Glowworm Fuelsaver F overheat tripping all the time

[muggo]

Hi
Since I drained down my central htg sys to replace a rad, the overheat trip switch needs resetting all the time.
I've added anti corrosion AND anti noise additives, but after 3 weeks it hasn't improved.
I know there's air in the sys (v loud banging at times when switching off) & the boiler is "kettling" which is causing the problem I suspect, but I've no idea how to cure it.
90% of the time, I have to press the red reset button underneath before the boiler will ignite. I effectively have a manual htg sys as the timer just switches the pump on. I have to remember to reset the trip every night or I wake up to a cold house.
I've bled the rads OK & once it is running, there's plenty of heat in the house. The system was installed over 20 years ago so unlikely be a system design fault & I don't have the money for a new boiler as, apart from this fault, it works fine. The system was flushed out after running cleaner through last winter. The boiler heat is set at 4 out of 5 in case it makes a difference.
My local plumber once told me the cutout sensor can be adjusted as it happened once before. Now he tells me it can't. He's too busy to take a look.
Can anyone help please?
Thank
Paul

 

[combispecialist]

its either air in your system still or maybe when uve filled the system up its caused a blockage in the cold feed pipe entering ure system,check to make sure the header tank is full in loft if thats ok cut the cold feed just before it enters the primary pipework and make sure theres no blockage

 

[muggo]

Thanks for the quick reply.
Assuming the header tank is full, what exactly do you mean by "cut the cold feed just before it enters the primary pipework and make sure theres no blockage"?