What is a Flue?


A chimney “Flue” refers to the inner liner of a chimney, which carries the smoke out of the house. In the case of masonry chimneys, most flues are made of fired clay, which is a terra-cotta or buff color. If you have a factory built pre-fabricated (“pre-fab”) fireplace, the inner liner (flue) is made of high temperature steel.

What are the most common Flue problems?

Lack of draft, smoky smell in the house, downdrafts, backpuffs and inferior fire performance. Most problems which are blamed on a stove or fireplace are actually Flue problems.

Which of these problems can Extend-a-Flue help with?

All of the problems mentioned in the last question!

Can your products help solve problems with factory built pre-fabricated fireplace flues?

No, our products are for masonry chimneys only.

I have more than one flue in the same chimney. When I build a fire, smoke is pulled down the adjacent flue. How can Extend-a-Flue help?

smoke crossover illustration

Smoke Crossover

Negative pressure within the home and / or vertical wind currents can force smoke from the top of one chimney flue to be sucked down an adjacent, unused flue. This is a common occurrence and may result in multiple problems . . . . from smoky smells in the house to actual poisoning of the residents! And, the installation of a chimney cap – whether two single caps or one multi-flue cap – can actually make the situation worse!


Masonry Institute of America Recommendation

The Masonry Institute of America, which is the #1 source for published information on masonry, states that adjacent flues should have a vertical height difference of between 4 to 12 inches. This prevents smoke from exiting one flue and being drawn down into the living area by an adjacent, unused flue which is down-drafting.


Our Boost-a-Cap provides a cost effective solution to increase the vertical height of one flue by 12 inches over an adjacent flue and allows for the proper installation of two or more chimney caps next to each other when the flues are close together.




Proper Topper and Matching Decorative Chimney Cap

For a more decorative look, our Proper Topper and matching decorative chimney cap not only look good, they bring your chimney up to spec and prevent down-drafting and smoke crossover. Made from cast aluminum and stainless steel, the Proper Topper and matching cap provide a stylish and unique look.

My flues are so close together, my chimney caps won't fit. What can I do?

Some homeowners use a multi-chimney cap to cover all the flues. This can promote smoke crossover. The Boost-a-Cap is designed to prevent smoke crossover and to also allow two or more chimney caps to fit when the flues are too close together.

Boost-a-Cap problem issustration

Flues too close together



My flue is level with the top of the chimney crown, so I can't attach my chimney cap. Can I use a Boost-a-Cap?

Yes! The Boost-a-Cap is inserted down into the flue and expands outward creating a tight fit. Therefore, even if your flue has been broken or cut-ofBrick-Chimney---No-Fluef even with the top of the chimney crown, the Boost-a-Cap will still work!

I think my chimney is too short because it doesn't draw properly. I get smoke and smoky smells in the house. How high should my chimney be?

Improper or inadequate draw of combustion exhaust gas and smoke can cause major safety problems. The flue’s job is to carry these gases out of the house. If the chimney is not tall enough, or if external structures are too close to the opening of the chimney flue, the chimney cannot do its job, smoke and exhaust gases are released into the house, and a hazardous condition is created.

Minimum Chimney Height Requirements

Minimum Chimney Height Requirement

Local building and fire codes require that a chimney be at least 2 feet taller than any structure within 10 feet of it and also at least 3 feet higher than the point at which it penetrates the roof. This is called the 3-2-10 rule and is a minimum requirement. A chimney that does not meet this minimum requirement is more likely to cause a roof fire from flying embers and less likely to draw well. Taller chimneys produce a stronger draft, so a properly designed chimney should have a minimum total height of at least 15 feet.


Installing an Extend-a-Flue Cast Classic CC1212 on a short chimney

Extend-a-Flue provides a decorative and cost effective means of extending your chimney. By adding additional height, Extend-a-Flue chimney pots create a better draft AND can help your flue outlet rise above certain wind and roof air currents that cause problems. Because Extend-a-Flue can be purchased with an insulated stainless steel inner liner, it keeps the flue gases warmer – warmer air rises faster. Our units feature specially designed caps which are proven to help with wind related problems such as down drafts. Our caps also keep unwanted moisture, leaves, sticks and other debris out of the chimney.

Extend-a-Flue products look like they're made of clay. What's the difference between a heavy clay chimney pot and Extend-a-Flue products?

Clay chimney pots are extremely heavy and weigh as much as 300+ pounds. A lifting crane and scaffolding is required to safely install. Clay chimney pots are not designed for retrofit (no fastening method) and therefore are best installed when the chimney is first built.

Extend-a-Flue products are made of lightweight and durable cast aluminum. Our largest and tallest products weigh less than 50 pounds. If the optional stainless steel EZ Mount & Liner System is purchased, our chimney pots can be installed in less than an hour with no special tools or masonry work required.


Installing a Cast Classic CC1212 with the patented EZ Mount & Liner System


Are there other ways besides Extend-a-Flue to safely extend my masonry chimney?

Here are the other safe options:

1.  Hire a qualified mason to extend your chimney with brick or block and clay flue liners. Depending on chimney height and style, this can be quite costly and also difficult to match existing brick.

2.  Install a clay chimney pot on top of your chimney. These units are extremely heavy and weigh as much as 300+ pounds. A lifting crane and scaffolding is required to safely install. Clay chimney pots are not designed for retrofit (no fastening method) and therefore are best installed when the chimney is first built.

Are there any Flue problems that Extend-a-Flue cannot solve?

Some, however we now offer other innovative products at competitive prices that will virtually help you solve any chimney related issue.

So if you have a question, call or write us. We’ll be glad to help!