DON'T Just "Do it" - Placing a wood stove into and existing masonry chimney could be hazardous to your health and house. Read this to learn more.

One of our biggest problems occurs when a customer enters the shop and states "I have an existing woodstove into a masonry chimney - I'd like to replace the stove with a more modern one".

In the old days, we thought this was a gravy job...these days I wonder if I should send them packing....why? because:

1. 90%+ or more of existing masonry wall-pass thrus are wrong

2. It is often difficult confirming or repairing these installations.

What to do? Most customers state "well, it's worked for 10 years". and look at us like we're trying to take advantage of them by being concerned about their safety. However, I can assure the public that these "wall pass-thrus" are responsible for a LARGE percentage of house fires resulting from woodstoves. Let's talk about the right ways and wrong ways to pass through a wall into an outside chimney. Most important - DO NOT ASSUME THAT YOUR WALL PASS-THRU IS OK - MOST ARE NOT.

The Problems:

1. Most, if not all, masons and building inspectors are not aware of the current standards.

2. Most existing wall pass-thrus do not meet these standards.

3. It is difficult to inspect these installations, since the pass-thru is often covered by the brick veneer on the inside of the fireplace.

Discussion of the solutions:

Most older chimneys use a "crock" , which is simply a piece of round terra cotta tile (similar to your chimney liner) which is cemented into the wall. The material that this crock is made from has little or no insulation value, which means the heat from the stovepipe is passed through to adjacent wall materials such as 2x4's, paneling and the kraft paper on wall insulation. Our installation crew has found wood studs as close as 2" away from this crock - many times they are charred! The only reason that the wall didn't catch fire yet is the lack of oxygen into the wall cavity to support the fire - not the type of close call that most homeowners welcome. If you do intend to use a terra cotta crock as your wall pass-thru, then follow the guidelines below.

These requirements have been specified by the NFPA (National Fire Protection Association) , and are accepted by most code officials.

An alternate method is also mentioned in the NFPA guide. See the drawing below:

In this case, a section of insulated, double wall chimney is used instead of the crock...and an airspace is used around it instead of the solid brick wall patch.

Another method is to use some specially manufactured, UL listed wall pass through devices that are available from many chimney sweeps and installation specialists. For a professional near you, check out the following links:
HearthPro...certified installers at:
Chimney Safety Institute of America at:

In any case, remember the following rules:

Back to HearthNets Information Section