<p>A furnace is heated by a burner that can be selectively operated by either air-fuel or oxy-fuel combustion. The burner comprises a conduit for fuel, a conduit for air, a conduit for oxidant, and control means for regulating flow through the air and oxidant conduits. An air-fuel fired furnace can be modified by addition of the oxidant and fuel conduits and the control means for regulating flow through air and oxidant conduits.</p>
