I have an open vented system with a cylinder in the airing cupboard. There is what I believe is called an air vent cap or capillary air vent cap tee'd from the top pipe coil pipe entering the cylinder. it has a small grooved part which is threaded and screws up and down. I am curious as to the operation of this. Is it an automatic vent or is it used manually to bleed air from the cylinder coil - for example after draining the system?