I have 7 people and 4 bathrooms with 3 high flow shower heads. What size water softener should I get? should I get a twin or a single?
I have city water so iron is not a problem and I have the following:
10gpg Hardness
PH: 7
chlorine: 0.4-0.6
Any large jetted or non jetted tubs?
What is the gpm rating of the shower heads?
Your softener has to have a constant service flow rate (SFR) higher than your peak demand gpm. That is based on the cuft of resin and then using a control valve that can service the size tank that volume of resin requires. That is going to size your softener.
IMO your peak demand is the total gpm of how you use water in the house (what fixtures re used at the same time) rather than a total of all fixtures in the house. That is explained at the link in my signature but you have to come up with the actual gpm.
Twin tank type softeners sound great if you don't learn their disadvantages. It is a rare twin tank that allows water flow through both resin tanks at the same time. That means you get water through only one tank and.... since a twin regenerates as you are using water, your flow rate is decreased by the gpm rating of the DLFC (drain line flow control). It also means much more expense to buy the correct size twin tank type softener.
Most folks selling twin tanks always go on about a regular softener wasting salt because of having to have a reserve capacity that is rarely used but the salt dose having to be set to regenerate that resin as if the capacity has been used between each regeneration.
What they don't tell you is that since the twin tanks use soft water to regenerate with that that requires salt use to regenerate the capacity needed to do that. And then they claim the twin uses less salt but..
If a regular softener and the tanks of the twin are the same size so both softeners have the same constant SFR, and both softeners are using the same type of resin, it will take the exact same volume of salt to regenerate the same K of capacity in the two types of softeners. Meaning both softeners will use the same volume of salt and thereby have the same salt efficiency.
That also means that the twin tank has to have the same size tanks as the regular softener has unless the twin allows water through both tanks at the same time. And IF that is the type of twin you get, you lose some of the flow from one tank while it is used to regenerate the other tank and you normally are using water during a regeneration of any type twin tank. That's because you have to be using water to trigger a twin to regenerate. That will reduce its constant SFR gpm until the regeneration is done.
The best twin type softener is two regular type correctly sized softeners (each large enough to provide the needed constant SFR gpm) ganged together with one providing the house soft water while the other independently regenerates. They are not very common and are quite expensive and really....
Unless your house is constantly using water 24 hrs a day, you don't need a twin tank type water softener.