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The ideal hydronic boiler would continually adjust its heat output to match the heating load of the building it served so that it never produced more heat than needed. Unfortunately, residential boiler technology hasn't yet progressed this far. For the time being, boilers have only two rates of heat output: totally on or totally off. Since boilers are often oversized for the loads they serve, once turned on, they almost always produce heat faster than the building needs it. The excess heat rapidly warms the water and metal within the boiler. In most systems, the temperature of a firing boiler will continue to climb until it reaches the temperature set on the boiler's limit control. In some systems, this can