Q: I just moved into my new home and it has a full masonry fireplace. It’s 55 inches wide and has an arched opening that’s 33.5 inches at its peak. It looks like regular brick mortar was used with the firebrick. For whatever reason, it has two flues and a damper on each one. It smokes like an old steam locomotive, sending clouds of choking smoke into the house! Cracking a window helps but doesn’t solve the problem. What’s wrong?

A: When I opened this email and saw the homeowner’s fireplace photo, I was flooded with a range of emotions: frustration, sorrow, mild anger and sympathy were at the top of the list. It breaks my heart to see this, knowing that it’s so easy to build things, even fireplaces, that work perfectly. The science of how fireplaces and chimneys work has been known for hundreds of years.

I’m sorry to say that this is the absolute worst fireplace job I’ve ever seen. It was destined to fail from the get-go.

The Brick Industry Association has published easy-to-understand information for decades detailing how to build a residential wood-burning fireplace that will never smoke. They cover sizing requirements, including how to size the flue properly to match the height of the chimney.

It’s important to understand that the size and shape of the firebox are directly related to the width and height of the opening to the fireplace. Unfortunately, this mason created a firebox that had two short stub walls that project back into the firebox, then go off at 45-degree angles to meet at a hard 90-degree corner in the rear. It’s no wonder smoke billowed into the house with this unbalanced design.

The shape and size of the firebox is critical, but what you don’t see above the firebox is even more important. The shape and size of the throat of the fireplace, as well as the design of the smoke shelf, will ensure that all the smoke and hot gases go up the chimney rather than into the room.

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Another common mistake in fireplace construction is the use of ordinary brick mortar inside the firebox. The firebrick that should be used in a firebox is a special refractory style that is meant for high temperatures. The temperature in a roaring fire can exceed 1,000 degrees, and normal brick mortar is not made to withstand that much of an increase from room temperature.

Fireclay is the best material to use to create the thin joints between the firebrick. It’s a special fine clay that is mixed with water to the consistency of a thick gravy. Skilled masons carefully dip the edge of a firebrick into the mixture to apply the perfect amount. The joint between the firebrick usually ends up being no wider than 1/16-inch.

With the wet fireclay on the edge of a firebrick, the mason then presses it against another brick, which creates a fireproof bond. This combination will easily withstand thousands of fires over decades of use without crumbling, as ordinary brick mortar will.

Tim Carter has worked as a home-improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.