I wouldn't bother with a twin tank unless you really don't think you have 2 hours available weekly when you won't be using any water . You will spend a lot more than a single tank with no real benefit.
Do you have any high-water-use fixtures, such as a multi-head shower, large bathtub or jet tub? If not, you would be okay with a 1 cuft softener now, which would be a bit small when your family increases to four. A 1.25 cuft softener would be ideal. That size can be difficult to find, so a 1.5 cuft will be fine, and most likely little to no difference in price.
Since you have some manganese, you will need to regen with 8 lb salt/cuft of resin, which will make your softener a bit less salt-efficient, but it's necessary to ensure the manganese gets stripped from the resin during regen.
Make certain you get the following:
High quality American or German made resin. This will provide a tight size distribution for optimal flow within the resin
Since you have WELL WATER: 8% crosslinked resin.
Top basket. This serves two purposes. It sets up a proper water distribution during normal operation and prevents resin loss during backwash.
Gravel underbed. The gravel underbed is there to set up proper flow patterns, improve backwash, keep the bottom basket in place, prevent basket failure, and prevent channeling. Many softener sales companies like to leave this out or sell softeners with a vortex system instead. Vortex systems weigh less than gravel so they cost less to ship. In addition, they are a more expensive item that adds profit for the softener salesperson, but provides no additional benefit to the homeowner . It simply adds another piece of equipment that can break.
Fleck or Clack valves. These set the industry standard. Be aware that you will not be able to purchase Clack valves online. This is not a problem if you purchase locally.
The valve should regen based on water usage, not time.
Noryl bypass. Most softeners are available with either Noryl or stainless bypass valves. Both are good valves, but the noryl tends to be more reliable when not used for long periods of time.
Install the softener with a three-valve bypass. This will make it so much easier if you ever need to remove the softener for repairs or wish to take it with you when you move. Use full-port, quarter-turn valves.
Because of the manganese in your water you will need to take some additional steps to ensure your softener stays in good working order. Softening resin has a high affinity for manganese and doesn't like to release it during regen. Even using a higher salt dosage, some of the manganese remains in the resin and will slowly foul it from the inside out. You have several options:
The easiest option may be to simply use Iron-Remover salt.
Alternatively, you can use Super Iron Out. You may choose to layer this in with your salt. If you choose this option, use 1/4 C per 40 lb bag of salt.
Super...
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