Water remains hard even though the Marlo softener appears to regenerate normally

Safety first: Place the softener in bypass before opening any plumbing or the mineral tank. Depressurize by opening a faucet.

Step-by-step checks:

• 1. Verify programming: Confirm that the hardness setting on the Marlo controller matches your actual water hardness (use a test kit or lab report). Increase slightly if needed.
• 2. Check regeneration frequency: Ensure the capacity and reserve settings are appropriate so the unit regenerates before resin is fully exhausted.
• 3. Inspect salt usage: Monitor salt level over several weeks. If salt is not being consumed, the brine system is not working correctly.
• 4. Perform an extra regeneration: Manually start two back-to-back regenerations to fully recharge the resin. Retest hardness afterward.
• 5. Evaluate resin condition: If the unit is older (10+ years) or exposed to high iron, the resin may be permanently fouled. Consider removing a small sample from the top of the Marlo mineral tank for inspection (resin should be uniform amber beads, not mushy or blackened).

When to replace parts: If hardness remains high despite correct settings and proper brine function, replace the Marlo cation resin media. For iron-fouled systems, also consider adding or servicing a Marlo iron reduction system upstream.