A modulating central heating boiler makes the water very hot once, after which the central heating boiler only has to keep the water at the required temperature. When a modulating thermostat makes a heat request at the central heating boiler, the central heating boiler draws from the already heated water. With this construction you save a lot more energy than with an on / off central heating boiler. Please note that a modulating thermostat is only suitable for a modulating central heating boiler.