Contra: Against or contrary. Flow: To issue or move in a stream. Circulate. Then, what is a contraflow design? Well, in a masonry heater it is referring to the route the heat takes as it moves through the heater. Simply put, if left to itself, heat tends to rise. In a masonry heater, the heat rises, then goes down through a masonry channel and then rises again to the chimney where it exits the heater. So, think up, down, up again, and in some cases it can also include around. The around portion can occur when a warmed bench is included in the design.
The importance of the contraflow design is evident. It allows the entire masonry portion of the heater to warm. In turn, the masonry radiates the heat into the surrounding area in a gentle manner. Radiant heating and the contraflow design does not tend to dry out the air, your nasal passages or your skin, as a convection heater does.