Same valve body, completely different control head. Here's what changed and why it matters for your salt bill.
The original Fleck 5600 used a mechanical timer that regenerated on a fixed schedule regardless of water use. The 5600SXT replaced that timer with a digital metered head that tracks actual gallons and only regenerates when needed. The SXT saves 20–40% on salt costs over a timer system. The price difference is minimal. There is no reason to buy the original 5600 today.
Consider two households with the same 48,000 grain softener. Household A uses 150 gallons/day at 20 GPG hardness — the resin hits capacity in about 16 days. Household B goes on vacation for 2 weeks.
Timer-based 5600: both households regenerate on the same fixed schedule — wasting salt in Household B's case and potentially under-treating in Household A's if the timer isn't perfectly calibrated.
SXT metered: Household A regenerates exactly when needed. Household B's system sits idle for 2 weeks and regenerates when the family returns and starts using water again. Over a year the SXT saves 30-40 pounds of salt in the vacation scenario alone.
Fleck 5600SXT on Amazon →