High-Efficiency Furnace Venting Options

High-efficiency furnaces are a marvel. They are so efficient you may not even notice the furnace is running. Moreover, modern high-efficiency furnaces are equipped with modern technologies that allow them to integrate with smart home solutions for maximum convenience and usability.

Unfortunately, the long list of bells and whistles sometimes diverts attention from critical requirements such as proper furnace installation and ventilation. This guide outlines why proper installation is vital even for high-efficiency gas furnaces and explains five common ways to vent your high-efficiency furnace.

Do High-Efficiency Gas Furnaces Need to Be Vented?

Let’s begin with a question we get a lot – do gas furnaces need to be vented? The short answer is – yes, gas furnaces must be vented, including high-efficiency models. You see, heating with a gas furnace involves fuel combustion that produces several toxic gases. This applies to traditional low-efficiency and modern high-efficiency furnaces.

The only difference is that modern high-efficiency furnaces generate lower quantities of exhaust gases because most, typically 90% to 99% goes towards useful heating. Thus, only 10% or less of the fuel is lost to exhaust byproducts. By comparison, low-efficiency furnaces lose up to 40% of their fuel to exhaust byproducts, meaning massive amounts of toxic fumes.

For instance, if you own a natural gas furnace, the combustion process produces carbon dioxide, carbon monoxide, nitrogen dioxide, sulfur dioxide, volatile organic compounds, and many other hydrocarbons.

Many of these gases are poisonous and even toxic. For instance, when inhaled, carbon monoxide displaces oxygen in the blood, thus depriving vital organs in your body, including the heart and brain, of critical oxygen. This can quickly result in organ malfunction, causing loss of consciousness and, shortly after, suffocation.

Sulfur dioxide is even more dangerous. Inhaling moderate amounts of sulfur dioxide can cause severe throat and nose irritation. At high concentrations, it causes life-threatening fluid accumulation in the lungs, a medical condition is known as pulmonary edema. A single exposure can also cause asthma, while extended exposure can cause death.

Gas furnace ventilation is a must for the above reasons. Otherwise, you’d be dangerously exposed to several dangerous gases.

Benefits of Venting a High-Efficiency Gas Furnace

The following are three crucial reasons why you should vent your gas furnace and always ensure that the vent is working correctly;

Protection from unsafe gases

As we’ve seen, gas furnaces generate several unsafe gases, including carbon dioxide, carbon monoxide, sulfur dioxide, and volatile organic compounds. These gases can cause various health complications when inhaled, even in small quantities. Properly venting your furnace protects you and your loved ones from these dangerous gases.

Ensures healthy burner flame

You’re probably aware that flames require oxygen-rich air to burn healthily. Otherwise, your burner flame will be yellow, sooty, and weak. Unfortunately, this usually happens when the furnace isn’t properly vented. Exhaust gases fill the burner chamber, depriving the combustion process of vital oxygen-rich air.

Thus, you end up with a weak, yellow, and sooty flame. The flame may even go out if the problem persists.

Ensures efficient heating

A weak flame is bad news for the heating process. Although yellow flames can appear bigger and wilder, they carry less heat than the stable cone-shaped blue flame. Moreover, soot is actually wasted carbon from your gas, meaning that the fuel goes to waste.

Therefore, you will incur higher heating bills without generating enough heat to keep the home at the desired temperature.

It helps prevent rusting and corrosion

Finally, most people overlook it, but venting exhaust gases out of the furnace is critical to corrosion and rust prevention or slow-down. Remember that fuel combustion primarily generates carbon dioxide and water vapor. Water vapor (moisture) can cause rusting, thus weakening the furnace.

Rusting in the heat exchanger is particularly common and extremely dangerous. Meanwhile, the moisture and carbon dioxide can also mix, creating carbonic acid that may corrode the inner linings of the furnace. Proper venting helps slow down or even prevent rusting and corrosion in some cases.

High-Efficiency Furnace Venting Requirements

The main requirements all furnace venting installations and maintenance procedures in the US must comply with are captured in Section 211 of the National Fire Protection Agency (NFPA).

NFPA 211, titled “Standards for Chimneys, Fireplaces, Vents, and Solid Fuel Burning Appliances,” came into effect in 2000 and currently serves as the reference point for furnace ventilation requirements for all states in the US. In other words, you’re good to go if furnace ventilation installation is in accordance with NFPA 211.

The main gas furnace ventilation requirements under NFPA 211 Section 10 are;

  • Gas vents must also be sized and configured following NFPA 54, National Fuel Gas Code, approved methods, and the appliance and manufacturer’s directions.
  • All vents shall terminate above the roof surface. Those installed with mechanical exhausters shall terminate at least 12 inches above the roof surface, while those without exhausters must terminate at least two inches above the roof surface.
  • The exit terminal must be arranged such that flue gases don’t enter adjacent buildings or interfere with other people’s lives and activities.
  • Through-wall vent systems must not terminate over public ways or an area where the condensate or vapor may pose a hazard to the public.
  • Vents passing through floors of buildings requiring the protection of vertical openings must be enclosed appropriately. Similarly, single-wall vents passing through an exterior wall must be guarded with a thimble at the point of the passage.
  • The terminal of a direct vent appliance must be 6-12 inches from any opening into the building through which exhaust gases may enter the building. Meanwhile, the bottom terminal and air intake must be at least 12 inches above grade.

You can read more about the requirements in NFPA 211 Section 10. The Section also touches on the need for installation procedures for vent hoods, clearance requirements for all gas furnace systems, and special venting arrangements.

What Are Some Ways of Venting a High-Efficiency Furnace?

Now, let’s discuss the five most common ways to vent a high-efficiency gas furnace. We’ll focus on through-the-roof venting and through-the-wall vents but also mention other innovative ways to vent your furnace.  

Venting a High-Efficiency Furnace Through the Roof

Roof ventilation means letting the gas furnace remove combustion byproducts from the house through the roof. There are two main ways to vent a high-efficiency furnace through the roof – via a direct vent and through non-direct vents. Direct vents comprise two pipes, both of which lead outside the house.

The first is an air inlet pipe that draws fresh air for combustion, while the second is an exhaust pipe that removes combustion byproducts from the house. Two-pipe direct vents are typically pipe-in-pipe configurations.

Meanwhile, non-direct vents only have one pipe exiting the house – the exhaust vent. That’s because non-direct vent furnaces draw fresh air for combustion from the room where the appliance is installed.

The main benefit of roof venting is that it allows manufacturers to take advantage of natural forces to ensure efficient exhaust venting. Additionally, most furnaces vent vertically. So, you won’t struggle to find a good technician for roof installation. Unfortunately, roof venting accelerates roof system disintegration.

Venting a High-Efficiency Furnace Through the Wall

Newer high-efficiency furnaces vent through the wall. As the name suggests, wall venting is any gas furnace venting configuration where the vent(s) terminate through an exterior wall. This method is also known as horizontal or sidewall venting.

Sidewall venting is strictly for high-efficiency furnaces and may involve one or two pipes. Similar to roof venting, two-pipe configurations allow the furnace to draw fresh air for combustion from outside the house, while single-pipe systems utilize indoor air for combustion.

However, beware that sidewall venting is regulated even more strictly than roof venting. For instance, the exhaust vent terminal must be at least three feet above any forced air inlet within a 10-feet radius. This is one foot more than is required for roof vents.

Additionally, the vent terminal must be 12 inches below, 12 inches horizontally from, and 12 inches above any window, door, or gravity air inlet. NFPA 211 Section 10 covers all these requirements.

Concentric Venting 

Concentric venting is a modern approach to furnace and water heater venting that provides the convenience of direct venting with just one termination. This means that you can bring in fresh air for combustion from outside the house with just one pipe terminating through the wall.

It’s most popular in sealed flue systems where air for combustion must come from outside the house. These are also known as room-sealed appliances. Room sealed appliances reduce the risk of flue gases spilling back into the room due to poor operating conditions.

A concentric venting system provides a compact room sealed flue that manages supply air and exhaust gases within one piece of concentric ducting. The most common configuration is the duct-within-a-duct arrangement, where flue gases are expelled through the inner duct, while fresh air for combustion is drawn into the furnace through the outer annulus.

Concentric venting systems can be vented horizontally through the wall or vertically through the roof as long as you follow NFPA requirements and local building codes.

Using Power Vents

Up to recently, furnaces and other residential heating systems relied on natural forces to remove exhaust gases out of the furnace. Even modern units vent through the roof to take advantage of the stack effect and buoyancy. So, what happens if the natural conditions don’t permit smooth ventilation? That’s where power venting comes in handy!

A power vent uses a special fan to blow the gaseous byproducts of combustion from the furnace even when natural forces are against you. It’s the same way the furnace uses a fan to force fresh air into the unit through the return air plenum. Likewise, the power vent uses a fan to force exhaust gases out of the furnace.

The main application of power vents is in high-efficiency furnaces that produce cold, dense exhaust fumes. Venting dense fumes naturally is a significant challenge because cold air tends to settle around the floor. However, a power vent can quickly force the air out of the furnace.

High-Efficiency Furnace Venting Problems 

Unfortunately, high-efficiency furnace venting often presents a few challenges that installers and homeowners must keep in mind at all times for maximum safety and heating performance.

Obstructed air intake vents

An obstructed air intake vent in a direct vent two-pipe furnace is a major problem because the burners depend on air from outside the house to stay lit.

Therefore, the furnace cannot get oxygen-rich air for combustion if the intake vent is blocked. This can often result in backdrafts, resulting in poor burning, incomplete burning, or failed combustion.

Improper exhaust gas recirculation 

Improper direct vent system installation can also cause “short-circuiting” that may allow exhaust gases to flow back into the air intake pipe. This often happens when air intake and exhaust vents are installed too close together. As a result, exhaust gases may exit the exhaust pipe only to be drawn back into the home via the adjacent air intake pipe.

Clogged condensate drains 

High-efficiency furnaces produce tons of condensate, mostly made up of condensed water. This presents several challenges to efficient heating.

For instance, what happens if the condensate pipes are clogged? What about if the condensate pan is blocked? Additionally, the water can condense into ice on freezing winter days.

Pressure switch failure

Blocked vent pipes in high-efficiency furnaces may leave you dealing with frequent pressure switch failures.

Although you can avoid this problem with regular maintenance, the exhaust vent may occasionally become obstructed by bird droppings, pebbles, dust, soot buildup, etc. This will automatically trigger the pressure switch to shut off.

Power vent failure 

Finally, power vents can fail for various reasons, causing significant health and safety risks to you and your loved ones. The most common reason for failure is the lack of enough intake.

Lack of enough intake air can cause a slight vacuum inside the vent system, causing the motor to work harder in an already inhospitable environment, ultimately resulting in fan damage. Another potential problem is if the fan fails due to mold or dirt buildup.

How Do You Tell If Your Furnace is Venting Properly?

You can use two common signs to tell whether your high-efficiency furnace is venting correctly. First, monitor moisture levels in your home. You should not have moisture issues in the home if your furnace is venting correctly.

Secondly, monitor oxygen levels in the home. You can do this by installing a carbon monoxide sensor. A rise in carbon monoxide levels is a telltale sign that your furnace draws too much oxygen from your rooms, possibly because it’s not venting correctly.

FAQs about High-Efficiency Furnace Venting

Does a high-efficiency furnace need outside air? 

Yes. Fortunately, the architecture of high-efficiency furnaces means that they have their own fresh air intake systems that bring in fresh air from outside the house. 

How far can you vent a high-efficiency furnace? 

The maximum vertical distance that you can run a furnace exhaust is about 15 feet. Also, you need an additional three feet above any forced-air inlet within 10 feet of the exhaust vent.

Can a high-efficiency furnace be vented vertically? 

Yes. Depending on the venting type, a high-efficiency furnace has at least once and sometimes two PVC pipes exiting the home. These vents usually exit the home through the wall or roof.

Does a high-efficiency furnace need a cold air return? 

Not necessarily. High-efficiency furnaces draw intake air directly from the outside, meaning they don’t require a fresh air intake inside the room where the appliance is installed.

Can PVC be used to vent a furnace? 

Yes. Whereas traditional gas furnaces require metal vent pipes because of the high temperatures of the flue gases, high-efficiency furnace exhaust gases are much colder. Therefore, plastic pipes, specifically PVC, ABS, and CPVC, are permitted for high-efficiency furnace venting.

Can a high-efficiency furnace and water heater share a vent? 

Yes, technically. A high-efficiency furnace and water heater can share a vent with special adjustments to the venting system. However, not water tanks. A tankless water heater, for instance, cannot share a venting pipe with any other appliance.

Can a high-efficiency furnace be vented through a chimney? 

Unfortunately, no. A high-efficiency gas furnace must never be vented through a chimney. However, it’s permissible to use an abandoned chimney as a chase for a metal venting system, such as a B-vent.

What’s the difference between a direct vent and a power vent? 

The main difference between a direct vent and a power vent is that power vents use an electric-powered fan to propel the exhaust through the vent. In contrast, traditional direct vent systems rely on natural forces to remove exhaust gases from the furnace.


The two main ways to vent a high-efficiency furnace are through the roof and wall. However, you can also vent your 90%+ furnace using a concentric or power vent, provided you follow local building codes. Alternatively, just follow NFPA 211 Section 10 guidelines.