This brief explores practical and proven troubleshooting and assessment techniques for identifying and solving pump system problems commonly encountered in existing buildings during retro-commissioning processes. It focuses on pump and system interaction, with a primary focus on the parallel pump arrangements that are common in heating, ventilation, and air-conditioning (HVAC) applications.

A case study format is presented for readers. Each example is presented in the following context:

• What were the indicators of the problem and how can the problem be corrected?
• Are there any "ripple" effects associated with the problem?
• How can the costs and benefits be assessed and can persistence be ensured?
• How can the problem be prevented in future projects?