How to Guide: Using Flexible Venting for a Direct Vent Fireplace

 Napoleon's Freedom Flex venting can simplify gas fireplace installations and reduce costs compared to rigid venting systems.  I have included lots of links throughout this post to make finding the right parts easier. 

What is flexible venting?

Manufactured by Napoleon a.k.a. Wolf Steel, flexible direct vent piping consists of an inner exhaust pipe, usually 4" or 5", and an outer intake pipe, usually 7" or 8", made of rolled aluminum.  The flex pipes are available in 5 ft or 10 ft lengths, compressed to 20" and 40", respectively.  Also included are spacers to make sure the inner pipe stays centered within the outer pipe to provide appropriate clearances.  From here on out we'll call the two common sizes 4x7 and 5x8, indicating the inner and outer pipe size. 

Take a look at the Freedom Flex Brochure to see all the different available components.

What size piping do I need?

That depends on the model of the fireplace you are looking to install.  For example, the Napoleon BL46 Linear gas fireplace uses the smaller 4x7 vent size, while the Napoleon GX70 requires the 5x8 vent size.  The reason for the difference is based on the design of the fireplace and the BTU's.  A larger burner needs a greater amount of intake air as well as exhaust air. Check the installation manual of the fireplace you are interested in to find out the correct size venting and to note any restrictions.  There are some fireplaces available which have the starting collar for a 5x8 vent size, but can be reduced to 4x7 venting with the appropriate adapter.  

Most common installations

The Straight Back

Certain fireplaces have a rear vent option.  For a fireplace with a rear vent, installed on an exterior wall, you can use the Rear Vent Kit.  This kit includes only 20" of flex venting and a horizontal termination.  The reason for the short length is that if the vent was any longer than 20", there would need to be a vertical length.  Again, refer to the specific fireplace manual for venting design options. 

The Up and Out

Probably the most common venting installation and often the easiest.  This option is used with a top vent fireplace on an exterior wall.  The vent is run vertical for a certain length and then bent to pass through the exterior wall and connect to the horizontal termination.  There is a 4x7 Top Vent Kit and a 5x8 Top Vent Kit.  Each kit contains 5 ft of flex venting and a horizontal termination.  What makes this installation the easiest is that you have 5 ft of flex venting to work with and can easily avoid wall studs, wires or exterior elements.

The Roof 

Also very common, for fireplaces not installed on an exterior wall or for situations where a horizontal termination on the outside wall is undesirable.  Determine the full distance from where the top of the fireplace will be to the underside of the roof sheathing, including any offsets or bends, then add a few feet for ease of installation.  Let's say you have a fireplace being installed on a two story house with an attic.  The fireplace is being installed on the first floor with the top of the fireplace at about 4 ft off the floor, each floor is 9 ft including the joist spaces and there's 6 ft in the attic.  Without offsets we would need at least 20 ft of venting and may be able to get away with two 10 ft lengths.  I suggest instead getting the two 10 ft lengths plus a 5 ft length, this way you are sure to have enough to complete the job in one day rather than realizing too late that there is an offset or bend you didn't account for and need to purchase additional venting.  

For the vertical termination you will need to determine the pitch of your roof, expressed in inches of rise over 12 inches of run.  For example, if your roof rises 4 inches vertically every 12 inches horizontally, then you have a 4/12 pitch roof.  There are three roof kits available for different pitch roofs.  A low pitch roof kit, for roofs between 1/12 and 6/12 pitch, a high pitch roof kit, for roofs between 7/12 and 12/12 pitch roofs and a flat roof kit.  Each kit contains a 23 3/4" section of rigid pipe, a high wind vertical termination, storm collar, flashing and mounting hardware.  You can see all of the available kits by clicking here. 

Clearances

One of the most important steps in the installation process is making sure clearances to combustibles are maintained throughout.  The most common clearance for direct vent components to combustible materials is 1" on every side.  Please note that this does not apply to all fireplaces and situations.  Some fireplaces and their respective vent components require 3" clearance from the top side of a horizontal pipe and 2" from the side and bottom of the pipe.  Again, reference the manual specific to the fireplace for specific clearance requirements.  

Support

Flex venting is required to be supported every 3 ft whether installed vertically or horizontally. This is to prevent sagging in horizontal installations and for weight distribution in vertical installations.  The easiest way to support flex venting is with the appropriate wall support available for 4x7 vent or 5x8 vent.  Another alternative is to use perforated metal strap or "plumbers tape" available at hardware stores.  It is okay to use short self piercing screws to attach the metal strap to the 7" or 8" pipe as this is the air intake.  

Sealant

Each connection or joint needs to be sealed with a high temperature silicone sealant, such as Napoleon's Millpac. A small bead of sealant around the male section of the exhaust and intake will do.  

Couplers

When your venting installation will need more than a single length of venting, you can connect them with couplers, available in 4x7, 5x8 and other widths.  The couplers consist of two pieces a connector for the inner pipe and a connector for the outer pipe.  Both get a bead of sealant and are attached with short, self piercing screws.  

Firestops

Firestops are used whenever passing through an exterior wall or from one floor to the next.  As the name implies, firestops are designed to slow a fire moving from one floor to the next.  A firestop is basically a piece of sheet metal with a 7" or 8" hole cut in it.  Let's say a 4x7 section of vent is passing through from one floor to the next.  Since we need at least 1" clearance on all sides to combustibles, the easiest way to do this is through a 9" x 9" square.  The firestop is secured to the wood of the square and the venting passes through.  It is also important to apply high temperature sealant to the vent where it passes through the firestop to seal all gaps.  Every Napoleon direct vent gas fireplace includes one firestop.  

Important takeaways

• Read the manual for the model you are installing
• Size the vent pipe according to the model's installation manual
• Plan ahead, check for potential obstacles and clearance requirements
• Maintain required clearances throughout the vent route
• Seal every joint with high temperature sealant
• Support the vent to prevent sagging
• Install firestops wherever vent passes through a wall or floor
• This is just a basic guide for vent installations, there may be additional requirements
• We're here to help, call or email with any questions

I know there are a lot of factors to consider, particularly for more complex venting installations, but not to worry.  We are happy to help design venting systems specific to your house and fireplace model, just let us know.  

Thanks for reading our blog, let us know how we're doing or if there's a blog post you'd like to see written in the comments section below!

Geoff