe assembly. Thus, it offers an aesthetic and more straightforward furnace ventilation solution with just one hole through the wall or ceiling. The design is most common in water heaters.

However, many high-efficiency furnaces have recently adopted it. It’s most common among direct-vent furnaces. Concentric venting can be used for the entire vent run, i.e., connecting the heating and exhausting gas outside the house. However, this is only allowed in furnaces explicitly designed for concentric venting.

Alternatively, some furnace manufacturers opt for concentric vent termination. For instance, while installing a direct vent furnace, the technician may terminate the vent with a concentric pipe. The setup provides the benefits of a single-wall or roof penetration with an aesthetic vent termination.

A concentric termination system utilizes separate intake and exhaust vent pipes between the furnace and wall or roof. Then the concentric pipe takes over.  

We must clarify a few things about concentric venting, though;

  • Never attempt it on your own. Concentric venting is a strictly professional job.
  • If you must extend a concentric vent piping, please use only the pipe type recommended by the manufacturer.
  • Never install multiple concentric vent pipes directly above one another. If you must do so, ensure a minimum of three feet distance between the pipes.  

5. Using Power Vents

Finally, you can also vent a gas furnace using power vents. Power vents are equipped with an electric blower, typically installed on the water heater, which helps push exhaust gases and excess heat through the exhaust venting.

Thus, it’s entirely different from standard furnace vent systems that rely exclusively on natural forces, typically convection and buoyancy, to remove exhaust gases.

Standard power vents use just a single pipe, i.e., indirect venting, where the furnace draws inlet air from inside the house and exhausts waste gases outside the house. However, a few furnaces now use powered direct vents that draw inlet air outside the house. The most significant advantage of power venting is that it works in nearly all furnace types.

It can run horizontally or vertically, meaning you can use it for wall, roof, or even chimney venting under the right circumstance. Better still, power vents allow longer venting distances, offering invaluable flexibility. Ensure to consult your HVAC technician before opting for power vents.

How to Tell If Your Furnace is Venting Properly 

Gas furnace fumes are typically moist. Therefore, if you notice “sweat” on your windows, it’s a telltale sign that your furnace isn’t venting properly. Two other signs of poor gas furnace venting are backdraft (meaning the furnace is sucking exhaust gases down the flue) and low oxygen levels in the home. A carbon monoxide detector will sound an alarm when the oxygen levels drop dangerously low.


Can a gas furnace be vented with PVC? Yes, you can use PVC for gas furnace venting. The National Fire Protection Agency (NFPA) approves PVC, ABS, and CPVC plastics for furnace venting, provided you follow your local codes.

Can a gas furnace be vented horizontally? Yes, high-efficiency gas furnaces can be vented horizontally through the wall. However, low and mid-efficiency gas furnaces are typically vented vertically through the furnace or roof.

Can a furnace vent pipe have an elbow? Yes, gas furnaces vent pipes can have elbows. In fact, you can have more than one elbow inside and outside the house for the same vent system. However, you must strictly follow your local building codes.


You can vent your gas furnace in many different ways, with chimney venting, roof venting, and wall venting the three most popular venting approaches. It all comes down to the type of furnace and what your local building codes say. Consult an HVAC professional if you have more questions.