It runs a cascade of two loops.

The outer loop works in temperature.

It produces a flow demand rather than a position.

The inner loop holds that flow against duct pressure.

That is what makes the box pressure independent.

It is why a flow input exists at all.

More than most people expect.

Minimum and maximum flow for that zone.

The flow sensor factor for that box size.

Reheat sequence and occupancy behaviour.

It is per terminal, not per building.

Defaults are almost never correct.

Flow is read wrongly for the whole life.

The controller still appears to work.

The zone is simply always over or under supplied.

Nothing alarms at any point.

Only measured commissioning catches it.

Check the factor against the physical box.

Because observation cannot detect these faults.

A reversed flow pickup looks normal.

A wrong calibration factor looks normal.

A duct never connected can look normal.

Instruments at minimum and maximum find all three.

Compare the controller reading against measurement.

Speed at commissioning and visibility afterwards.

Forty terminals can be set from one place.

That is a day rather than a week.

Facilities can then see zone behaviour.

Boxes stuck at minimum all winter become visible.

That diagnosis is impossible box by box.

Only after flow has reached minimum.

Throttling comes first as the zone cools.

Reheat starts once no further throttling is allowed.

A badly sequenced controller heats and cools together.

Check the sequence, not just the setpoints.

This is a common source of wasted energy.

A well-specified controller carries on locally.

It holds its last strategy and setpoints.

Only supervision and scheduling are lost.

Some products degrade to a fixed position.

Confirm the behaviour before purchase.

Test it during commissioning.