Abstract: This invention relates to a system for reclaiming or recovering heat which would otherwise be lost from a heating plant. The invention accordingly relates to a heating plant incorporating a plurality of boilers, including a stand-by boiler, there being means for supplying fuel and air to at least one of said boilers, except for the stand-by boiler, and flue gas exhaust ducts associated with each of such boilers. The improvement according to the invention is a waste heat recovery system wherein the flue gas exhaust duct from said at least one boiler is operatively connected to the stand-by boiler for the circulation of hot flue gases therethrough in contact with its heat exchange surfaces thereby to effect the heating of water passing through the stand-by boiler thereby to recover a portion of the heat energy in the flue gases.