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N°6 – Filling pressure, installation in off and on mode training (VTC to A level)

Installation in off mode, the filling of a heating system with water consists of:

  • raising the water to the highest point. During this water loading, to allow the water to enter the circuit, there must be an air vent at the top. Large systems are equipped with one or more manual bleed valves at the highest points.
  • adding a little water after closing the manual bleed valves to have a minimum pressure of 0.5 to 1 [bar] at the system head. This pressurizing of the high points allows us to easily carry out the supplementary air-bleeding necessary.

We must not increase the pressure as any excess pressure will have consequences on the lower points, where pressure is obviously higher.


What pressure range on the manometer below will show us that the filling of the system is complete?

1.7 to 2.2 [bar]

A pressure of between 0.5 and 1 [bar] at the system head allows for easy bleeding of radiators.
The height of water above the filling manometer being 12 [m] (39.4 ft), we need to see a reading on the manometer a pressure of between:
1.2 + 0.5 = 1.7 [bar] and 1.2 + 1 = 2.2 [bar].

To ensure good functioning, a pressure of at least 1 [bar] is required at the pump inlet.
An insufficient pressure provokes the phenomenon called pump cavitation which is water vaporizing at the pump inlet.
Cavitation can deteriorate the high-speed wheel turning inside the pump casing.
The risk of cavitation increases with temperature and lack of pressure and particularly affects glandless circulating pumps.
In consequence:

  • A filling pressure of an apartment block system (with a low water height) should be approx. 1 [bar] when cold.
  • In the case of a system-head installation, a filling pressure at the boiler of approx. 1.5 [bar] is required. It is not advised to plan for more in the case of systems functioning with water at less than 100 [°C] (212°F) because any excess will have consequences on the low points of the system.

Nevertheless, for system head installations equipped with glandless circulating pumps in which the risks of cavitation are more important, it is very important to check the minimum pressures recommended by the manufacturer.


Determine the scale of pressures in the system on stop (system-head installation), below, at the end of water filling.

In the case of a system-head installation, a filling pressure at the boiler of approx. 1.5 [bar] is required.

Nb : During the temperature warm-up of a heating system, water pressures increase all over by 0.5 to 1.5 [bar] more, due to expansion (except with open expansion tanks).


The system below is on stop. The pressure at the boiler is 2 [bar].
Indicate a range of normal pressure on the filling manometer of the system when it has reached its operating temperature.

2.5 to 3.5 [bar] (increase of 0.5 to 1.5 [bar] of pressure during the expansion phase).

Start the film below (switch on the sound) and watch the fill-up and temperature increase of a heating system equipped with a closed expansion tank.


Indicate a scale of pressures on the manometer below when at operating temperature (system-head installation).
What is the pressure on the other manometers (ignoring the influence of the pump)?

For a pressure at cold of 1.5 [bar] on the system-head boiler, the pressure at the end of expansion is between 2 and 3 [bar].


The system below is at operating temperature.
The pressure at the boiler is 1.9 [bar]. Is the system correctly filled?

This system is insufficiently filled.
When cold the pressure at the boiler should be at least 1.7 [bar] (0.5 bar at system head + 1.2 [bar] due to the height of water = 1.7 [bar]).
At operating temperature, the pressure on the manometer should be approx.:
1.7 [bar] + (0.5 to 1,5 [bar]) due to expansion = 2.2 à 3.2 [bar].
The pressure of 1.9 [bar] is therefore insufficient. When the water retracts the pressure in the system will fall and there is a risk of a lack of water at the system head.

  N.B. :

  • On small heating systems supplied by wall-mounted boilers, the pressure variation between cold/stop and operating temperature is in general low, approx. 0.5 [bar]. This is due to the boiler being fitted with a high capacity interior expansion vessel.
  • Chilled water distributors (for air conditioning) at stop must be more pressurized than hot water heaters. Indeed, when switching on chilling units, water retracts and pressure drops by approx. 0.5 [bar]. As a consequence, at the end of filling a chilled water circuit a pressure of 1 to 1.5 [bar] is required at the top. With system-head installations, we plan a pressure of approx. 2 [bar] at the top.
  • Closed thermal solar installations with forced water circulation at stop must be well pressurized so that the water pressure at the sensors is approx. 2 [bar]. This high pressure is necessary to prevent the water vaporization phase of 2 [bar] which takes place above 130 [°C] (266°F).

Note that when the sensors are no longer in contact with water, the temperature can rise to more than 200 [°C] (392°F). Pressure will be limited by a safety valve pre-adjusted to a maximum of 6 [bar]. Due to this, at the end of filling or cold degassing, the automatic bleed valves of the sensors must be isolated by means of a tap, because they could well not support the high pressures and could have high temperature steam leaks.