A fuel-fired high capacity liquid heating appliance, representatively a boiler or a water heater, has a fluid heat exchanger extending around a combustion chamber into which first and second fuel burners extend, the first and second burners respectively having associated blowers for supplying combustion air thereto. Illustratively, the combustion chamber is oval-shaped, with the burners extending into opposite ends the combustion chamber If one of the burners is not firing while the other burner is firing, a control system starts the non-firing burner's blower, to protect it from overheating by the firing burner, if the control system senses an excess temperature in the non-firing burner. The heat exchanger comprises a series of fluid receiving tubes extending between baffle-free header structures iteratively sized to equalize fluid flow rates through the heat exchanger tubes over a wide flow rate range.