How & Why Chimneys Draft

Chimney Physics 101:

Why Some Chimneys Draft, And Others Do Not

Many variables can restrict the proper flow of air, smoke and vapors as they draft up the flue, including: excessive creosote deposits; closed or plugged dampers; improper construction; animal nesting; debris from crumbling mortar; structural damage, or even a dirty chimney cap.

But the most common culprit is simple physics:

Neutral Pressure Plane:

Somewhere in your house is something called the Neutral Pressure Plane (NPP).

The location of the NPP is dynamic, since it constantly adjusts in response to changing conditions.

Above this theoretical plane, the air pressure is slightly positive compared to the outdoor air, meaning the air has higher pressure and is therefore trying to force its way out of the house (the “stack effect” described above).

Below the neutral pressure plane, the air pressure is slightly negative, meaning that part of the house is trying to draw air in.

This is an annoying problem anytime a fireplace or fuel-fired heating appliance is situated below the NPP, because at that location air will have the tendency to flow into the house (meaning smoke will, too).

“Wind-loading”:

When wind strikes a building, it creates high pressure on the side that it hits and low pressure on the downwind side. This means that any open windows or doors on the windward side will help to pressurize the house, improving chimney draft; however, any windows open on the downwind side will worsen a chimney's draft.

"Back-drafting"

Interior mechanical devices such as clothes dryers, kitchen fans, bathroom fans, attic fans and, especially, forced-air furnaces compete with the chimney for the same air, and often win out (because chimney drafting is passive, whereas these devices use motors, fans and blowers to affirmatively capture and re-distribute air).

So the solution is simple: turn off competing devices while a fire is going, or slightly open a window that is near the fireplace opening to ensure the chimney has a proper air supply so support proper drafting.

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