Oversizing a softener usually only results in a longer interval between softener regeneration cycles, but if you oversize too dramatically it can have some negative unintended consequences.
Oversizing Softeners & Efficiency
When we talk about the efficiency of a water softener, we're talking about the amount of salt and water it takes to regenerate the system when it has become saturated. The size of softener you select will play into this to some degree. Almost all professional grade softeners (i.e. not the ones you buy from the Big Box guys) will have the ability to adjust the amount of salt used per regeneration and this directly affects how much softening the system can do before it needs to regenerate (learn more about that in our Ultimate Guide to Water Softeners).
If you choose a larger water softener - one that's built with a larger volume of resin - then you can use a lower salt dosage and still get enough capacity out of the system that it's not regenerating too often.
Oversizing Softeners & Iron Concentration
Iron (and it's cousin, Manganese) are present in some concentration in most water supplies, but can be present in quite high concentrations in rural well water. These elements are notorious foulants of water softener resin. They like to bind to the water softener and not let go. The longer they are adhered to the surface of the resin without being backwashed off, the more likely they are to cause permanent fouling. So in applications where iron or manganese are present oversizing a softener can cause the softener to have so much capacity that it regenerates too infrequently to wash these elements off the resin before they cause damage. So when iron and/or manganese are present it's often better to have a more modestly sized water softener that regenerates more frequently. The longterm success of the system will be maximized this way.
Oversizing Softeners & Day Override
All the softener sold by Aquatell and by most professional dealers are demand regenerated softeners. This means that they measure the amount of water used and regenerate based on that consumption. Most digitally controlled water softeners will have a Day Override function. The day override function is a number of days that if elapsed will cause the softener to regenerate whether the all the capacity of the system has bee used up or not. It overrides the normal cycle of the softener regenerating based on the amount of water consumed. This function helps to ensure that the resin is being regenerated (cleaned) often enough that it does not become damaged, which can happen from prolonged periods of low water throughput. If you too dramatically oversize a softener you may accidentally make the system operate in a very inefficient manner: the system will use salt and water to regenerate to its full capacity, but there's a risk that it's so much capacity that your home won't consume all of it by the time the day override kicks in. So you can end up having 20, 30, even 50% of the capacity of the system still remaining yet having the system force a regeneration to protect the resin life.