Water quality
Water is often used as a heat transfer medium in heat generation and heat distribution systems. With 4.19 kJ/kgK, it has a high specific thermal capacity. It is also characterised by high availability, environmental compatibility and low reactivity. However, the quality of water found in nature varies from region to region. Metals or salts, but also gases, are often dissolved in water. These may cause problems and damage in the hot water system, which is why the water must undergo water treatment. The required water quality depends on the heat generator output, the maximum operating temperature and the materials used in the system.
Required water quality
The values for water quality in hot water and boiler systems are specified in VDI 2035 and AGFW worksheet FW 510 (or TECH 1466 from VdTÜV). The table “Water quality overview” provides an overview of the respective values. The VDI also has an additional classification, divided according to system volume per kilowatt of heating capacity. However, this shall not be considered here because the values listed in the following table apply to systems with an output range of > 1,000 kW.
| VDI 2035 | AGFW FW 510/TECH 1466 (VdTÜV) | ||||
| Range of applications | Heating water systems | Heating and hot water systems | |||
| Temperature range | Up to 100 °C | From 100 °C | |||
| Conductivity at 25 °C | Low-salt: | Containing salt: | Low-salt: | Containing salt: | |
| 10 to 100 | 100 to 1,500 | 10 to 30 | 30 to 100 | 100 to 1,500 | |
| Appearance | Clear, free of sedimenting substances | Clear, free from suspended substances | |||
| pH value at 25 °C | Without aluminium alloys | With aluminium alloys | 9.0 to 10.0 | 9.0 to 10.5 | 9.0 to 10.5 |
| 8.2 to 10.0 | 8.2 to 9.0 | ||||
| Hardness (alkaline earth) [mmol/l] | < 0.05 | < 0.02 | |||
| Oxygen [mg/l] | As low as possible | < 0.1 | < 0.05 | < 0.02 | |
Overview of water quality (own representation based on VDI 2035, 2021, p.24 and TECH 1466, 2014, p.11)
Beyond this table, the manufacturer may impose higher water quality requirements, which must be strictly adhered to. In the case of Bosch Industriekessel GmbH, operating instructions B004 apply as a guideline. It is based on AGFW and EN12953-10, also for temperature ranges below 100 °C. Due to the high water volumes in Bosch hot water and boiler systems, a long service life is paramount, which can only be ensured by imposing stricter requirements on water quality, particularly with regard to hardness and deposits. We recommend using a desalination plant that removes all salts from the water in order to ensure that the low salt content in the circulating water also results in very low corrosion tendencies in the boiler and network. Furthermore, constant monitoring of the water quality is an important factor for the economical and trouble-free operation of the boiler system. Continuous monitoring of the relevant parameters enables a dynamic response to changes in the circulating water and protects the heat generator and heat consumers from damage.