Furnace Vent Piping – Types of Gas Furnace Vent Pipes

All furnaces produce combustion gases, some of which are unsafe, which must be expelled outside the house for health and safety reasons. Additionally, these combustion gases can be harmful to your furnace. Some cause corrosion, others rusting, and a few tend to accelerate dirt buildup, potentially causing blockages and increasing wear and tear. 

Therefore, it’s only logical to remove these gases outside the furnace and outside the house into the open air. However, EPA. NREL, EERE, and the DOE have stringent furnace venting requirements that can easily result in penalties and even jail time for non-compliance. Thus, you cannot just install your venting system however you wish. 

This guide seeks to help homeowners, and DIYers better understand furnace venting requirements so you can purchase the right venting systems, install them correctly, and maintain them properly in line with the law and for maximum impact. 

Does Every House Have a Furnace Vent Pipe?

Let’s begin with a simple question – is a furnace venting system mandatory for every house? Must you install one? The short answer is – yes! Most states and even the federal government turn to the Uniform Mechanical Code (UMC) on furnace installation requirements, and the UMC clearly states that venting is mandatory for gas furnaces.

These requirements are captured in Chapter 8 of the UMC document, which correlates with the National Fuel Gas Code (NFGC) found in the National Fire Protection Agency (NFPA) 54. Section 802.3 of the UMC, which addresses design and construction requirements of gas-powered appliances, explains that venting systems are designed to convey combustion fumes, i.e., the byproducts of burning gas, to the outdoors.

However, section 802.2.1 lists all gas-powered appliances for which venting is a must and appliances that don’t require venting. 

Gas Furnace Venting Options

You can vent a gas furnace in three main ways, i.e., natural venting, sidewall venting, and concentric venting.

1. Natural Venting

Natural venting, also known as Type B venting, was the most common venting method back in the day. It involves attaching a natural vent pipe to the furnace and running it vertically through the roof.

This way, hot combustion fumes easily rise and exit through the roof naturally. Most natural vents are built into the existing chimney. However, you can also install a through-the-roof vent without or outside the chimney. 

2. Sidewall Venting 

Sidewall venting uses one or two pipes (and intake and exhaust pipe) that run through an exterior wall to bring fresh air for combustion into the furnace and remove dangerous exhaust gases out of the furnace. 

Sidewall venting systems that utilize one pipe are known as indirect venting systems, while those that use two pipes are known as direct venting systems. 

3. Concentric Venting

Finally, you can also opt for concentric venting. Concentric venting designs provide a compact room sealed flue, where air supply and flue gases are managed within a single piece of the concentric ducting system.

It’s a duct-within-a-duct design where flue gases are expelled via the smaller inner duct while fresh air for combustion is drawn into the furnace via the wider outer annulus. 

A concentric venting system can be installed horizontally and routed through an exterior wall or installed vertically and vented through the roof.  

Types of High-Efficiency Furnace Venting Systems

High-efficiency furnaces typically use sidewall venting systems. If you feel a little lost, the NFPA and UMC provide clear guidelines on which venting system you can install for each gas heater type. Generally;

  • Category one appliances, comprising old-fashioned gravity vent and induced draft furnaces, are vented vertically, typically through the roof or chimney. 
  • Category two and three appliances, including boilers, vented water heaters, and tankless water heaters, are vented per the manufacturer’s guidelines.
  • Category four appliances, including high-efficiency gas furnaces and water heaters, are vented horizontally through the sidewall. 

Direct vs. Indirect Venting

High-efficiency furnaces can be vented via the sidewall using one pipe (indirect venting) or two pipes (direct venting). Single-pipe venting only includes one pipe to remove exhaust fumes out of the furnace as these furnaces draw air for combustion from inside the house. It’s similar to how your wood stove works.

This type of venting is best where there’s no need for separate combustion air intake, such as in the basement, attic, or crawlspace. Meanwhile, two-pipe venting systems comprise two pipes to bring in fresh air for combustion and the second to exhaust combustion fuses out of the furnaces. 

Furnace Vent Pipe Types

Let’s now look at the common pipe materials used in gas furnace venting. Most furnaces use one of the following pipe types;

  • Copper pipes
  • Galvanized steel pipes
  • Iron pipes
  • Concrete pipes
  • Aluminum pipes 
  • Carbon fiber pipes 
  • Stainless steel pipes
  • Plastic pipes

Copper, galvanized steel, and iron pipes are the most common piping materials in traditional gas heating systems. Why? Because exhaust fumes from traditional vents are very hot, typically exceeding 460°F. However, all three present corrosion concerns that often leave homeowners with massive maintenance burdens. Freezing is another common concern.

Recently engineers have started drifting from metal exhaust systems to fiberglass and plastic vent pipes. Both materials don’t rust and are resistant to freezing. Additionally, fiberglass can withstand extremely high temperatures, while plastic is resistant to freezing. 

Why Furnaces Use Plastic Pipe

Modern furnaces use plastic pipes for venting because today’s high-efficiency furnaces produce relatively cool exhaust fumes. This means that the risk of melting is nonexistent.

With that covered, plastic seems an excellent choice considering that the only other requirement for category IV appliance venting systems is water tightness and airtightness. Plastic venting pipes pass these two tests with flying colors. Above all, plastic is cheaper than all other vent pipe materials.

What Type of PVC is Used for Furnace Venting?

Beware, however, that not all plastic is good for furnace venting. Why? Because high-efficiency furnaces produce significant amounts of water and carbon dioxide, which typically mix to form carbonic acid within the vent pipes. Carbonic acid is highly corrosive, thus can quickly eat up certain plastics.

For this reason, HVAC engineers only recommend polyvinyl chloride (PVC), Chlorinated PVC (CPVC), and Acrylonitrile Butadiene Styrene (ABS) plastic pipes, depending on the gas furnace type. CPVC is considered the best, followed by ABS plastic.

What Size Vent Pipe Do I Need for My Furnace?

Gas furnace vent sizing is a critical matter determined by the National Fire Protection Agency (NFPA) and National Fuel Gas Code (NFGC). So, we recommend checking out the two resources. Alternatively, check out this valuable guide from Ameri-Vent. It has all the relevant tables to help you determine the ideal vent sizes for all types of gas furnaces.

Generally, the sizing requirements depend on the type of furnace (natural venting or fan-induced) and the size of the furnace in BTUs. The ideal vent pipe diameter also directly depends on the total distance between the exhaust outlet on the furnace and the exit point on the wall or roof. This means you need to account for both the horizontal stretch and vertical distance. 

That said, you’re allowed to install a total vent pipe length of up to 150 feet, 100 vertically and 50 feet horizontally, with vent pipe diameters ranging from 3.0 inches to 24 inches. The minimum allowable vent diameter, i.e., 3.0 inches, is reserved for small furnaces up to 78,000 BTUs (draft fan assisted) and 46,000 BTUs (natural venting). The venting distance must also be short, typically zero feet horizontally and a maximum of 6.0 feet vertically. 

Meanwhile, the maximum allowable vent pipe diameter is 24 inches, which only applies for Type B double-wall vents connected directly to a single Category One gas furnace. More importantly, it only applies to natural vent furnaces rated up to 8,100,000 BTUs or draft-induced furnaces rated between 752,000 BTUs and 13,354,000 BTUs. The trick is in reading the tables.

First, make sure to identify the correct table for your furnace, then move down and across the table until you find the correct vent pipe size for your appliance. A few additional rules are applicable depending on the installation type. For instance, you need to keep the following in mind;

  • If the vent size determined from the sizing tables is smaller than the appliance’s draft hood outlet or flue collar, you are allowed to use the minimum capacity/size.
  • Single-appliance venting systems with zero lateral length have no elbows. However, all venting configurations with lateral/horizontal length require at least one elbow. Thus, the total length listed in the table is reduced by 10% for every 90° turn. Remember that 2 x 45° turns = one 90° turn.
  • If the vertical vent has a larger diameter than the vent connector, you’re allowed to use the vertical vent diameter to determine the minimum vent capacity. 

What is the Pitch Required on a PVC Venting Pipe?

Gas furnace vent pipes must have a minimum of ¼ inch per foot of slope towards the furnace. This is very important, and something many DIYers overlook. More importantly, PVC intake and exhaust vent pipes must be supported all the way from the furnace to the discharge points outside the house to prevent sagging. Sagging can lead to condensate accumulation, airflow restriction, water pooling, and ultimately furnace shutdown. 

Support system recommendations depend on the diameter of the pipes. However, most manufacturers recommend support hangers every four feet for 2-inch vent pipes and five feet for 3-inch vent pipes. 

Why Condensing Furnaces Need a Condensate Pipe

High-efficiency furnaces implement dual-stage heat extraction to pull as much heat energy as possible from the burning gas. The heat from the burner(s) passes through the first heat exchanger, as in all furnaces. However, after the first heat extraction cycle, the combustion fumes go through a second heat exchanger that draws even more heat from the gases. 

Now, remember that moisture makes up almost 70% of exhaust fumes. This moisture usually condenses to water after the second round of heat extraction, necessitating a means to get the liquid out of the furnace. That’s where the condensate pipe comes in handy. 

How to Inspect and Clean Furnace Intake and Exhaust Pipes

Fortunately, it’s a straightforward process. Begin by locating the vent pipes. They are typically black or white pipes that run from the furnace and exit your home through the wall or the roof. You may need a ladder if the pipes are high on the wall or the roof. Once you’ve found them, proceed as follows;

  • Ensure the intake vent pipe points downward while the exhaust pipe points up. If not, contact a professional.
  • Ensure at least five feet of clearance all around the vent pipes. So, cut back any branches or plants growing too close.
  • Look inside the pipes to make sure nothing is blocking them. You need to do this at least once every two weeks. If you notice debris or ice blocking the pipes, contact an HVAC professional. 
  • Consider vent screens: Vents screens help protect the vents from debris, ice, and other large particles. So, consider speaking with your HVAC technician about it. 


Can you vent a furnace with PVC? Yes, you can vent a furnace with PVC. However, only PVC and CPVC are allowed. 

Can you use PVC for furnace exhaust? Yes, you can use PVC for both the gas furnace inlet vent and exhaust vent. 

Can ABS pipe be used for furnace exhaust? Yes, you can use Acrylonitrile Butadiene Styrene (ABS) plastic pipes for gas furnace exhaust vents. 

Can I use a flexible duct for furnace exhaust? Yes, you can safely use flexible ducts for furnace exhaust venting. Indeed, you can use B-Flex pipes anywhere B-vent pipes are needed. 

Can you vent an 80% furnace with PVC? No, you cannot vent an 80% (low-efficiency) furnace with PVC as low-efficiency furnaces produce very hot exhaust fumes that would instantly melt the plastic. 

Does the furnace vent need to be sloped? Yes, furnace vents are sloped by a minimum of ¼ inch per foot towards the furnace.

Should the furnace exhaust pipe be covered? Yes, though it’s not absolutely necessary. The covers, known as vent screens, prevent obstructions and pest infestations. However, you need to speak to your HVAC technician before installing one. 


Venting is a critical step in gas furnace installation. The choice of vent pipe determines many factors, such as venting efficiency and the lifespan of the venting system. Therefore, you should select your venting system carefully and install it per industry regulations. Alternatively, let an HVAC professional handle the entire process.