Thermostatic mixing valve for centralised solar thermal systems


The thermostatic mixing valve is used in solar thermal systems that produce hot water for domestic purposes.
It is designed to maintain the preset temperature of the mixed water delivered to the user outlet, when there are variations in the temperature and pressure of the hot and cold water at the inlet, or in the drawn-off flow rate.
This particular series of mixing valves has been specifically designed for systems requiring high flow rates and they can work continuously with hot water supplied at high temperatures from the solar water storage.

High temperature and solar systems

In solar thermal systems with natural circulation in the primary circuit and a water storage with heating jacket, the temperature of the domestic water in the storage can vary considerably depending on the degree of solar radiation and can reach very high temperatures over long periods. In summer, and if there is little water usage, the hot water at the storage outlet can actually reach temperatures around 98°C before the temperature and pressure safety relief valves are actuated.
At these temperatures, the hot water cannot be used directly, because of the danger of scalding for users. Water temperatures over 50°C can cause burns very quickly. For example, at 55°C, partial burn occurs in about 30 seconds, whereas at 60°C partial burn occurs in about 5 seconds.
It is therefore necessary to use a thermostatic mixing valve able to:

  • reduce the temperature of the water distributed throughout the domestic water system to a value lower than that in the storage and suitable for the end user. For reasons of safety, it is recommended to set the temperature of the mixed water sent to the consumer to values no higher than 50°C.
  • keep mixed water temperature constant despite variations in inlet temperature and pressure.
  • continue functioning over time without any loss in performance and without problems due to the continuously high temperature of the inlet water.
  • enable the high temperature water in the storage to last longer, by distributing water to the user circuit at a reduced temperature.

Operating principle

The regulating element in the thermostatic mixing valve consists of a temperature sensor fully immersed in the mixed water outlet pipe which, by expanding and contracting, continuously ensures correct proportioning of hot and cold water at the inlet.
The water proportions are regulated by a piston that slides inside a special cylinder placed between the hot and cold water passage channels.
Even if the pressure drops due to hot or cold water drawn-off by other consumers or temperature variations at the inlet, the mixing valve automatically adjusts the water flow rate until the set temperature is achieved.


Serie 2521 ... Thermostatic mixing valve for centralised solar thermal systems

Tehnical documetation

Serie 2521 ... Tehnical documentation