Flow balance: why the last node runs hottest

One of the most common findings in a commissioning report is a single node several degrees warmer than its neighbours, sitting at the far end of the manifold. It is almost never a bad cold plate.

What is actually happening

A manifold is a pipe with holes in it. Fluid entering one end loses pressure along its length, so the last branch sees a lower differential pressure than the first and passes less flow. With a poorly proportioned manifold the spread between first and last node can exceed twenty per cent, which shows up as several degrees of junction temperature.

Three ways to fix it, in order of preference

Oversize the manifold relative to the branches. When the header's cross-section is large compared with the sum of the branches, the pressure gradient along it becomes small and the branches see nearly the same driving pressure. This costs volume and almost nothing else.

Feed from both ends, or return from the opposite end to the supply. Reverse-return plumbing equalises total path length for every branch and is usually the cheapest change on a drawing.

Add restriction at the branches. Deliberately making every branch resistive swamps the header's variation. It works, and it costs pump power permanently, so it belongs last.

Verify it on site

Balance is measurable. At commissioning, run the rack at design load and compare inlet-to-outlet temperature rise across nodes. Equal power in and equal temperature rise means equal flow. If one node is warmer and its rise is larger, it is a flow problem — not a plate problem, and not a silicon problem.