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N°7 – Bleeding and draining training (VTC level)

After water filling the technician must carry out air-bleeding of all units and especially those situated at high points.

Several stop/starts of the pump will assist in pushing the air pockets to the high spots where they are evacuated by manual or automatic bleed valves.

Installed at system high spots, automatic bleed valves evacuate the gases in the pipe


Explain the working of the air purger below.

When the gas enters the air purger below, the float (1) descends and the valve (2) opens. When the water level climbs the float (1) rises and the valve (2) closes.

These degassing operations must be carried out throughout the first heating season, because dissolved gases escape from the water during the rise in temperature phase. Due to this it is strongly recommended to replace heating or chilling system water as infrequently as possible.

To empty a system, it is necessary to allow air to enter the system at the top, if not draining will be very slow.
We try to limit water draining to a strict minimum, because each draining is a source of oxygen entering the system, and thus corrosion.


The system below is at stop and is full of water. The technician must replace the top radiator of the right-hand column. To do this he will carry out a partial draining.
At what pressure on the filling manometer (at the boiler) must he halt this draining?

A pressure on the filling manometer of approx. 0.9 [bar], corresponding to a height of 9 [m] (29.5 ft) of water above the filling point will ensure that the radiator to be changed at the top of the right column is empty.