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District energy · campus HVAC

Sub-metering a district cooling campus without draining a riser

The problem

A central plant supplied chilled water to a dozen buildings across a campus and needed to bill each building for the cooling it actually consumed. The buildings were occupied and operational; draining risers to install inline BTU meters was neither affordable nor acceptable.

The approach

Permanent clamp-on BTU metering at each building's chilled water entry — flow measured non-invasively, plus matched-pair temperature sensors on supply and return. The METRI IC-UPF was specified for its 1:2500 turndown, which matters enormously here: a chilled water loop in August and the same loop in a mild spring are different orders of magnitude, and a narrow-turndown meter goes blind at the low end without telling anyone.

What mattered in the detail

The ΔT was the accuracy bottleneck, not the flow. On these loops the temperature difference was modest, so matched-pair sensors were specified rather than generic RTDs — because the relative error in a small ΔT dominates the energy calculation. The flow measurement was never the limiting factor.

The lesson

Campus energy billing is where clamp-on BTU has no real competition — you cannot drain an occupied building's riser. Spend the accuracy budget on the temperature sensors, and choose a meter with enough turndown to see the winter flows.

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