There is an atmospheric condition, known as air inversion, which causes high-density air to be trapped at fluetop altitudes normally occupied by the low-density air that creates ambient updraft. During periods of air inversion, chimneys in the affected area simply don’t draw properly. One way to tell if air inversion is causing temporary draft problems is to look at the smoke that exits the flue: if it eddies around the top of the chimney or flows downward onto the roof instead of rising as heated air normally would, an inversion layer is most likely present.
Having established that a draft problem is being caused by air inversion, several solutions present themselves:
(A) Don’t attempt to start a fire during inversion days. These don’t happen very often in most areas, and seldom occur during the long periods of winter cold when we use our woodstoves most.
Air inversion episodes occur most often when cold weather turns warmer, or when warm weather suddenly turns cooler, as sometimes happens in the Spring or late Autum. Some areas are more subject to air inversion than others: if your house is totally surrounded by tall trees, hills or buildings, you may experience local “inversion” every time the wind blows across the top of the taller obstruction, pressurizing the air below.
(B) During air inversion episodes, remove all possible draft resistance at the bottom of the chimney. Today’s woodstoves have very small air intakes and very restrictive baffle systems through which air must be drawn by the chimney. Opening a nearby door or window a crack often reduces this resistance considerably, and may allow the stove to be burned even on heavy inversion days.
(C) Elevate the top of the chimney to a point above the inversion layer. This is kind of a hit-or-miss solution, for three reasons:
(1) nobody can accurately predict exactly how high the inversion air tends to stack over a given neighborhood.
(2) the density of an inversion layer can vary from one episode to the next.
(3) there is a limit to how high a chimney can extend before it gets too top-heavy to support. If there’s a chimney in your neighborhood that is taller than yours, you might ask the owners if the additional height overcomes the effects of inversion you are experiencing. If so, try extending yours to the same height.
Note: never extend your chimney with uninsulated metal pipe, or excessive creosote formation will result.