Farewell to the Chimney?
Bidding an old friend adieu
For thousands of years, the chimney has been closely associated with our concept of home. Upon spying smoke curling from a distant chimney, the weary traveler ends his journey with lightened steps.
When I built my house in Vermont, as a much younger man than I am today, I designed a house with two chimneys. The house has a cellar, first floor, second floor, and attic; because I wanted the chimneys to rise five feet above the ridge, they had to be 40 feet tall.
Since I couldn’t afford bricks in those days, I spent a lot of time gathering, sorting, and cleaning stones. Three years later, when the two 40-foot stone chimneys were finally complete, my financial situation had improved, so I flashed the chimney penetrations with copper and lead. Each chimney has a copper cricket.
I love my chimneys. Like travelers of old, I love to see a wisp of smoke rising from my chimney-top as I trudge homeward. Much as I love this image, however, I fear that the days of the residential chimney are numbered.
An escape hole for conditioned air
Whether serving a fireplace, wood stove, furnace, boiler, or water heater, a traditional chimney can be problematic. What’s wrong with chimneys?
- When not in use, your chimney flue is a big hole in your house that allows conditioned air to escape into the atmosphere.
- The large thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. of a traditional masonry chimney acts as a thermal bridge penetrating your home’s envelope.
- Traditional chimneys are incompatible with exhaust appliances, especially large range hoods. Exhaust fan operation can cause chimneys to backdraft.
- Older masonry chimneys are often incompatible with modern combustion appliances, which usually require small flues. When such appliances are connected to an oversized flue, a variety of problems — including poor draft, chimney corrosion, and freeze/thaw damage to the chimney — can occur.
For these and other reasons, energy experts now advise builders to choose sealed-combustion appliances. Instead of a chimney, sealed-combustion appliances usually require a PVC vent through a home’s sidewall or roof. Sealed-combustion appliances have the following advantages:
- When burners are off, these appliances close the flue with a motorized damper, limiting the undesirable exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. of conditioned air.
- Since the burners of sealed-combustion appliances have a ducted supply of outdoor air, they aren’t affected by house depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. caused by exhaust fans.
Yet old-fashioned chimneys have at least one advantage over PVC flues: they don’t require any electricity. (Sealed-combustion appliances are usually power-vented.) Reducing a home’s electricity use is always a good thing. Moreover, old-fashioned chimneys can enhance a home’s “passive survivability,” defined by Alex Wilson as “a building’s ability to maintain critical life-support conditions in the event of extended loss of power, heating fuel, or water.” There’s nothing like a wood stove and a masonry chimney to get you through an ice storm. Yet this useful role may not be enough to save old-fashioned chimneys from near-extinction.
Before I end my essay on chimneys, I’ll explain the curious photo above, showing a “suspended chimney,” also known as a “bracket chimney.” A suspended chimney is a brick chimney supported by joists or a projecting wooden shelf. Once common on simple rural homes or lakeside cabins, these chimneys were built by masons who couldn’t afford to pay for the bricks that a full-height chimney would require. (Been there.)
I mention suspended chimneys for only one reason — to give me the opportunity to quote from a poem by Robert Frost. You’ve got to love a writer who chooses a construction detail as the theme for an entire poem. Frost’s 1923 poem, “The Kitchen Chimney,” includes these lines:
Builder, in building the little house,
In every way you may please yourself;
But please please me in the kitchen chimney:
Don’t build me a chimney upon a shelf.
However far you must go for bricks,
Whatever they cost apiece or a pound,
Buy me enough for a full-length chimney,
And build the chimney clear from the ground.
It’s not that I’m greatly afraid of fire,
But I never heard of a house that throve
(And I know of one that didn’t thrive)
Where the chimney started above the stove.
Last week’s blog: “Passivhaus For Beginners.”
- Martin Holladay
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