Abstract: A liquid conducting heat exchanger is mounted above a combustion zone burning combustible gas supplied by a fan through a combustible gas and air mixing chamber and a plurality of separate burner units spaced adjacent to the combustion zone. Each burner unit is specially designed to provide a gas conducting channel of diminishing cross-sectional area which conducts gas from the mixing chamber to a perforated screen so that gas is supplied to the combustion zone at a velocity greater than the rate of burning by the flame. Each perforated screen provides a convex surface facing the combustion zone while a first screen edge is movably retained within a recess for permitting thermal screen expansion and contraction while the remaining screen side edges are connected to a housing by a gas impervious seal. Each burner unit includes an outwardly extending lip which provides a removably sealing connection with the mixing chamber housing.