Open vented system,advised to cap off expansion pipe??

Hi,
im a GSR engineer but not from a plumbing background.
I have a customer with an open vented system that is having to bleed radiators downstairs and upstairs every few weeks.
I asked a plumber about this and he said it was either
1. A leak allowing air into system
2. The expansion pipe rises above the F&E tank by only 300mm and that this would allow air into the system.(there is almost no extra room to extend the expansion pipe above the tank due to limited space)
The solutions are to obviously find and stop the leak if there is one.
Or he also advised (and this were I have serious doubts!) to put a stop end on the expansion pipe and any expansion in the system will travel back up the return pipe into the F&E tank.
If after a few weeks the problem reoccurs then I obviously have a leak and could take the stop end off the expansion pipe.
This is my first time posting here so only helpful advice please,be gentle!

 

Do not cap off the expansion

 

Difficult to say exactly without inspection.
Could be a small leak somewhere esp if ground floor suspended, does expansion tank seem to be filling regularly?
Could be build up of sludge in system creating Hydrogen in the rads, check for cold spots usually bottom of rad cooler than the top.
Take a peak inside the expansion tank, if its really dirty with sludge that could be the problem.
As Gasmk1 said do not cap off the expnsion pipe

 

Thanks I new that didn't sound right but this guy that advised has been a plumber/heating engineer for 20 years...makes you wonder!!

 

It never ceases to amaze me !
As above, don't stop end the vent.
It could be pulling air in the vent. A lot depends on the position of the pump in relation to other things.
You could send photos of pump, tanks, etc

 

Cheers will do