In the present study the effects of air preheating and dilution on stabilizing MILD (Moderate or Intense Low-oxygen Dilution) combustion of natural gas in a 100 kW jacket type water heater are investigated experimentally. The effects of FGR (flue gas recirculation) and flame heat loss on the self-ignition and extinction temperatures are analyzed numerically using a non-adiabatic WSR (Well-Stirred Reactor) model in the CHEMKIN software. Experimental results showed that the mechanical-process retrofitting techniques were successful to reach positive characteristics of the flameless combustion; e.g. uniform temperature distribution, improved efficiency (>10% increase), and lower NO emission (̴ 13% decrease), without the need to replace the conventional burner with modern flameless type or retrofitting the structure of combustion chamber. Based on the simulation results, a new combustion regime map for practical conditions encountering heat loss is developed. The new T-FGR-heat loss regime map shows that as heat loss increases the quasi-MILD region expands, whereas the MILD and MILD-like regions shrink and beyond a specific heat loss the HTC (high temperature combustion) zone disappears. The method of numerical visualization of experimental conditions on the combustion regime map is a simple powerful numerical tool to design practical flameless systems or retrofitting conventional devices.