Limescale is calcium carbonate deposited from hard water onto hot surfaces, and the products sold against it work in three genuinely different ways, plus a fourth category whose claims do not hold up. Knowing which is which prevents a good deal of money going to the wrong place.
Removal, which fixes everything
Ion exchange softening removes the calcium and magnesium from the water entirely, so no new scale can form anywhere and existing deposits lift gradually over months. It is the only approach that also fixes the other hardness complaints: poor lather, soap scum, spotting on glass and stiff laundry. It costs a drain connection, salt, space for two tanks, and it adds sodium. The USGS classes water above 180 mg/L as calcium carbonate as very hard, where the case for removal is strongest.
Conditioning, which fixes the scale only
Template assisted crystallization passes water over a media bed whose surface nucleates microscopic hardness crystals that then travel through the system rather than bonding to hot surfaces. Scale in the heater and appliances drops away substantially. Hardness remains in the water, so lather, soap scum and spotting continue unchanged, and a hardness test reads the same as before. It needs no drain, no salt and no power, which is why it suits apartments and septic systems.
Sequestering, for one appliance
A polyphosphate cartridge adds a small dose of sequestering agent that binds the minerals and holds them in solution past the point where they would deposit. It is cheap, small and fitted on the feed to one appliance such as a combination boiler or a coffee machine. It depends on continuous correct dosing with no warning when the cartridge is spent, and polyphosphate reverts at sustained high temperature, so it is thin cover on very hard water.
The category that does not work
Magnetic and electronic descalers clamped to a pipe are widely sold and the evidence for them is poor. Independent testing has not established a reliable, reproducible reduction in scale, and the mechanisms proposed vary between vendors. That is not the same as saying nobody has ever seen an improvement, but it means the effect cannot be relied on when specifying a system. Spend on one of the three approaches above instead. Our whole house filter guide covers the options.
Questions people ask about limescale water filter
Which approach stops limescale completely?
Ion exchange softening, which removes calcium and magnesium outright, so no new scale forms and existing deposits lift over time.
Do salt free conditioners work?
Against scale on hot surfaces, substantially. They do not remove hardness, so lather, soap scum and spotting continue.
Are magnetic descalers effective?
The evidence is poor. Independent testing has not established a reliable reproducible reduction, so they cannot be specified with confidence.
What protects just one appliance?
A polyphosphate cartridge on its feed, which sequesters hardness so it deposits less. Cheap, small, and thin cover on very hard water.