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The thought of adding a complete central heating system to an existing house conjures up images of hours of "Sawzall surgery" followed by more hours of repairing drywall and finishes. This is especially true for forced-air systems, with all their associated ductwork. But with the right approach, central hydronic heating can now be added to many homes with minimal disruption to existing finishes. Advantages of Water When it comes to retrofit systems, hydronic heating has a basic advantage over forced air. The water used to convey heat is almost 3,500 times more "thermally concentrated" than air. A cubic foot of water absorbs about 62.4 Btu for each degree Fahrenheit its temperature increases. By comparison, a cubic foot of air only absorbs