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N°5 – Pressures in closed tanks and circuits pressurized training (VTC level)

In “pressurized” tanks, the pressure at the highest point is added to that of the liquid.


Indicate in [bar] the pressures in the tank below.

Hot water heating systems can be equipped with a closed expansion tank. The heating circuit is no longer in contact with the outside atmosphere. It is therefore possible to increase its pressure even at the highest point.

On systems equipped with closed expansion vessels, water filling is carried out by raising water to the highest point and then adding a little more to have a minimum pressure of 0.5 to 1 [bar].
This pressurising of the high points facilitates the necessary air and water bleeding.

The knowledge of the pressure at any particular point of the system (for example at the boiler) allows us to estimate it everywhere else:


Knowing the pressure at the boiler below (2.9 [bar]), evaluate the pressure at all the points of the installation.
Check that the system is correctly filled.

With 1.6 [bar] at the top, this system is too full.

A pressure of 0.5 to 1 [bar] is sufficient. When the system is functioning the pressure will increase and could perhaps become excessive