--- title: "What is Total Hardness As CaCo3 mg/L?" description: "Total Hardness measures the amount of hardness-forming minerals, mainly calcium and magnesium, dissolved in the system water. High hardness can increase the risk of limescale and other deposits forming on hot surfaces, reducing heat-transfer efficiency and potentially affecting system performance and component life." url: https://help.fernox.app/en/articles/152-what-is-total-hardness-as-caco3-mgl collection: "Chemical Parameters" updated: 2026-10-02 language: en alternates: [es, de, nl, pl, fr, fr-be, nl-be, cs, sv, ro, sk, tr] --- # What is Total Hardness As CaCo3 mg/L? Total Hardness measures the amount of hardness-forming minerals, mainly calcium and magnesium, dissolved in the system water. High hardness can increase the risk of limescale and other deposits forming on hot surfaces, reducing heat-transfer efficiency and potentially affecting system performance and component life. ### Total Hardness — as CaCO₃ mg/L **Technical definition** Total Hardness is a measurement of the combined concentration of **dissolved multivalent metal ions**, principally **calcium (Ca²⁺) and magnesium (Mg²⁺)**, present in the heating system water. It is expressed as **milligrams per litre of calcium carbonate (CaCO₃)** to provide a standardised measure of hardness. Water hardness is primarily determined by the mineral content of the original filling water. In a heating system, total hardness is important because calcium and magnesium compounds can become less soluble as water is heated and may form **scale deposits**, particularly on high-temperature heat-transfer surfaces. Total Hardness therefore provides an indication of the system water's overall **scale-forming potential**, particularly when assessed alongside alkalinity, pH and temperature. **Simplified definition** Total Hardness measures the amount of hardness-forming minerals, mainly **calcium and magnesium**, dissolved in the system water. Hard water contains higher levels of these minerals. When the water is heated, they can contribute to the formation of **limescale and other deposits** inside boilers, heat exchangers and pipework. ### Why is Total Hardness important to test? Total Hardness is important because elevated hardness can increase the likelihood of **scale formation**, particularly within components where the system water reaches high temperatures. An abnormal Total Hardness result may indicate: - **Hard mains water** being used to fill the heating system. - An increased potential for **limescale formation** on heat-transfer surfaces. - Repeated topping-up of the system with fresh mains water. - Ineffective or incomplete use of softened or demineralised filling water where this is expected. - Changes to the system water following draining, refilling or maintenance. - A possible leak or pressure-loss issue if hardness progressively increases because mains water is repeatedly being introduced. - Water chemistry that may require additional treatment or consideration under the applicable system-water guidelines. When hard water is heated, calcium and magnesium compounds can form deposits on internal surfaces. **Calcium carbonate scale** is particularly important because it tends to form on the hottest surfaces within the system, including boiler and heat pump heat exchangers. Scale accumulation can contribute to: - Reduced heat-transfer efficiency. - **Higher energy consumption** to achieve the required heating output. - Localised overheating of heat exchanger surfaces. - Reduced flow through narrow waterways. - Increased system or boiler noise. - Restricted valves, pipework and other components. - Reduced appliance performance and service life. - Premature component failure where deposits become severe. Total Hardness can also provide useful information about the system's filling history. For example, a system originally filled with **demineralised or low-hardness water** would normally be expected to retain relatively low hardness. A subsequent increase may indicate that untreated mains water has been introduced through topping-up or refilling. Total Hardness should therefore be considered alongside parameters such as **calcium, M-Alkalinity, P-Alkalinity, pH and conductivity** when assessing scale risk and overall system water quality. ### Important interpretation note Total Hardness should **not normally be assessed in isolation**. A high hardness result indicates that scale-forming minerals are present, but it does not automatically mean that scale is currently forming. Scale formation depends on several interacting factors, including **calcium concentration, alkalinity, pH, temperature and overall water chemistry**. The risk is generally greater on high-temperature surfaces where mineral solubility can decrease and deposits can form. It is also important to distinguish between **Total Hardness and Calcium**. Calcium testing measures calcium ions specifically, whereas Total Hardness represents the combined contribution of calcium, magnesium and, to a much smaller extent, other hardness-forming ions. The acceptable hardness level may also depend on the **heating appliance manufacturer's requirements, system volume, filling-water treatment method and applicable water-quality guidance**.