First, he found that the existing boiler’s gas input was too high by shutting down or off some number of the gas burners, while still satisfying the thermostat. He also was keeping track of the water temperature, which he told us was up to 150°F on the cold nights. When I asked the homeowner with the coal boiler when the house was built, he replied 1913 or so. There is a gas boiler sitting just to the left that also feeds into that same header. Natural ModulationWhen operating, a coal boiler is naturally modulating, since the coal fire cannot be controlled exactly.