The condensate drain on the aftercooler, receiver, or dryer is stuck, dirty, or broken. Water that should be drained accumulates and gets pushed into the air lines.
What you'll see
Verify the dryer is actually working (check dewpoint). If the dryer dewpoint is correct and the drain is working, the water may be condensing in the piping after the dryer -- see: piping issues or no point-of-use filtration.
How to diagnose
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Check each drain point visually
Disconnect the water drain hose from the condensate trap and watch. Is water actually being discharged? A working aftercooler drain on a typical 50 kW compressor in humid conditions should discharge several liters per day. If nothing comes out, the drain is blocked or stuck.
Result: No water discharge = drain is stuck. -
Manual drain test
Most auto drains have a manual test button or valve. Activate it -- does water come out? If yes, the drain mechanism is faulty but the path is clear. If no water even manually, the condensate trap itself may be blocked or plumbed incorrectly.
Result: Manual drain works = timer or float mechanism failed. No flow at all = blockage.
How to fix it
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Clean or replace the drain valve
Disassemble the auto drain, clean all parts, check the float or timer mechanism. Replace worn seals. If it's a timed drain, verify the timer interval and duration are set correctly. If it's a float drain, make sure the float moves freely.
Timer drains can waste compressed air if set to drain too frequently or for too long. Float-type (zero-loss) drains are more efficient. In either case, verify they're actually draining condensate, not just blowing air.
Replacement auto drain valve or repair kit. Manual ball valve as temporary workaround.