How to Choose the Right Venting for Your Fireplace

Getting Started

Before we get into the meat of the post, let’s take a look at the basics for needing to pick a new venting option.

I’ll break this down based on if you have a new fireplace or you’re keeping an old fireplace.

Whichever you have, it’s extremely important to make sure you know what you have because mistakes in the venting industry can be deadly when dealing with hearth products and vent pipes.

New Fireplace, Insert, or Stove

Every new vented gas fireplace, wood-burning fireplace, vented gas, or stove will have specific requirements for the proper venting components.

These come straight from the manufacturer. Specific brands or types of pipe apply to a particular category.

For example, you wouldn’t use the same pipe on a wood stove as you would for a vented gas fireplace.

Purchasing the right chimney components is essential, too.

A common misconception we hear is a contractor assuming that an existing chimney pipe can be used on a new model. Especially if the pipe diameter is already in place.

This is a rare occurrence since most old chimney and vent systems are no longer used with the new units.

It’s always safer to check with the manufacturer and see what their thoughts are.

Old Fireplace, Insert, or Stove

Older fireplaces and stoves may not have all the listed options or may require a particular brand to be used. 

So the ideal venting components will simply be determined by the size of the combustion chamber and the flue collar on the unit. 

If you’re trying to replace part of the venting system, you’ll need the same brand of pipe. This is usually pretty difficult and sometimes impossible if those parts are discontinued and no longer available. 

Chimney venting is not exactly collector material. 

The fireplace and stove manufacturing industry is filled with companies that have gone out of business.

A Warning

Whatever you do, you should always consult the owner’s manual before purchasing or installing venting components.

If you don’t have the manufacturer’s installation instructions, you can call the manufacturer and they’ll usually be able to provide that for you.

Misinformation and mistakes can be deadly when dealing with hearth products and vent pipes.

If a system doesn’t meet the manufacturer’s requirements, you could face carbon monoxide leaks and chimney fires.

Both can result in death, serious illness, and significant property damage.

Fireplace vs. Insert

A common point of confusion in the hearth industry is about the difference between a “fireplace” and an “insert.”

A fireplace is a manufactured unit that can be framed in a wooden enclosure, a wall, or even a mantel cabinet.

Whereas an insert is a model that must be installed inside an existing and operational fireplace.

The key difference, for the purpose of venting, is that the fireplace needs to have a dedicated chimney or vent system that can be located within 2 or 3 inches of combustible materials.

But an insert requires the use of a chimney liner that runs within an existing, functional, fully combustible chimney, such as a brick chimney with a terracotta flue or a steel pipe.

Vent Pipe Category

Each hearth appliance requires a particular type of vent pipe.

For example, a solid fuel appliance, such as a wood-burning or a coal-burning stove, uses Class A chimney pipe, which is made to withstand the higher flue gas temperatures.

Direct vent fireplaces on the other hand use natural gas or propane as their fuel and the exhaust fumes aren’t nearly as hot. Therefore, the direct vent gas pipe used on these units can be an aluminum inner wall.

Class A Chimney Pipe

A Class A chimney pipe is often referred to as a double-wall chimney pipe, a triple-wall chimney pipe, all-fuel pipe, or insulated chimney pipes.

Class A pipes are used to vent high-temperature exhaust released from wood, coal, and oil-burning appliances, such as fireplaces, stoves, boilers, and furnaces.

Stainless Steel Chimney Liner

Stainless steel chimney liners can come either rigid or flexible and are used to repair or replace an existing chimney flue that has been compromised.

It’s commonly used inside a masonry chimney when the clay tiles have cracked or mortar joints have deteriorated.

Stainless steel liners are also used in masonry chimneys that were simply built without flue tiles.

They’re the best material for the application so most liners come with a lifetime warranty.

Direct Vent Gas Pipe

A direct vent pipe is made specifically for a direct vent gas appliance.

The term ”direct vent” actually refers to an appliance that uses only outside air for combustion and vents all exhaust directly back outside.

The appliance is sealed off from the room by a glass door that should never be opened while the unit is in use.

Type B Gas Pipe

A B-vent fireplace is also called a Type B Gas Pipe or a Natural Vent. It’s a prefabricated, double-wall metal pipe used to vent gas appliances listed for use with Type B gas vents.

Type B pipes are becoming less prevalent for venting hearth products but many furnaces and water heaters still use them.

Type L Pellet Vent Pipe

Type L vents are designed for venting oil-fired and natural gas appliances that produce flue gasses that don’t exceed 570 degrees F for 10 minutes in an over-fire situation.

The minimum combustible material is generally 3 inches.

Type L vents should not be used to vent coal or wood-fired appliances.

Clearance to Combustibles

In any hearth appliance installation, for the unit itself and the chimney system, maintaining the proper clearances to combustibles is the most important aspect.

With regard to the chimney, the different types of vents have varying clearances.

Class A Chimneys usually have a 2-inch clearance to combustibles and a Type B gas vent normally has only a 1-inch clearance.

The vent pipe for a direct vent fireplace is unique because it has a 1-inch clearance when it runs vertically. But the clearance varies when it runs horizontally from 1-inch up to 3-inches, due to the hot exhaust fumes rising within the vent.

Flue Diameter

The diameter of the vent pipe used is also very specific to each model.

Every fireplace, stove, and insert will use a pipe or liner that will need to be sized according to the diameter of the flue collars on the unit.

You should know that verifying the diameter needed should never be the only consideration when looking for vent pipe because the components used must fall within the correct category as listed above.

A common mistake is measuring the flue diameter without verifying the proper pipe category listed and the compatible components.

We’ve seen this a lot when other contractors or homeowners try to retrofit new parts to an old chimney system.

Never mix and match chimney sections of different brand manufacturers, even if the diameter of the pipes match.

Different types and brands of pipe use proprietary connections. So it’ll be nearly impossible to properly connect two different kinds of vent pipes.

Additionally, you shouldn’t just “make it work” by crimping, sealing, or welding two pipes together.

Flue pipe increasers and decreasers, which are components that change the flue diameter are never recommended unless absolutely necessary or if they’re being used for a particular configuration.

Chimney Height

The overall height of the venting system is another factor to be considered for fireplaces that will terminate vertically over the roof.

Each fireplace, stove, and insert will have a listed minimum and maximum chimney height based on the unit’s need for sufficient draft.

Therefore, even if the manufacturer’s requirements for a vertical termination are met, the minimum chimney height must be satisfied for the proper operation of the appliance.

Offsets/Elbows/Horizontal Runs

Each appliance will also have guidelines dictating the acceptable offsets allowed within the chimney system. Just like with the vent pipe category and flue diameter.

For example, Class A Chimneys don’t allow for elbows greater than 30-degrees, and the length of each offset is limited according to the model.

Elbows can’t be combined, either, to create larger offsets.

The only exception to this is with freestanding wood stoves or coal stoves that use a chimney connector (AKA a stovepipe) that runs horizontally through a wall. They can connect to a Class A tee, which serves as a 90-degree elbow to turn the chimney vertically.

Vented gas fireplaces are much more tolerant of offsets.

But there are restrictions listed within each model’s installation manual regarding the permissible offset lengths, the number of elbows used, and the length of horizontal runs.

Most of the time, vented gas fireplaces allow the use of 90-degree, 45-degree, and 30-degree elbows.

Plus, most gas fireplace vent systems allow the use of flexible vent pipes, which can create any offset from 0-90 degrees.

Vent Terminations and Chimney Caps

Every hearth appliance also has restrictions and requirements about where and how the chimney or vent system ends.

All solid-fuel fireplaces and stoves must terminate above the roof, following the 3-2-10 rule.

This rule is that the chimney must be at least three feet above the roof at the point of penetration. Two feet above any obstruction, and within a 10-foot radius from the top of the chimney.

This doesn’t apply to vented gas fireplaces.

B-vent fireplaces must terminate over the roof like a wood-burning unit. But the height needed above the roof can be as little as one foot. It all depends on the model’s requirements and the roof pitch.

Direct vent fireplaces, on the other hand, can utilize a vent system that either terminates vertically over the roof or horizontally on an exterior wall. This depends on the model’s requirements, too.

At the very end of the vent systems, a proper termination cap must be used.

Class A chimney pipes have their own caps that are specifically designed for a particular brand and diameter.

While there are universal caps available for these chimneys, the use of anything other than what is approved by the manufacturer could void the warranty of the appliance or chimney components.

However, in the case of old chimneys, when a cap needs to be replaced and the replacement no longer exists, you’ll need to use a universal retrofit option.

Direct vent fireplaces, absolutely need to have the proper termination component as each one is designed to allow the intake of combustion air while safely removing the exhaust from the system.

Different caps are used for vertical and horizontal terminations, with both types having several different options to choose from for aesthetic purposes.