In a contraflow masonry heater a bypass channel is a mechanically operated iron damper, which when open allows the smoke path to feed directly into the chimney, bypassing the side channels, manifolds and chimney connection. In certain cases it can be used to bypass a bench, or other external extension to the smoke path.
When open the bypass offers the gases a constantly rising unobstructed path directly into the chimney. When closed the gases are forced to follow the full smoke path of the stove.
The function of the bypass is to assure that a newly lit fire would draw easily, by feeding directly into the chimney, and become established before being drawn through the stove. This is also to avoid having the carbon deposits of the initial stages of the fire before secondary combustion is achieved, deposited on the inner surfaces of the side channels, in areas where it would not necessarily burn off, and could be difficult to clean.
The bypass is intended to be opened only for the first 10 to 15 minutes of the fire and then closed for the duration of the fire.
The first 4 contraflow stoves I built had bypass dampers, which were particularly difficult to install correctly. After operating all these stoves it became apparent that, once cured and in regular use, the stoves would all draw perfectly during start up without opening the bypass. Since then I have never built a stove with a bypass channel, and never had a problem with draw, or channels becoming blocked. It is on this fact that the critique below is based.
That the bypass can improve efficiency during start up by offering a stronger draw and so arriving faster to the point of secondary combustion is incorrect. In heaters without bypasses, even seconds after lighting a bottom up fire, the draw is sufficient to arrive as rapidly at the point of secondary combustion as if drawing through a bypass.
With the top down burn, which is widely considered to be the most efficient way to start the fire, the time between start up and temperatures high enough to enable secondary combustion is not important.
That the use of a bypass can prevent deposits on the inner surfaces of the side channels is not a valid reason for its installation. Carbon deposits in the side channels will mostly burn off when the fire becomes established. That there are never examples of contraflow stoves, that are used properly, having blocked side channels indicates that this is not an issue.
A bypass damper may be used to engage and disengage a section of extended smoke path. An example of this would be a stove with an extended horizontal smoke path in the form of a bench on all four sides of a stove. On light up the chimney may not always have a strong enough draught to draw through such a long bench. Installing a bypass damper directly from the stoves manifold to the chimney would enable the stove to be fired directly into the chimney. Then when sufficient draught had been established the bypass would be closed and the gases drawn through the bench before entering the chimney (at the same level as the bypass channel). In this instance the bypass is installed at the bottom of the stove, exiting one manifold, and not in the upper chamber. Damage from heat and flame bite would not be a problem in the manifolds and so malfunction less likely. Also the upper chamber,having no openings, maintains its structural integrity.