Situation -

Walkin box - about 30' by 60' by 12'

Two evaps - hung at about 10' on one long wall.

Two condensing units outside at ground level.

The line sets are vertical and external to the walkin box.

Suction line is insulated - liquid line is not insulated.

One unit is running - the other unit is not running. The 'off' system is pumped down and has an isolation ball valve closed at the condensing unit.

95 day. The suction line sweats from the closed ball suction up towards the evaporator.

Why is that? What is happening to produce refrigerating effect in a dead-ended suction line?