Do You Need a Twin Tank Water Softener?

The Short Answer

If you're a residential homeowner you probably don't need a twin tank softener. The exceptions are when water hardness is extreme or where salt efficiency is the primary system selection criteria.

If your water hardness is so high that it's unlikely a traditional single tank system can make it through a 24-hour period without becoming fully depleted, a twin tank system is an ideal choice. Usually this means a hardness value north of 40 grains per gallon (684 mg/L) for a residential application.

If salt efficiency is the main thing you're looking for in a softener, twin tank systems offer a unique advantage. Single tank softeners are set to regenerate when they still have a little capacity remaining, where twin tank systems fully deplete their capacity before regenerating. This gives twin tank systems an extra edge in salt utilization.

What Is a Twin Tank Water Softener?

Twin tank softeners are systems that have two softening tanks instead of one. The softening tank is the tank that contains the water softener media (often called "resin"). Most softeners have a single softening tank and then a brine tank for the salt. Twin tank softeners have two softening tanks, and then the brine tank. Twin tank softeners are sometimes referred to as "twin alternating" softeners which provides a hint as to how they operate.

How Is a Twin Tank Different From a Single Tank Softener?

Aside from the physical differences between the two systems (two softening tanks vs one) there are operational and application differences between twin tank and single tank systems.

Single tank water softeners put all of the softening resin into a single tank and it's in this single tank that all of the softening occurs. Twin tank systems split the softening resin into two tanks that sit side by side. In a twin tank system only one tank is softening the water at a time, and the second tank sits in standby.

In a single tank water softener all the water in the home passes through this single softening tank and the controller monitors the water consumption and the remaining capacity. As that capacity approaches zero, it coordinates a regeneration to replenish the capacity. For homes with normal water and normal water consumption, the regeneration of the softener typically happens once every 3 to 14 days. The harder the water and the greater the water consumption, the faster the softener capacity is depleted, and the more often the system needs to regenerate. When the system is regenerating, which takes from 60 - 120 minutes, home water can be used, but that water won't be softened. So most modern softeners regenerate themselves in the middle of the night when water demand is typically low

Situations can arise where the water hardness in a home is so extreme that a single tank water softener can't make it through a single day before the softener is depleted. In these situations a single tank system would need to regenerate mid-day to continue to deliver soft water, but during the regeneration any water used in the home would be unsoftened. When water hardness is this extreme even small quantities of hard water can cause the rapid formation of hard water scale on surfaces and can be very hard on water using appliances and hot water heaters. It's in these situations where a twin tank system shines: it can seamlessly switch to softening from the backup tank as it regenerates the primary tank creating an uninterruptible supply of soft water no matter how hard the water is, or how much water is required.

It's for these same reasons that twin tank softeners are used in many commercial applications: 24-hour carwashes, bakeries, and factories are common use cases.

When Do You Actually Need a Twin Tank Softener?

You need a twin tank softener when your home water is so hard that a single tank softener can't get through a single day without running out of capacity.

If we use a large residential single tank softener as an example, we can look at the "softening math" in more detail:

A 2.0 Cubic Foot Water Softener has a capacity of 48,000 grains when the medium salt setting is used. If a household water hardness level is 100 grains per gallon (GPG), this system can soften about 480 gallons of water before it's depleted. In many homes 480 gallons of water is enough to get through a day, but for higher water-use homes it is not. And if the hardness is 125 grains per gallon, that water capacity number drops to 384 gallons. You can see that with an increasing hardness value or water-use value a point is reached where even a large residential system won't be able to deliver soft water for 24 hours without running out of capacity.

Another use-case for twin tank systems are situations where a consistent regeneration window is hard to find. Consider a home with extremely hard water where one of the residents works shift work. This home doesn't have a consistent time of day when a single tank system can regenerate where there won't be home water demand. In these cases a twin tank system is useful as it removes the need to have that consistent regeneration time for the softener.

When Is a Single Tank Softener the Better Choice?

Almost always. Outside of the use-cases described above, single tank softeners are the go-to design for a number of reasons. The most obvious advantage of single tank softener is price. Twin tank softeners use two softening tanks and are significantly more expensive than single tank for that reason. The second advantage of single tank systems is much less obvious.

The water softening process is not instantaneous. The longer the water is in contact with the water softening resin the more thoroughly it is softened. In other words, the more softening resin that a softener is built with, the better it softens the water under all flow conditions.

Consider a single tank water softener built with 2.0 cubic feet of resin. All of the water used in the home will pass through this 2.0 cubic foot volume. If we take this same volume of resin and split it into a twin tank design, each tank will contain 1.0 cubic foot of resin and since in a twin tank design only one tank is "active" the water will be treated by only 1.0 cubic foot of resin all of the time.

Two cubic feet of resin can support a flow rate of about 10 gallons per minute, while 1.0 cubic foot of resin can support a flow rate of about 5 gallons per minute. In other words, at any given flow rate, the larger 2.0 cubic foot single tank softener will deliver much better softening performance than the twin tank. Under all conditions it will minimize the amount of hardness breakthrough, and under high flow conditions it will continue to effectively soften the water where the 1.0 cubic foot tank will fail.

So residentially the logic is quite simple: if you have a consistent window when you can regenerate a single tank system it will deliver better softening performance than a twin tank system built with the same total volume of resin.

Do Twin Tank Softeners Use Less Salt and Water?

Twin tank water softeners will use slightly less salt than single tank systems but there won't be any considerable water savings.

When a single tank water softener regenerates, it usually does so with some amount of softening capacity remaining. This is to ensure that the supply of soft water never runs out. But this remaining capacity was previously "paid for" in a previous regeneration. It used up salt to generate that leftover capacity and this has a cost. Twin tank systems run the capacity down to zero, or very close to it, so all of the capacity generated gets used and not wasted.

As for water use, this is proportional to the water softener resin capacity regardless of whether it's a twin tank or single tank. So there's no advantage or penalty in water usage between these two different designs.

How Much More Does a Twin Tank Softener Cost?

Twin tank softeners are about double the cost of the single tank equivalent. In other words, if a 1.0 cubic foot single tank systems costs X, a twin tank system with 1.0 cubic foot of resin in each tank costs 2X. The extra cost is for the extra softening tank, the hardware that connects Tank 1 to Tank 2, and the twin alternating style control valve which can operate and coordinate the two softening tanks.

Which Twin Tank Softener Should You Buy?

For residential and small commercial/industrial applications we love the Clack WS1EE Twin Tank control valve and we whole-heartedly recommend twin tank systems that are built with it. We use this control valve extensively in our twin tank softener lineup. This control valve is an extension of the renowned Clack WS1 platform and like the rest of these products it's rugged, easy to install, easy to maintain, and the "EE" controller has flexible programming that allows for fine-tuning of system performance. It's also exceptionally easy to troubleshoot, clean and fix should the need arise. This platform maintains a 1" flow path throughout the entirety of the softener making it ideal for higher-flow residential applications and opening up its use for many commercial industrial applications.