Abstract: This invention relates to A contra-flow masonry heater comprising: a firebox having a front opening, an air intake communicating with a source of fresh air and an upper flue outlet; at least one vertical contra-flow heat exchange channel communicating between the flue outlet and an exhaust port, the firebox and channel having walls of refractory material for mass absorption of heat energy from a burning fuel and combustion gases contained therein and subsequent radiant release of heat energy to an adjacent space, a down draft hood communicating between the exhaust port and outside air, the down draft hood having a plenum communicating among an exhaust inlet, a chimney and a relief port, the relief port being located below the elevation of the exhaust inlet and communicating with the source of fresh air; and an air tight door secured to the front opening of the firebox.