Thursday, May 28, 2009

Gentlemen, we can rebuild it. We have the technology.

May 22

OK, I'm going to see how many references I can make to 1970's TV shows in the titles of my posts going forward...

Today, the masons started rebuilding the chimney. The flue for the second floo
r fireplace was reconstructed along its original path (which had been altered in a previous rebuild of the chimney).

They had to parge the flue as they went. If they rebuilt all the way then tried to parge the flue, they might not be able to reach all the way down inside the flue. sounds obvious... but its one of those things that I can see myself doing wrong if I was attempting something like this on my own.

As they rebuilt the chimney the areas that previously were voids are instead solid masonry construction now.

Rafters that had been "free wheelin'" near the chimney now tie in to a header that rests on the masonry of the exterior wall.

Once they brought both flues to essentially the same height, they parged the exterior of the chimney.

Remember the earlier post when I promised gory details....well, here comes the math part (I promise it's not hard...maybe boring...but not hard!).

To insure that the fireplaces don't smoke the flue needs to be sized correctly. A rule of thumb is that the area of the flue opening should be about 10% of the area of the fireplace opening
(from my sophomore geometry class I remember that area = length x width). If the flue is too small, the fireplace won't draft correctly and smoke will fill up your room - that's bad. If the flue is too big, it may not warm up enough to create a good draft and smoke will fill up your room - that's bad.

Before we started our project the kitchen fireplace was 64.5" x 58.5" so the area of its opening was 3,773 square inches (feel free to check my math!). That means the flue size needed to be 377" square...but it was only 308" sq. Uh oh.

The fireplace did smoke a bit when we had cooking demonstrations in years past but opening windows seemed to help somehow (besides allowing people to breathe!).

The corner fireplace is 31" x 41" (1,271" sq.) so its flue needs to be 127" sq. (see how easy this is!). But its flue was 168" sq. - its too big!

The point here is that if we rebuilt the flues exactly as they were, the fireplaces wouldn't work right. When we add the fact that we have now increased the total area of the kitchen fireplace opening (back to its original size), we definitely can't use the previous kitchen flue dimensions.

The kitchen fireplace opening is now back at its original size of 88" x 58.5" (5,148" sq.), so it needs a flue opening of 514" sq. to avoid smoking. If the original flue was just 308" sq. they must have had a smoking problem! (more on this in a second.) That might explain why the fireplace opening was reduced in size. It also might explain why a brick arch was added behind the lintle though, because of its design, it probably didn't help much.

So what are we going to do?

Well we have some wiggle room here. Remember that the flue for the corner fireplace is oversized. Additionally, the partition between the flues was two bricks thick. If we reduce the size of the corner fireplace flue, and make the partition one brick thick (which is normal anyway), we can make the flues upper dimension 16" x 32" resulting in a flue area of 512" sq. The result? the ability to breath in the kitchen while a fire is going! The corner fireplace flue will still work too. And the chimney will still be the same size as it was from the outside so we won't be changing its appearance. Whew!

But there's a nagging question here...Why did the original mason who built the chimney make the kitchen flue undersized? Did he not know what he was doing? We don't think its likely that an incompetent mason was hired by the owner. Instead, could it be that the corner fireplace on the second floor was added after the original chimney was built? To compensate for forcing two flues into a chimney meant for one, the kitchen fireplace may have been reduced in size. We're not sure but its something we'll continue to explore and look for evidence as we continue to work in the attic.

Onward and upward!

No comments:

Post a Comment