No Smoking Indoors
Chimney expert Dan McLaughlin explains the basics behind the bricks in the McCues' new fireplace system in Manchester.
Looking at architect Stephen Holt's drawings in early July, it was clear
that his vision of an addition to the west side of the McCues' house
would beautifully restore the balance and grace of its original design. The new fireplace, placed at the end of the new wing, would also become the natural focal point of the new music room, with its re-creation of a
Shingle-style inglenook. This design is very popular in new homes, where
a great-room addition is built off one side of the house, with a fireplace at the end of it. The fireplace is the natural focal point as you walk into the room, and an exterior chimney can help soften the sometimes "boxy" look of a big new house.
But overall, the thing to remember is that a chimney is really a machine
whose job it is to create a "vacuum" at the fireplace so that smoke gets
sucked in, up and out of the house. Since this machine has no moving
parts, its design has to be right on from the beginning, especially given the tiny pressure differences it's working with. A good chimney will give a "vacuum" of 5 pascal (the standard unit of pressure) when
starting up, and reach 25 pascal when burning strongly. (By comparison, air pressure at sea level is about 100,000 pascal.)
Design vs. Function
The design of the McCues' new fireplace made a great deal of sense from an aesthetic point of view. However, long before a brick was laid it was also clear that the chimney was likely to suffer from the problems common to many chimneys of this design — smoking, poor draft, and a persistent smoky smell in the spring and fall. These functional problems stem from two major design issues with the new chimney: 1) that it would
be outside the heated envelope of the building; and 2) that because the
new wing is only one story high, the chimney would end up being
substantially shorter than the main house.
Exterior chimneys work well in places such as Virginia, or in England, where it's typically 40 degrees in the wintertime. In fact many Colonial-era buildings in the South exhibit this design. But here in New England, where the winter climate is quite a bit colder, you can see that most surviving Colonial-era buildings have interior or center chimneys. An exterior chimney poses a problem in very cold weather because, until the fire has heated up the six tons or so of masonry, that masonry is continually cooling the smoke, causing it to rise more slowly, or perhaps not at all. And as you can imagine, if the outdoor
temperature is 20 degrees, for instance, warming the chimney will take a long time.
Second, a house itself acts as a chimney, with warm air rising up the
stairwells. This air has to be replaced from somewhere, and if the
chimney is short, it will simply become a "battle of the chimneys," with air (plus its accompanying wet, sooty smell after rain) being drawn
downward within whichever is shorter — the chimney or the house.
To make matters worse, this problem is actually exacerbated by modern "tight house" construction. In the old days, windows didn't fit as
tightly, nor were air-leaks in the basement, attic and elsewhere sealed as well, so a fireplace, which on average extracts around 350 cubic feet of air per minute from a house, had no trouble getting fresh air to feed the flames and help carry away the smoke. But nowadays, with
energy-efficient doors and windows, and insulation that can seal every
tiny crack, much less air infiltrates into a house, and the fireplace has to fight for its air with bathroom extractor fans, range hoods, clothes dryers, and other household amenities. Often the solution to this issue in newer homes is to install glass doors, or an extractor fan that sits on top of the chimney and provides a mechanical draft. And the industry in general has moved towards air-to-air heat exchangers to help replace stale inside air and reduce "low pressure" syndrome that causes
fireplaces to smoke.
Pumice to the Rescue
But we needed to address the McCues' chimney problem more fundamentally. One possible, and seemingly straightforward, solution to the exhaust problem would be to build the chimney taller. However, if all we did was to add more cold masonry, that probably wouldn't help much. And since the new wing was only one story high, bringing the chimney inside the house wasn't the answer either.
What we really needed was a way to insulate the chimney, which in turn
would also allow us to make it taller. The key to the solution was in constructing the fireplace and chimney out of pre-engineered volcanic-pumice blocks, manufactured in Denmark. In appearance the pumice blocks are not dissimilar to concrete, but they have much greater insulating properties while being much lighter. They're also conveniently pre-shaped to form a fireplace and chimney.
Using this "system" also had the desired effect of solving the chimney's height issue. Because the blocks come with four corner holes, pre-formed to accept re-bar and grout — mainly for use in earthquake-prone areas — we were able to confidently build the chimney higher than would otherwise have been possible without some support measures. The final step was then to clad the pumice with a water-struck brick veneer to make it indistinguishable from a conventionally built chimney. And as a welcome bonus, this also happens to be a very cost-effective way to build chimneys.
So while from the outside, the chimney appears to be of conventional, locally-produced brick construction, the material for the "working" parts of the chimney was formed 2,500 years ago in an Icelandic volcano, mined, shipped to Denmark, formed into blocks and shipped again to New England, finally to be built into the fabric of the McCues' house. There it will play its part, quietly and invisibly, in warming the inglenook and its occupants for many years to come.
McLaughlin was the chimney contractor for the Manchester project.