AbstractHydronic heating and cooling systems are among the most common types of heating and cooling systems installed in older existing buildings, especially commercial buildings. According to the 2012 Commercial Building Energy Consumption Survey (CBECS) data set, hydronic heating systems in the United States include two main systems: (i) boilers inside the building represented with a boiler system and (ii) district steam and hot water systems represented with district heating, which are connected to seven different types of zone-level equipment. Similarly, there are two main hydronic cooling systems: central chillers inside (or adjacent to) the building and district chilled water piped in from outside the building. Chiller systems are investigated based on three different classes: (1) water-cooled, (2) air-cooled, and (3) absorption chillers. This study presents a deep analysis of the 2012 CBECS microdata to characterize hydronic heating and cooling systems by year of construction, census division, building area, building site hydronic system energy use index (EUI), and the types of mechanical systems. The results show that nearly 65% of commercial buildings built before 1990 utilize hydronic heating systems. Hydronic heating and cooling system design are a function of a building area. District heating systems are considered as the main heating systems in buildings with an area greater than 18,600 m2 (200,000 ft2). In addition, systems with central chillers inside the buildings are responsible for providing cooling for more than 50% of the commercial buildings with areas greater than 9,000 m2 (∼100,000 ft2). Among the types of chiller systems, the chiller systems connected to the central air handling units, fan coil units, and duct reheats are the most common systems for large buildings. The results of this building stock characterization provide useful insights into the characteristics of hydronic heating and cooling systems in US commercial buildings.