Back to Back masonry fireplaces

Old 10-08-12, 09:31 PM
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Back to Back masonry fireplaces

Building a back to back fireplace - both masonry. One is on an outdoor porch, the other in a master bedroom. I've designed it where each side has it's own damper, smoke chamber, smoke shelf, flue, etc. Basically like two separate fireplaces where the flues just happen to wind up in the same chimney, but smoke from each fire would be completely separate all the way to the top of the chimney. I have a mason who is telling me that you can basically have both dampers open into the same smoke chamber/smoke shelf area, then if sized correctly, we can even share the same flue - and that he has done this multiple times without issue. This would save some money I suppose, but i feel like it could be a source of problems. Am I overthinking this? I can find information all day long on how to design fireplaces, but nothing that tells me about this particular situation with two fireboxes back to back. What's the right way? No one in this area builds masonry fireplaces anymore, so I'm having a hard time figuring it out.
Old 10-09-12, 06:25 AM
Join Date: Oct 2008
Location: New England
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I'm not an expert on fireplace construction, but I can add some details for you to consider.

From my experience, a damper never closes perfectly tight. Whether that is designed in to always have some escape for those last few embers buried in the ash or just what happens over time, I can't say.

If this is a chimney exposed to the outside over much of its height, cold chimneys do have more issues than inside (warmer) ones. There have been a couple recent posts on backdrafting and smoke issues, so check them out.

I'm sure you are aware that a fireplace is extremely inefficient as far as adding heat to a home, they are primarily for enjoyment. IF at some point in the future you or the next owner want to convert the inside one to a more efficient unit, having its own chimney could make it easier.

Having two dampers requires remembering to close the one not in use while running the other. I'm sure you will remember, but you are not going to be the one using this combination all of the time.

Now some safety. One of the primary concerns with a natural draft appliance is finding enough inside air to burn and maintain a draft. If the home that this will be in is tight and energy efficient (our government wants all homes to be) then an outside air source should be provided for the FP. Don't just rely on someone opening a window, because it will get closed as the fire goes out and everyone heads off to bed. That leaves smouldering coals that produce lots of CO and no place for it to go.

For current use, for future use, and for safety, follow your plan and not your mason's. But, consider an energy efficient, glass front, air tight wood stove for the inside. They are just as enjoyable as an open fireplace and much more energy efficient. Plus they are cleaner and safer. And if you change the design now, the savings from the fireplace could pay a big chunk of the cost of the wood stove.

I hope some of this helps.


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