Electric field effects (EFE) on combustion characteristics, heat energy production, and composition of polluting emissions have been investigated experimentally for different types of fuels (natural gas, biomass) providing experimental study of the EFE in a district heating boiler and complex modeling experiments in a small-scale pilot device. The DC field-induced variations of the produced heat energy, efficiency of heat energy production, flame characteristics, and the composition of polluting emissions have been studied for a positively biased axially inserted electrode and negatively biased (grounded) heat surfaces by varying the applied DC voltage, net current, and consumed electric field power. Experiments in the district heating boiler have shown that electrodynamic control of the heat production and combustion characteristics depends on the applied field voltage, power, and on the flame region, where the top of the axially inserted electrode is located. The most pronounced EFE was observed when the top of the electrode was placed in the primary mixing zone intensifying the mixing of the flame compounds and thereby completing combustion. The mechanism of the electric field effects on the combustion characteristics is discussed with reference to the analysis of electric field effects on the flame characteristics observed in modeling experiments.