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Two wood burning fireplaces sharing a flue

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I have never come across this before, but it is my understanding that two wood burning fireplaces are never permitted to share a single flue. I had a house this morning with a zero clearance fireplace in the living room (16" raised hearth), and the same type of fireplace raised waste high on the back porch. They are back to back, but different heights.

As luck would have it, this was the one house in 500 that didn't have hardly any attic access. A funky truss-cathedral thing that prevented me from getting a look at the flue from the inside. All I can really tell for sure is that there are two fireplaces, and only one spark arrestor exiting the chase.

I tried the DESA sight, but it wasn't much help.

Are there exceptions to the sharing flues rule, or am I just imagining that it is a hard fast rule to begin with?

I always recommend a level II inspection, and have on this one too.

As always, any help will be greatly appreciated.


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Here's a start:

http://www.inspectapedia.com/chimneys/S ... ues_OK.htm

Smoke Entering House Through Unused Fireplace

Q: We read your explanation of why the smell of smoke can enter a house through an unused chimney, but our problem is a little different. We have two fireplaces that share the same chimney, one upstairs and one downstairs (which we never use). Whenever we have a fire in the upstairs fireplace, smoke (not just the smell) comes billowing in through the downstairs fireplace, even when we close the damper! Can you explain this one?

A: Actually, the explanation is based upon the same principle: whenever air travels to the outside of a house, an equivalent amount of air attempts to enter somewhere to replace it, and an unused fireplace flue is often the path of least resistance.

In houses where two fireplaces share the same chimney, each has its own flue, running side-by-side inside the masonry housing. Whenever you have a fire in one of the fireplaces, that flue becomes "charged" with rising hot wood exhaust, and vacuums large amounts of air out of the house. If the adjacent flue is the path of least resistance to the replacement air, smoke from the chimney in use can be drawn down the unused flue and into the house with the makeup air. As discussed in the smoke smell answer, most fireplace dampers don't close tightly enough to stop this flow. This phenomenon is called cross-drafting, and there are several possible solutions:

1) Open a window near the fireplace that is being used. If you can provide a path of less resistance to the incoming makeup air, you can eliminate the backflow down the unused chimney. Drawback: you won't want to be sitting in the cold draft between the open window and the fireplace.

2) Add an extra flue tile extension to the upstairs fireplace flue, so the smoke exits at least 18" above the downstairs flue. This won't stop the downstairs chimney from acting as a makeup air return, but should stop it from vacuuming smoke into the house. Possible drawback: if the downstairs fireplace has ever been used, the replacement air might still carry a smokey smell from the chimney into the room. Another possible drawback: this trick probably won't keep the upstairs fireplace flue from vacuuming smoke down into the house if you ever decide to have a fire in the downstairs fireplace.

3) Consider a pair of top-sealing dampers. These mount at the top of the chimney, and are opened and closed via stainless steel cables running down the inside of each flue. Caution: these are much tighter sealing than standard fireplace dampers, but are not airtight: we have experienced cases where some smoke was still drawn into the house through the closed top damper.

4) The ultimate fix: provide a source of combustion air from outside the house to both fireplaces, and install good, tight-fitting glass firescreens to cover both openings. This will stop the vacuum effect from whichever fireplace is being used, and will also inhibit the flow of heated air OUT of the chimney when you use the fireplaces. This fix will require the services of a Mason to install the outside combustion air intakes, and usually can't be accomplished unless the fireplace/chimney structure is on an outside wall.

From: http://www.chimneysweeponline.com/library.htm

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Are you sure there's only one flue in your chimney.?

Did you get a look at the top of the chimney?

The number of spark arrestors visible from the ground is not always indicative of how many flues are in the chimney.

Click to Enlarge

84.14 KB

Here's an old stone chimney serving two wood burning stove inserts (for kicks).

Click to Enlarge

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First off, no offense but let's please eradicate that 'zero clearance' term from everyone's lexicon. Factory built fireplaces typically are zero clearance only to the floor. The use of this term has led to many unfriendly fires as there comes a perception they do not get hot or have other clearance requirements, which they do.

Each factory built fireplace must have its own chimney. In fact, there is no way to interconnect two fireplaces. If you clearly have two fireplaces but only one termination visible outside you have one that dead ends inside the building, which is an extreme fire and carbon monoxide hazard and should be red tagged.

The advice in that linked article deals with masonry fireplaces which does not apply to factory built appliances.

Any pics?

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