Water softening: hardness, salt use, Simplex or Duplex

Scale in the boiler, white deposits on taps and excessive detergent use almost always come down to a single water parameter. A softener removes it completely, but sizing the unit by flow rate alone is not enough: two sites with the same flow may need different tank volumes and different valves. Here is what to calculate before buying.

Hardness is not "bad water" — it is a specific number

Hardness comes from dissolved calcium and magnesium salts. Heat the water and they come out of solution on the walls of heat exchangers and pipes as a dense layer that conducts heat many times worse than metal: the boiler ends up heating scale rather than water.

Hardness is a measured figure — milligram-equivalents per litre (meq/l), the unit that reports also call a degree of hardness. Take the value from a laboratory analysis report, not from how the water feels and not from a neighbour's borehole: within a single village, water from different aquifers can differ a great deal.

An ion exchange unit works like this. Water passes through a tank filled with resin, and calcium and magnesium ions swap places with sodium ions given up by the media. Once the resin has used up its exchange capacity, the controller runs a brine solution through the tank; the resin sends the accumulated hardness to drain and goes back into service. The cycle repeats for years, and the resin itself is not consumed.

What to calculate before choosing a tank

The key figure is not the hourly flow rate but how much hardness the unit has to remove between two regenerations. That is the volume of water multiplied by its hardness.

So the sums go like this:

• ✓take the site's daily consumption in cubic metres;
• ✓multiply it by the raw water hardness in meq/l — that is the daily load on the resin;
• ✓decide how often a regeneration is acceptable (usually at night, every day or two);
• ✓divide the working capacity of the chosen media volume by the daily load — that gives the length of the cycle.

Then comes the second condition: the flow velocity through the resin bed. A tank sized tightly on capacity but too narrow in diameter lets water through the media too fast, some of the ions never get the chance to exchange, and hardness shows up at the outlet long before the cycle is due to end. Diameter therefore sets the peak flow, and height sets the media volume and the length of the cycle.

Both dimensions are there in the unit designation: 0817 is an 8×17 inch tank, 1054 is 10×54 inches. The first number governs peak flow, the second how far the tank will run.

Timer or meter

The second part of the designation is the Runxin control valve: 0817/F117Q3, 1054/F73A. What separates them above all is what they use to time the interval between regenerations.

A valve with a water meter counts the volume that has actually gone through. Regeneration starts once the calculated number of cubic metres has passed the resin, whether that took a day or a week. A valve without a meter counts time instead, and regenerates after a set number of days.

Choosing between the two means choosing between saving salt and keeping things simple:

• ✓Steady, predictable consumption (a house with a settled number of residents, a small boiler room) — a timer is enough.
• ✓Seasonal or uneven load (a hotel, a café, a site that stands idle at weekends) — you need a meter, or the unit will either regenerate for nothing and burn salt, or let hard water through at the end of the cycle.

The valve also sets the connection size and the throughput: on small tanks that means 3/4 and 1 inch at up to 4.5 m³/h, and both grow with the tank. Valves in this range are top-mounted — they screw into the neck of the tank from above.

Simplex or Duplex

Simplex is a single tank. It drops out of service while it regenerates, and for that stretch the site gets either raw hard water or nothing at all. That is usually acceptable, because regeneration is scheduled for the night.

Duplex is two tanks working in turn: while one regenerates, the other keeps supplying softened water. The unit never stops.

The rule is simple: if the site can go an hour or an hour and a half at night without softened water, choose Simplex. If it cannot, choose Duplex. The extra tank pays for itself the first time a shift does not stop because of a regeneration.

Salt, drain and space for the unit

Three things most often left out of the design:

• ✓The brine tank. Regeneration runs on a solution of tablet salt, and the tank has to be topped up from time to time. The larger the media volume, the more salt each cycle takes; the harder the water, the more often those cycles come round.
• ✓The drain connection. Backwash water carrying the accumulated hardness goes to the sewer, so the drain point has to be close by, with an air gap.
• ✓Ceiling height. A top-mounted valve needs clearance above the tank so it can be lifted off for service. On tall tanks such as 14×65″ that adds up.

Check separately that mechanical filtration sits ahead of the softener. Suspended solids and iron from a borehole clog the resin bed and cut its capacity, so the unit starts regenerating more often than it was designed to and the media wears out sooner.

What next

Sizing needs four figures: hardness from the analysis report, daily consumption, peak flow, and whether a break in supply during regeneration is acceptable. Iron and turbidity are worth having too — they decide whether pretreatment is needed.

With those figures in hand you can turn to the catalogue: single-tank Simplex units cover most of what a house or a small production site needs, twin-tank Duplex units cover sites that draw water round the clock. If you do not have a water analysis yet, start there: sizing without hardness figures is guesswork, and getting it wrong costs a whole tank.