A compact modular heat distribution unit for a hydronic heating system connects to a hot water source, to supply and return pipes that lead to heating zones of the building, and, optionally, to an indirect water heater. The module is preferably assembled off-site in a factory setting to contain all components of a typical hydronic heating system except for a hot water source and the building heating pipes. The module includes heating zone distribution sub-modules for the different heating zones of the building. A three-dimensional frame supports the heating zone distribution sub-modules. The module may be used in any multi-zone system including hot water baseboards, hydronic radiant heat floor pipes, swimming pool heaters, ice melt systems or in any combination of these zones.