The AAV as it is also known is fitted to the Chaffoteaux boiler to ensure that all the air is removed from the central heating system and the boiler. This ensures air can't get into the boiler which could cause air locks and possible damage to the boiler. The auto air vent is normally located on the pump housing ensuring air is removed from the boiler at this point, the point where the water is being circulated and where air bubbles would occur.
The pump circulates water around the boiler and also around the central heating system but if air was present within the pump the pump would cease to be effective, the Chaffoteaux boiler would overheat which would cause the boiler to turn off on an overheat fault. Auto Air Vents are fitted to every Chaffoteaux boiler and should be checked on a regular basis to ensure they are working and in good working condition.