Modern low-power boilers with automatic burners require electricity for proper operation. The electricity voltage in the network is not constant and is subject to fluctuations. Variations in voltage will have the most significant impact on the operation of electric motors since their speed is controlled by changing the voltage. The purpose of this research was to assess the impact of supply voltage deviations within the range allowed by the EN 60038:2012 standard (230 V ±10%, i.e., 207 V and 253 V) on boiler operation. This study analysed the effects of these variations on flue gas and dust emissions during boiler operation at full load, as well as on the boiler firing process. Tests were conducted on a boiler with a nominal output of 25 kW. Changes in voltage significantly influenced the blower fan speed. For the nominal boiler output, at 253 V the speed increased by 17.6%, and at 207 V it decreased by 20.4%. Variations in voltage affected the volume of air supplied to the combustion chamber, altering the excess air ratio (λ): 1.8 at 230 V, 2.1 at a higher voltage, and 1.4 at a lower voltage. Changes in voltage translated into changes in exhaust gas temperature and flue gas and dust emissions. Boiler operation at 253 V increased CO emissions by 77.2%, NOx by 31.2%, and dust by 12.5%. In contrast, at 207 V, emissions were lower, with CO decreasing by 17.3%, NOx by 11.7%, and dust by 18.8%. Fluctuations in voltage further influenced the boiler’s ignition time; the ignition process was four times longer at a higher voltage and twice as long at a lower voltage. The results of these studies underscore the necessity of adapting boiler designs to fluctuating voltage conditions.