Make a Chimney Cap

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Assembly diagram for the chimney cap.

One chilly morning, as my family hacked and coughed and
waved away the plumes of smoke that the woodstove was
belching into our kitchen, it occurred to me that building
a new top for the chimney might cure the heater’s draft

And as I explored that notion, I became convinced that a
chimney cap designed to create a venturi effect would be
the answer to my problem. Basically, a venturi is a tube
with a constricted, throatlike passage that increases the
velocity and lowers the pressure of whatever moves through
it. I reasoned that if I were able to incorporate such a
passage into my chimney, the exiting flue gases would be
speeded up to such an extent that the once sluggish wood
smoke could overcome the pressure exerted by the outside
air and be expelled efficiently from the top — instead
of backing up and plaguing members of my household as they
sat around the kitchen table trying to eat.

Then, entirely by chance, I literally unearthed
the means of translating my design from imagination to
rooftop reality. While cleaning out the pig yard, you see,
I found several disk blades that had broken free from a
harrow. The idea of using the smooth, curved
earth-cutters to form the cap took shape.

I used two lightweight 14″-diameter
disks (you could substitute the convex and concave ends of
a water heater tank, with the edges smoothed to reduce air
drag) and spaced the rounds about five inches apart by
welding three 1/2″-diameter steel rods in place near the
outside edges of the disks. (Since the blades are usually
made of high carbon steel, I’ve found that low-hydrogen
welding electrodes are necessary to produce good, solid

Next, I made a steel collar that barely slipped
over my existing flue and gouged a hole out of the
bottom blade to match the size of the new support. Then I
welded the collar to the bottom blade and pressed in some
1/2″ wire mesh to discourage the birds. With that done, I
plugged the former axle hole in the upper disk using
a wide, thick rubber washer and a nut, bolt, and flat

Finally, I installed the whole cap on the flue and secured
it with three sheet metal screws. (Guy wires and
turnbuckles could be used to help stabilize the setup.)

I’m happy to report that my jerry-built flue cap has
eliminated the woodstove’s noxious backpuffing. And if your
woodburner is giving you similar trouble, I’d suggest that
you consider constructing a simple little device like mine.

EDITOR’S NOTE: Although Mr. Ford’s design does not
constitute a true venturi, a chimney cap of this kind can
be effective in some circumstances.