Designers and operators typically do not include distribution losses when analyzing the performance of heating hot water systems. Though these losses are very small compared to design-day loads, they are not zero. As the losses occur continuously whenever the systems operate, and these systems spend the vast majority of time operating at low part loads, the losses can still comprise a substantial fraction of annual heating energy consumption. When the entire building requires heating, these distribution losses have little negative effect. However, the losses unnecessarily waste heat whenever the air handlers operate above minimum outside air. In warmer weather, when temperatures are above air handler economizer lockout conditions, the losses are doubly detrimental as they both waste heat and increase cooling plant loads. We reviewed the literature and could not find measurements of hot water distribution losses in real buildings. Here, we report results from 7 large commercial buildings at 5 different organizations, all located in California, climate zones 3B and 3C. For each building, we commanded the valves closed on heating hot water end-use components and shut down the air handlers. We operated the heating hot water system to maintain a constant flow and a constant hot water supply temperature setpoint typical for each building. We then measured the steady-state heating power required to maintain that setpoint. Building characteristics varied widely in terms of size (5,100-15,000 m2 or 55,000-160,000 ft2), type (e.g., city administrative office, college lab and classroom), HVAC design (VAV reheat or dual duct systems), and year of construction (1917-2000). Despite this, the results were reasonably consistent when normalized to building conditioned floor area. The median loss rate was 1.2 W/m2 (0.37 BTU/hr.ft2) with a min/max range of 0.8 - 2 W/m2 (0.25 – 0.63 BTU/hr.ft2) across all buildings at typical supply temperatures for each building. For comparison, this is roughly a third of average office plug loads, and though it is low on a conditioned floor area basis, it ranged from 6% to 60% of the annual HW energy consumption for the five buildings for which we had long term data. We discuss the methods, buildings, and results in more depth in this paper, as well as the opportunities for improving system design and operation.