Lab Testing of Balancing Valve Performance in a Multi-Family Central Recirculation System

In central domestic hot water (DHW) systems, recirculation based hot water distribution (HWD) contributes to approximately one-third of system energy use due to pipe heat loss1. Improving recirculation performance represents a major efficiency improvement opportunity for multifamily buildings, where central hot water systems are prevalent. The Statewide Codes and Standards Enhancement (CASE) team has developed and advocated multiple updates to California Title 24, Part 6 Building Energy Efficiency Standards (Title 24) to improve energy efficiency of central HWD systems. For 2008 Title 24, the statewide CASE team developed the initial Alternative Calculating Method (ACM) for modeling central HWD systems. For 2013 Title 24, the statewide CASE team conducted a CASE study to develop requirements on recirculation controls and enhance the ACM model for HWD systems. For 2022 Title 24, the statewide CASE team developed a CASE study on pipe insulation and pipe sizing requirements for central HWD systems. The CASE study for 2025 Title 24 code cycle proposed further requirements on pipe insulation and a compliance option for using thermostatic balancing valves to improve HWD energy efficiency2.

Additional Title 24 improvements on HWD systems can be made if relevant performance information is available. Field studies cannot fulfill all the data collection needs to support future building code updates due to limited accessibility to recirculation pipes. Most of the recirculation pipes are usually installed behind walls, making it nearly impossible to assess temperature distribution in the recirculation systems and evaluate hot water delivery quality. Without knowing the actual piping configurations and insulation conditions of installed recirculation systems, field performance data cannot reveal the impact of recirculation designs on distribution performance. To overcome these limitations, PG&E Code Readiness Program conducted a laboratory testing study on a full-scale central recirculation system, aiming to provide a comprehensive understanding of recirculation system performance to support future updates to Title 24 requirements and ACM.

This report focuses on the investigation of recirculation system balancing. Recirculation systems usually include parallel flow paths between supply and return pipes to distribute recirculation flow across all dwelling units. In an unbalanced system, water tends to travel through flow paths with low flow resistance. These flow paths will have excessively high flow rates, leading to potential pipe erosion. Conversely, other paths may suffer from small or nonexistent recirculation flows, resulting in lower recirculation temperatures, prolonged waits for hot water, and increased user complaints. In response, building operators may increase the supply temperature setpoint and/or increase the recirculation flow rate, both of which increase recirculation heat loss and decrease water heater operational efficiency. Various balancing valves, available in the market, are designed for recirculation system balancing. This study evaluated their performance through laboratory testing. Preliminary test results were provided to support the 2025 Title 24 CASE study development. This report provides the complete testing and analysis results.