Abstract: A boiler for generating steam and/or hot water is provided having a tank containing water, an outlet located proximal to the top of the tank, and a return inlet located proximal to the bottom of the tank. A substantially horizontal combustion conduit in heat transfer relationship with the water extends through the tank proximal to the outlet. A heat source in communication with the combustion conduit heats and drives the heated gas into the combustion conduit. A set of fire conduits in heat transfer relationship with the water and in communication with the combustion conduit, extend though said tank below the combustion conduit. When heated gas is driven into the combustion conduit, the heated gas flows through the combustion conduit, and then flows downwardly and then in a substantially transverse through the fire conduits, and heat is transferred to the water as the heated gas flows through the combustion conduit and fire conduits.