Home & Garden

About the House: The Hidden Chimney

By Matt Cantor
Wednesday February 04, 2009 - 06:28:00 PM

A lot of us live in wonderful old homes built in the days before modern furnaces, metal flues and self-venting stoves. These are houses built before 1935 that contain—among their many other dated and cherished facets—brick flues which have nothing whatsoever to do with fireplaces. Brick flues that are sure to crumble or crash when that much lauded earthquake finally makes its, somewhat overdue, appearance here in the East Bay. 

The age of the modern engineered flue pipe began with the patented ceramic-lined flues of the 1920s (though brick can be seen well into the ‘30s in some houses) and gave way to the double-walled metal flues still in use today (though some flues in use today are even plastic). Brick flues (or, if you prefer, chimneys) were used for furnaces, cooking stoves and water heaters. They were also used for coal-burning heaters that are often still present in the dining rooms of our Victorians, Classic Boxes and Craftsman bungalows.  

These small chimneys are generally quite small in cross-section because, unlike the earlier fireplace chimneys, they had only a tiny interior space for the evacuation of hot waste gases from our appliances. 

The earliest brick flues have no lining, as is true of our early fireplace chimneys too. This can lead to problems. When natural gas is burned in stoves, water heaters and furnaces, it creates an acidic, moist gas that eats away at the alkaline mortar between the bricks. Over time, this gas destroys the structure of the chimney (creosote in chimneys is also acidic and does the same thing, which is one reason you want to get them cleaned and keep a rain cap on top).  

Many of these chimneys get so soft through this process that I often can remove bricks by hand or indent the side of one of these chimney with a little push (which I do very delicately). This is scary when you think that lots of these are just hanging there in space, waiting for a modest shaker (or a very big one) to take them down into your kitchen, dining room or the upstairs bedroom. And we’re talking about a lot of weight. 

As awareness of this type of mortar deterioration grew, masons began using terra cotta linings for brick flues. Most of the brick flues still around today are of this improved type. These flues are often found in the walls between the kitchen and the dining room in homes from the 1800s up to about 1930. In these houses, cooking was often done on one side and heating (with a coal burner or gas heater) done on the other, with both components plugged into a double-barreled brick chimney stack.  

Although they experience less mortar deterioration, these lined flues are still extremely inefficient (as flues) and present a similar level of seismic risk. These flues tend (due to their mass) to cause the exhaust gases within, to reach dew-point and rain down along the inside, rather than escape above the roof. This means that the exhaust isn't escaping and some part may actually travel in vapor form into the dwelling. 

This is especially true when these brick flues are used for water heaters or furnaces. As noted earlier, this often leads to the brickwork's becoming soft and unstable.  

So what makes sense? What do we want to do with these forgotten serfs of smoke and heat? There's no appliance that requires a brick flue today. For any item that is still connected to one of these, a lighter, smaller and more efficient flue can be installed.  

Since many of the brick flues I see in houses today are doing no work at all, I'm strongly inclined to recommend that these be called to work as herringbone-patterned patios in our backyards. Bricks really want to be lying down here in California anyway in keeping with our laid-back modus-and our earthquake awareness. If we leave them running up through these hidden spaces in our houses, they're destined to come down through the plaster and the light framing of our houses when the big one arrives. The taller the stack, the greater the pressure at the base and the more dramatic the effect when the earth moves. 

I think it's a powerful act of seismic planning to remove one of these things, especially if your house is two or more stories high. Keep in mind that most fireplace chimneys run up the exterior of the house and are likely to end up lying in the side yard while the ones we're talking about are sure to end up inside the house, causing a lot more damage. Couple that with the lack of real usefulness and you've got a darned good argument for demolition. Even removal of an upper portion and blocking off of the points of entry is a good start.  

Removal of a chimney should be done by a qualified professional in gradual steps. It usually involves removal of some interior wall material, but perhaps not much. Chimneys of this kind seldom provide any structural support for the building, so they rarely need to be supplanted with framing as they are removed. Your contractor should be able to take it out and give you some nice choices for the use of the remaining space. For example, removal of a brick chimney can also provide additional space in the interior for storage or a slightly larger kitchen. Think about doing this job prior to replacing a roof so that you end up with fewer leaks from the roof/chimney junction (a common place for leaks to occur). 

Your newly liberated brick might take shape as that Arcadian patio I mentioned or could be transported to the local salvage yard. A pile was recently left out by a neighbor on our block and it nearly led to a fight by wheelbarrow wielding neighbors vying for possession, so it might not even be necessary to cart them away yourself. 

Old brick is generally of some value and if you have Clinker brick, even more so. Clinker, usually seen only on the more visible portions of upscale residential chimneys or facades, is malformed, bubbly and oddly colored, suggest a forgetful baker … which is pretty much the case. 

When I think of the effects that our inevitable earthquake are likely to evoke, the removal of needless brick (especially tall stacks of same) gets placed high on my to-do list. I hope it'll be on yours, too.