You could go with a 1.25 cubic ft softener, but it will be less slat-efficient than a 1 cubic ft. A 1 cuft would regen every 7 days at 6 lb salt per cuft of resin. A 1.25 cuft would theoretically regen every 9 days. However, your softener will operate better for a longer period of time if you regen every 7 days. The difference between the two, in terms of salt usage would be 3 lb per month. So, if you want the ability to have a slightly higher flow, go with the 1.25.
The Fleck 5600sxt you mentioned is a good, reliable valve that should provide many years of service. Once your softener arrives I can help with programming for best efficiency.
Things you need:
1) Top distribution basket
2) Gravel underbed
3) High quality American or German resin, 10% crosslinked (because you are on city water)
4) Noryl bypass.
5) Plumb the softener with a separate bypass so you can remove it, if necessary, without disrupting water flow to your home.
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