Abstract: A heating boiler has a substantially horizontally extending combustion chamber for combustion of liquid or gaseous fuel, a boiler water chamber surrounding the combustion chamber, a thermostat probe having a sensor section extending into said boiler water chamber, an auxiliary heating channel positioned within the boiler water chamber, a return pipe extending into the boiler water chamber for the discharge of return water thereinto, an outflow chamber surmounting one end of the combustion chamber and separated from the boiler water chamber by a separator wall, the outflow chamber being in communication with one end of the combustion chamber and the inlet of the auxiliary heating channel communicating with one end of the combustion chamber through the separator wall and the outflow chamber, the sensor section of the thermostat probe being disposed above the auxiliary heating channel adjacent thereto and proximate the inlet thereof, and the discharge end of the return pipe being located within the boiler water