A salt bridge is a hard crust of salt that forms across the brine tank and leaves an empty space beneath it. Water can only dissolve salt where it touches it, so when the crust blocks the water from reaching the salt below, no brine forms. The control may still run its regeneration cycle, but the resin does not get the brine it needs. Hard water can return without any alarm, so the early signs are worth learning.
Why it forms
Salt bridges tend to form when the salt sits in humid air, when the salt has absorbed moisture, or when the tank is filled above the level the manufacturer recommends. Some salt forms are more prone to bridging than others. Keep the lid closed and store salt bags in a dry place, because damp air and open tanks encourage crust.
Signs to watch for
The most useful sign is salt that stays high in the tank for longer than you expect. If the salt level does not drop between refills while the softener is in regular use, brine is probably not forming. Other signs include water in the brine tank that looks low or does not seem to reach the salt, hardness spots or scale returning shortly after a regeneration, and a control that shows a cycle while the water still tests hard. A hardness test after regeneration is useful, but it tells you about the problem only after hard water has already reached your fixtures.
Check the tank safely
Wear gloves and do not climb into the tank. Use a broom handle or a similar wooden or plastic rod and push it gently down through the salt in a few spots. If the rod hits solid salt a short distance below the surface and then stops, with an empty pocket beneath, you likely have a bridge. Also check the water level in the brine tank against the mark the manufacturer shows, and confirm that it rises to the right level after a regeneration. Do not guess at the level. Use the marking on your own unit or the manual.
Clear it and prevent it again
Breaking a bridge usually means carefully loosening the crust so the water can reach the salt, then removing the crust pieces and refilling to the recommended salt level. Ask the company whether they want to handle this on a service visit, especially if the brine tank has sediment or the valve shows other problems. To prevent a repeat, use the salt form the manufacturer names, keep the level at the marked line, keep the lid closed, and store salt dry.
Questions to ask a technician
Ask how the control reports brine levels and whether your unit has a salt level warning. Ask whether the brine tank should be inspected on each service visit or only when a problem shows. Ask which salt form they recommend for your unit and how often they expect you to check the salt level. The answers tell you whether the problem is likely to be caught early or only after hard water has returned.