When a water softener is recharging, it is designed to disconnect from the water system. For this reason, the regeneration cycle is usually set to occur at night. Large families or those who live where water is particularly hard—where the water is rated more than 10 grains per gallon (GPG), for example—may want to consider a unit with two resin tanks. With these units, when one tank is in use, the other is regenerating. As a result, you never run out of softened water. And because these units operate on demand, they can be sized smaller than single-tank units.

Several models are made. One popular product, the Fleck 9000, has a fully adjustable valve that is controlled by a meter. When it starts backwashing one tank, it switches the water supply to the other tank, offering a continuous flow rate of 21 gallons per minute. With this model, you can buy various tank capacities—24,000, 32,000, 40,000, 48,000, 64,000, 80,000, and 110,000 grain capacity per tank. These range in price from about $950 to $2,000.

Why we like it: • Treats water for the entire house • Relatively easy to install • Does not require salt or periodic backwashing

When shopping for a dual-tank water softener, keep in mind the space it will require. It should be installed where it can serve the main inbound water line so it can serve the entire house. It will also require a drain for backwashing. If you choose a model that requires electrical power, a nearby circuit will be required.

Also look for features such as NSF certification and a solid warranty on the control valve and mineral tank, such as three years for the valve and 10 years for the mineral tank. A good softener should last at least 20 years.