HVAC retro-commissioning (re-tuning existing equipment without replacing it, for example optimising AHU sequencing, room setpoints and schedules) is typically evaluated under IPMVP Option C with a 12-month baseline model built from pre-commissioning data.

Retro-commissioning has a very small capital cost (consulting fees plus technician hours) but produces broadly distributed savings across many small adjustments, which is exactly the situation Option C (whole facility) was designed for. A 12-month whole-building energy baseline is regressed against routine drivers such as heating and cooling degree-days and occupancy, then 12 months of reporting-period data are compared at adjusted conditions. Typical savings are 10 to 30 percent on HVAC in poorly tuned buildings, ROI is often under two years, and demand is strong in the French tertiary sector under the Decret Tertiaire decarbonisation pressure for 2024-2026.

Keep a change log of every setpoint and schedule that was modified during the commissioning: when next year's facility manager 'fixes' a complaint by undoing one of them, you need to be able to prove what changed and when.