Disclosed is a boiler apparatus useful for the heating of living and working spaces. The disclosed boiler apparatus incorporates a burner element and controller element that cooperate to provide a modulated firing rate. The modulated firing rate serves to increase the efficiency of the disclosed boiler apparatus. The boiler apparatus of the invention incorporates a heat exchanger and a combustion system. In operation, the control element collects and analyzes information from a multiplicity of sensors in order to modulate the firing rate of the burner element so as to maintain a selected output temperature for the water being heated. This monitoring activity by the control element also avoids short cycling of the boiler apparatus of the invention. The boiler apparatus of the invention also includes an indicator light that provides a visual indication the invention's operational status.