Danger – Do you smell smoke in the house when you use your fireplace or stove? If you do, and you have more than one flue in the same chimney, please read this carefully!

smoke crossover illustration

5.17.  Masonry Institute of America –

“The pouring of smoke from one flue to an adjacent flue occurs where downdraft from interior suction or vertical wind currents force smoke down an inactive flue as it exhausts from the adjoining flue”

5.14.  Masonry Institute of America –

“The top of a flue should have a height differential…….as much as 12 inches to prevent smoke from pouring from one flue into the other flue”

The Masonry Institute of America, which is the #1 source for published information on Masonry, states that adjacent flues should have a 4″ to 12″ in difference in vertical height, as shown above. The reason is to avoid smoke from exiting one flue and then being drawn down into the living area by an unused flue which is downdrafting. This is a common occurrence and may result in multiple problems…….from smoky smells in the house to actual poisoning of the residents!

Yet most chimneys do not meet this standard. To make matters worse, the installation of a chimney cap – whether two single caps or one multi-flue cap – can make the situation much worse.

Quotes from those who know!

“Flow Reversal or Down drafting chimney — This is the most common problem I have run into in my 20 years of business. A typical phone call comes in from a customer saying “I’m trying to light my stove and the whole house is filling up with smoke” – Craig Issod, Publisher of Hearth.com

“I have a problem with reverse flow on my chimney when I crank down my wood stove. The smoke comes down the other flue for the fireplace even though I have a damper cap on top” – Todd W, Chippewa, WI.

Multiple Flues and Downdraft Hazard – Separation of 4″ to 12″ in Vertical Height

Millions of multiple flue chimneys exist that are not up to current masonry standards – which calls for a separation of 4 to 12 inches in vertical height for each flue. Our patented products solve multiple problems including:

The Boost-a-Cap Solution! The patented stainless steel Boost-a-Cap increases the vertical height by 12 inches, preventing smoke crossover and allowing for the proper installation of two or more chimney caps!

The Boost-A-Cap is available in 3 sizes: 8 x 8, 8 x 12 and 12 x 12. The chimney caps are not included, but can be purchased from us at discounted prices. If you already have a chimney cap, it should fit. 

Typical comments from customers who purchase and install a Boost-A-Cap:

“I received the Boost-a-Cap on Wednesday. Great service and incredibly fast delivery. Exactly what we were looking for. It was easy to install and solved our smoke problem – – and for a very reasonable price! Terrific!”

“I just wanted to drop a quick note to outline my experience with your product.  Last year I had three new chimney caps installed. You can imagine my surprise the first time I fired up my upstairs fireplace to find smoke coming out my downstairs fireplace and from around my hot water heater. Upon searching the internet, I came across your site which outlines why this problem occurs (three chimney caps of the same height). I purchased the Boost-A-Cap product and followed the included direction (very simple) and installed the Boost-a-Cap myself. I am happy to say the problem has been solved and I am once again able to use all my fireplaces safely. So again thank you for the great product. I will definitely recommend your product to all my neighbors!”

More Information:

Video Demonstration

Document #1 from Masonry Institute of America
Document #2 from Masonry Institute of America
MIA Web Site Masonry Institute of America Web Site

Note:  Our testing has shown that the 4″ minimum is not enough and that a separation of at least 7″ is required to properly address smoke crossover. The Boost-a-Cap was designed to provide a 12 inch height difference so as to prevent smoke crossover and to allow for the proper installation of multiple chimney caps.