How Long Can A Furnace Vent Pipe Be?

Furnaces burn fuels like propane or natural gas to operate. The unit releases carbon monoxide and other toxic gasses when the fuel burns. It will need an exhaust or furnace vent with enough length and diameter to release these gasses properly. 

The standard length of the furnace vent pipe is 15 feet. The maximum length can be 150 feet vertical and 50 feet horizontal. The vent pipe should have a vent angle of 1.25 inches per foot to pull out the hot air from the bottom. However, the length can vary depending on the size of the furnace.

Knowing the right size and length of the vent pipe is vital as it helps pull the toxic gasses out of the furnace. This guide will share with you the importance of the right vent pipe length and also help you know the different types of vent and vent pipes.

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What is a furnace vent?

The furnaces and other heating systems burn fuel, such as propane or natural gas. 

When the fuel burns, it releases harmful carbon monoxide and many toxic combustion gasses and substances. 

The gas will need to be released from the furnace; that is when you need the furnace vent pipe to let the gasses escape. 

The furnace vent pipe is responsible for exhausting combustion gases from the furnace. 

The vents come in various shapes, types, and sizes.

The first one is a flue that is a pipe. 

The pipe goes up through the roof and out of the chimney or through the walls to the side of the house. 

The flue helps the exhaust gasses to travel through and get out of it. 

The second furnace vent is the draft fan. 

It is a fan found in the furnace close to the heat exchange. 

The draft fan takes the hot air out and pushes it out of the furnace. 

How long should the furnace vent pipe be?

When it comes to fixing the vent pipe in the furnace, the length and size of the pipe are very important. 

A proper pipe length and size will allow the furnace to release the toxic gasses without any delay or clogging.

Finding the right vent size for the furnace is quite critical. 

It is critically determined by the National Fire Protection Agency (NFPA) and the National Fuel Gas Code (NDGC). 

You must check out both resources before deciding on a particular vent length and size. 

The size can change depending on the furnace type and size in BTUs. 

The right diameter of the vent pipe depends on the distance between the exhaust outlet and the exit point on the roof or wall. 

So, you need to get the right size based on horizontal and vertical distance. 

Generally, it is suggested to use a vent pipe around 15 feet in the vertical direction. 

But, it will change based on factors. 

15 feet is considered the standard vent pipe length.

If you use a one-pipe system, the vent pipe length of the furnace should be 5 feet.

In the case of a two-pipe system, you need a 10 feet long furnace vent pipe. 

The maximum length of a vent pipe can be up to 150 feet, 100 feet vertically, and 50 feet horizontally. 

The diameter of the vent pipe should measure around 18 to 24 inches. The minimum size should be 3 inches. 

The minimum measurement is because of the small furnaces up to 78k BTUs and 46k BTUs. 

The maximum vent pipe diameter is 24 inches. 

It is generally applied to the Type B double-type vents connected to the single Category One gas furnace. 

The maximum diameter of the vent pipe also applies only to the natural vent furnaces rated around 8,100,000 BTUs or draft-induced furnaces around 752,000 to 13,354,000 BTUs. 

What happens if the vent pipe length is too small?

The vent pipe for the furnace should not be very small. 

As you know, the furnace will need to release toxic carbon monoxide and other harmful gasses. 

So to release these toxic substances, it needs a vent pipe. 

The pipe’s passageway must be big and long enough to allow the gasses to escape properly. 

Suppose the pathway in the vent pipe is very small compared to what the gasses need. 

In that case, the vent motor inside the furnace cannot generate enough pressure for the exhaust of the flue gasses. 

It will get clogged due to too much gas, preventing it from escaping. 

Even if it does escape, the toxins will take a lot of time to get out through the pipe. 

As a result, the furnace will trigger a safety pressure switch which will shut down the furnace. 

So, you need to ensure that the vent pipe is long enough and big in diameter so that the gasses can escape easily without clogging. 

What happens if the vent pipe is too big or too long?

If the vent pipe of your furnace is too big or too long, the pipe will take forever to move the air out of the unit. 

Do not use very long or very big pipes. 

15 feet for a vertical length measurement is the standard size. 

But, it can change based on the furnace size. 

The maximum length should be 100-150 feet vertical and 50 feet horizontal. 

It will help the toxic substances get out of the pipe properly without being too late. 

Should the furnace vent be sloped?

The vent pipe can be slightly sloped if you want the venting system to work properly. 

The slope in the vent pipe is called a vent angle. 

The vent angles can vary depending on the position of the vent. 

The standard size of the vent angle should be around 1.25 inches per foot, leaning down to the furnace location. 

The vent angle gives a good drainage angle and makes a strong updraft to pull the hot air from the furnace’s bottom. 

The water in the venting system can cause several issues. 

But if you narrow down the vent system, it can help. 

The exhaust flow will slow down if you narrow down the vent system. 

For this, the pressure inside the furnace will reduce. 

When the pressure gets very low, the safety pressure of the furnace will shut down the system. 

How do you calculate the vent pipe for the furnace?

Calculating the furnace vent pipe depends on the unit and the exit point of the vent outside the building.

The vents are attached to a flange on the top or back of the furnace. 

The unit runs through the roof or outside wall. 

Consult your local building department to find the proper vent exit point. 

From that point, you have to start calculating the piping and fittings. 

Venting through the roof 

  • Measure the flange’s diameter at the top of the furnace to get the diameter of the vent pipes and collars, roof flashing, and other offset fittings. Measure the distance from the furnace top to the ceiling above the furnace’s position. Note the measurements and include the duct collar for the hole in the ceiling. 
  • Get to the attic using a stepladder or stairs. Walk on the upper edges of the ceiling joists and reach the area above the furnace’s position. 
  • Now measure the vertical distance from the upper side of the ceiling to the downside of the roof sheathing. Add 36 inches for clearance above the roof. Note the quantity of vent pipe required in the attic. Add a roof flashing and pipe strap measurement to secure the pipe at the side of the rafter. 
  • Determine the 45° offset fitting numbers needed to install the vent pipes. Avoid touching the framing support and AC ducts. Install a long vent section at a 45° angle to get the necessary clearance. Add another 45° offset at the upper end of the pipe section to transition vertically and visit via the roof. Since vent pipes cannot be within a 6-inch of wood framing and AC ducts, you must install the long vent. 

Venting through the outside wall

  • Measure the diameter of the circular flange where the vent is attached to the back of the furnace. It will tell you the size of the vent pipe, fittings, and flashings required to install in the vent system. 
  • Note the clearance distance between the furnace and the back wall at the heater closet, garage, or basement wall based on the manufacturer’s suggestions. 
  • Now measure the width across the face of the door jamb or window frame near the wall where you will install the vent. Add the clearance with the thickness of the wall to find the amount of vent pipe needed. Also include the flashing to install outside the wall and the vent collar to insulate the vent pipe passing through the wall. 

Different types of vents

There are different types of vents for different furnaces, like gas, high-efficiency, etc. 

Types of vents for furnaces

If you want to vent a gas furnace, there are three options – 

  • Natural venting
  • Sidewall venting
  • Concentric venting

Natural venting is a B-type being and was a common method in the old days. 

You must attach a natural vent pipe to the furnace and run it vertically through the roof.

In that way, the hot combustion fumes can rise easily and exit through your roof. 

Most natural vents are built in the existing house chimneys. 

However, you can still install one through-the-roof vent without a chimney or an outside chimney. 

Sidewall venting uses 1 or 2 pipes to run through the exterior walls, bring fresh air for combustion in the furnace, and expel dangerous gasses out of the furnace. 

Sidewall venting systems that use one pipe are called indirect venting systems. 

In contrast, those using two pipes are called direct venting systems. 

Concentric venting systems give a compact room-sealed flue, where the flue gasses and air supply are managed inside the single concentric ducting system.

The system is a duct-within-a-duct style where the flue gasses escape with the smaller inner duct, and fresh air can enter for combustion from the wide outer annulus. 

The system is either installed horizontally and passed through the outer walls or vertically and vented through the roof. 

Vents for a high-efficiency furnace 

These vents use the sidewall venting system where two pipes are used, like the intake and exhaust pipes. 

One pipe is used as a direct vent, and another as an indirect vent. 

If you need clarification, consult ht eNFPA and UMC for clear guidance. 

Different category appliances are vented in different ways:

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  • The category one appliances, like the old-fashioned gravity vent and induced draft furnaces, are vented vertically through the roof or chimney. 
  • The category two and three appliances, like the boilers, vented water heaters, and tankless water heaters, are vented as per the manufacturer’s guidelines horizontally through the sidewalls. 
  • The category four appliances include high-efficiency gas furnaces and water heaters vented horizontally through the sidewall.

Types of vent pipes for furnaces

There are multiple vent pipe options for the furnaces. 

Maximum furnaces use vent pipe types from the following options:

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

Out of so many options, copper, iron, and galvanized steel pipes are the most common pipe materials used for venting.

The exhaust fumes from the traditional vents are hot enough and exceed 460°F. 

Modern furnaces use plastic pipes because these furnaces produce cool fumes, thus reducing the chances of melting. 

So, plastic pipes are great for category IV appliances as they are watertight and airtight. 

Besides, plastic pipes are cheaper than others.

Can I use PVC pipes for furnace vents?

The vent pipes of the furnace connect the unit to the outside. 

The pipes should prevent the air from accumulating in the furnace. 

PVC (polyvinyl chloride) pipe is not rated for heat transfer and can melt easily and cause leaks. 

However, some contractors use PVC pipes for the furnace venting. 

But, it is better to use something other than PVC pipes. 

The biggest problem with these pipes is the phthalates. 

These are a class of chemical compounds used to make plastic flexible. 

Inside the furnace, these chemical compounds can drain into the air due to high temperatures and become cancer-causing agents. 

So, PVC is not considered safe for venting pipes in the furnace. 

Furnace vent replacement 

At some point, you might have to replace the furnace. 

Though it takes time, it is necessary to know the different things for the replacement. 

Research said that gas furnaces are the most efficient for home heating during cold weather. 

But, they do not properly release combustion exhaust fumes containing carbon monoxide. 

Here are some good furnace vent options for replacement based on different vent types:

Concentric venting

  • Diversitech CVENT-2 Concentric Vent
  • Rheem SP20245 Concentric Vent

Sidewall venting

  • Tjernlund SS1 Oil Side Wall Vent
  • Vent HoodVH1-6 Vent

Final thoughts

The vent pipe helps expel the furnace’s toxic gas and carbon monoxide. 

Allowing these gasses to be expelled will keep the furnace good. 

The standard size of the vent pipe should be around 15 feet at a vertical distance. 

But, the pipe length can reach 150 feet, 100 feet vertically, and 50 feet horizontally. 

The length also depends on the furnace type. 

The vent pipe length should be 5 feet if you use a one-pipe system and 10 feet long for a two-pipe system. 

There are multiple vent types natural, concentric, and sidewall. 

The pipe length will differ based on the type. 

Related Articles:


Should I insulate the vent pipes?

Vents in the HVAC system allow warm air to escape and allow cold air to enter the system. 

The leaky vents can make the hot air escape too much with some cold air. 

You need a thermostat to adjust to the situation. 

It is best to insulate the vents to avoid the pipe from losing too much air. 

How to know if the furnace is working properly or not?

The furnace’s job is to heat your house. 

The furnace must be vented outside to prevent moisture from entering the house. 

If there is moisture condensation on your windows, the furnace is not working properly. 

Your house will feel cold, damp, and musty. 

It is because the furnace is not properly venting. 

Reference: Furnace Science Direct, Effects of HVAC on combustion-gas transport in residential structures, HVAC System and Part of Indoor Air Quality.

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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