What hard water means
Hardness mainly reflects the calcium and magnesium content of water. These minerals come in part from the ground the water passes through. Their presence varies with the source and the network supplying your home.
In a French analysis, hardness is often shown as TH (total hardness) and expressed in French degrees, written °f. Always include the unit when sharing a measurement.
How deposits form
Hard water can leave deposits, especially in appliances that heat it. The marks seen in your home also depend on temperature, the volumes used and surface maintenance.
Identify the areas that need the most cleaning and check your appliances' instructions. This helps define your comfort and maintenance needs.
What resin water softening does
A resin water softener reduces calcium and magnesium ions by exchanging them for sodium ions. The resin regains its capacity during regeneration with salt water.
The outlet hardness setting depends on the water composition and the characteristics of the internal pipework. For drinking water, identify the outlets supplied and have the resulting composition assessed for the chosen configuration.
Useful information for sizing
Before choosing a system, gather three sets of information.
Inlet water
Find the hardness reported for your local supply and the date of that information. A measurement at the installation point helps plan the setting.
Household consumption
Check the volumes on your bills and note the number of occupants. Include simultaneous uses, such as two showers running at once.
Installation conditions
Locate the inlet of the circuit to be treated, the available space and the access needed for maintenance. Check connections against the manual for the proposed model.
A budget that includes ongoing care
Compare the price of the system and installation, then salt, checks and the water and energy used for regeneration. Ask for an estimate based on your installation's water volume and hardness.
The aim is practical: to limit new deposits in the softened circuit. Maintenance of existing appliances and checks on the settings still need to be planned over time.
Sources for this guidance
Eaufrance, Water Hardness glossary: definition of calcium, magnesium and French degrees, based on BRGM.
French Ministry of Health, The Quality of Tap Water in France report, 2012 data, section 3.1.5: origins of hardness and its link to scaling. This document is used here to explain the physical process.
Anses, report on water softening and decarbonation processes: ion exchange and resin regeneration. References accessed 7 September 2026.