Curious and alarming hot water problem

Sorry if I've posted this before - I'm easily confused!

I have a single cold water tank in the roof, an indirect hot water cylinder and a wall mounted Potterton gas boiler.

The central heating pump has a standard 1" pipe into the bottom, and a converter to a 1/2" pipe at the top. Actually, the pump seems to be mounted upside down...

Anyway; the bottom, 1" pipe circulates the centrral heating water AND the water into the cylinder coil. Both the cylinder and the main CH pipe have electronic valves on them.

The problem: periodically during the day what sounds like a large volume of air enters the system. The hot water vents into the roof tank to truly alarming noises.

I've changed the cylinder thermostat; changed the programmer; taken all the radiators off, flushed them through and refilled the system. But the problem remains.

Can anybody advise me please?

That could be a problem, there's usually a 50ish gallon cold water storage tank and a 4ish gallon feed & expansion tank for the heating. Unless the heating is a sealed system; is there a pressure gauge & filling loop on it?

Too many possibilities to list and it sounds very alarming. I'd recommend you try to find someone competent to look at it. Quick. And turn it off until they have.

There should be an open vent pipe from the boiler which has an open end over the F&E tank. If you can find it, does water discharge from this? The open vent should always be open, i.e., neither of the electric valves should be able to close it off. Can you figure out the pipework layout to ensure this is the case?