A low emission combustion system for use in a fuel-fired apparatus includes a fuel-fired burner (30) operative for generating a flame extending substantially axially outwardly from the outlet of the burner, a heat transfer tube (40) opposed to the outlet of the burner whereby the flame extending from said burner passes into a flame inlet section (48) of the gas flow conduit (46) of the heat transfer tube, a radiator body (50) disposed within the flame inlet section of the gas flow conduit of the heat transfer tube, and a catalytic converter (60) for oxidizing carbon monoxide to carbon dioxide. The radiator body (50) has a thermal mass sufficient to reduce peak flame temperatures in the flame inlet section to less than 2800 F. The catalytic converter is disposed within the gas flow conduit at a location downstream of the radiator body.