Twin tank vs Single tank water softeners, what’s the difference?

Harvey Minimax Water softener installed in kitchen cupboard
Twin tank water softeners, sometimes referred to as twin cylinder, have two water softening vessels whereas single tank softeners have one vessel.

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In the UK, the twin tank water softener is the most popular choice due to its compact size and, as a result, you will most likely find lots of companies saying that the twin tank is the best option.  Whilst they do have many advantages, a single tank softener can be a better option for some people. Here is an outline of the differences and pros and cons of both types:

Twin tank water softeners

Having two tanks allows the softener to soften water with one tank and at the same time also be performing a cleaning cycle through the other tank using a small amount of saltwater.

The biggest benefit of this design is it allows the machine to be more compact compared to single tank softeners where the tank must be large enough to last the day before completing a cleaning cycle (regeneration)

Twin tank softeners are more compact

Twin tank softeners typically use salt that has been compressed into blocks. This block salt makes loading the machine easier and is another contributing factor to it’s compact design. The block salt comes in 8 kg packets and typically costs around £5- £6 per bag.

Twin tank water softeners are also non-electric. As the twin tank water softener can simultaneously clean itself and soften the water at the same time they don’t need to know the time of day and can operate without electricity. They use water meters (similar to the type used by your water supplier) to measure the amount of water passing through. Then, using gears and the flow of water to drive the mechanism, the whole process is automatically controlled without any electrical components. We like to say it’s a bit like clockwork driven by water. Having no electrics means the machine is easier to install and cannot break down due to a fault with anything electrical.

Twin tank softener Pros

  • More compact allowing them to be fitted in more places
  • They use block salt which is easier to handle
  • No electrics makes installation easier

Twin tank softener Cons

  • Not as efficient as the best single cylinder
  • Will need more frequent salt top-ups than a single tank softener.

Single tank water softeners

Most water softeners sold worldwide use a single water softening vessel. This design works very well as long as the water softener is correctly sized. Having one vessel means the softener will go through its cleaning cycle at night time when there is no demand for wate. For basic models this could be every night, for the best-designed softeners they will run the cleaning cycle only when necessary. For the purposes of this article we will refer to the better-designed models. 

The best single tank water softeners are actually more efficient and are cheaper to run than a twin tank softener. Per cycle they use more salt but overall, as they are cycling less often, they can be use less salt compared to a twin tank compact softener. 

The best single tank water softeners are actually more efficient and are cheaper to run

Single tank softeners use tablet salt. Tablet salt is sold in 25 kg or 10 kg bags which per kg is cheaper to buy, but is a bit harder to handle and load. The reduced cost per kilo along with slightly less salt needed reduces the running costs of a single tank machine vs a twin tank.

They often have a larger salt storage area meaning they need to be loaded less frequently and therefore are easier to manage with a larger family or houses using lots of water.

Lastly is the lifetime of the softener. A single cylinder softener, if sized correctly, can have a longer lifespan. As they typically have more resin in the cylinder than a twin tank this means they will last a bit longer because resin, the medium the softener uses to work degrades over time. With more resin, the machine can be adjusted as it gets older to still ensure the water is still softened effectively –  something that isn’t possible with a non-electric twin tank machine.

Single tank softener Pros

  • Most have larger salt containers, so they need topping up less often
  • Tablet salt is cheaper, lower running costs
  • Better suited to houses using lots of water
  • Best single tank machines are more efficient than twin tank designs
  • Longer overall lifespan

Single tank softener Cons

  • Larger overall, they won’t fit in everyone’s home
  • Tablet salt is less convenient to use
  • Cheap single tank softeners aren’t as good

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