A contraflow heater, also known as a masonry heater, is a wood burning fireplace constructed of soapstone, brick, clay, tile or other masonry material that uses airflow and mass to heat a room efficiently. Hot gases from the fire are redirected through a series of heat exchange conduits, instead of directly venting directly outside through a regular flue.

Initially, the gases start out in the upper combustion chamber, then they are guided down, then out to the side channels, warming the stones of the heater. Simultaneously, the air in the room outside the heater heats up and moves up the surface of the stone in an opposing direction, which is called contraflow. This network of channels gives the heater more surface area through which to radiate the maximum amount of heat. A contraflow heater typically continues to radiate heat for 12 to 24 hours.

The benefit of a contraflow heater is that the hot gases are fully used instead of being immediately exhausted. The wood is fully combusted over a long period of time instead of a sudden, short fire. In addition, the fuel is burned more cleanly, with a low creosote buildup and minimal emission of smoke and carbon monoxide.

Another bonus of the contraflow heater is that the relative humidity of the room is maintained. The design is simple, efficient and practical, and doesn't rely on blowers, fans, burners or boilers which can be handicapped by power or fuel interruptions. Although a contraflow heater may be more expensive initially, the pay off comes over time.

A contraflow heater may be constructed using a variety of finishes, materials and designs. Soapstone is the most popular material due to its ability to retain and radiate heat efficiently. Extra features include bake ovens or cook-tops, making this hardworking heater a multi-tasker. In Sweden, contraflow heaters are installed in nearly 90% of new homes. Although the contraflow heater may not have seen widespread use in the U.S., the high efficiency and aesthetic values of the contraflow heater have been widely appreciated throughout Europe and Asia for centuries.