High Pressure in unvented system

[Syd98]

Hi. I hope someone can shed a little light as too what the problem we have just developed.
We have a Megaflo unvented system, with a 2 year old Worcester Bosch boiler, serviced every year.
The system as worked perfect for years with water pressure sitting at 1 bar with no problems.
I have noticed that with either just hot water or central heating the system pressure is creeping up over a few days, to a point where when the system is cold it as increased from 2 - 3 bar.
I have isolated the filling loop and the problem still occurs, have also had the expansion vessel checked and recharged..!
Presuming that mains high pressure is somehow leaking across to the pressurised hot water side and causing the pressure to build up.
I am an engineer by trade so I understand how the system should work.
Anyone shed any light on to what the other possible problems could be etc.
Once I have an idea, I can then call in the professionals to fix the problem.
Thanks in advance.

 

[ianmcd]

you don't say which Worcester boiler you have the model would help

 

[Syd98]

Opps sorry its a Worcester bosch Greenstar 18i. Just a standard boiler, two years old and fully serviced.

 

[halfway]

have also had the expansion vessel checked and recharged..!

When you say checked, has the air valve been pushed in to check that there inst water at the air side of the diaphragm? If air was just pumped into a leaking vessel, it may not reveal that problem. I assume that there is water coming from your safety pressure valve when your system is running. Could also be some sort of blockage near the expansion vessel, though unlikely.

 

[ianmcd]

try turning the cold water supply stop cock off to your megaflow over night and see what happens, your boiler passes the system water through a coil inside your Megaflow at what ever pressure your primary system is set at but the secondary water inside the megaflow can easily be at a higher pressure , you may have a pin hole in the transfer coil, common on cylinders like Albion but not as common on Megaflows but not unknown

 

[Syd98]

Thanks for replies.. Sorry not been back, only been away. What's the best way to check for a pin hole in the transfer coil. Presume if not the filling loop can only be transfer coil pin hole, is there any other way mains pressure could force up primary sys pressure. Some one mention the heat exchanger in the boiler, but if this was cracked surely pressure would drop.
How about the cold water combination valve.
Yes expansion vessel checked and no water leaking at valve..
Thanks .

 

[Boilerman2]

The only way to prove the filling loop is sealing shut is to remove the flexible hose, having done that if the pressure continues to rise do as has already be suggested, overnight Turn off the cold feed to the megaflow, open a hot tap to release the stored pressure in the cylinder and in the morning see what effect it has had on the boiler pressure

 

[Syd98]

To all who replied, just a quick note to say problem sorted.
Problem was the filling loop, bought one from W.....s and thought this was ok, however after trying all the solutions offered the only explanation could be the filling loop, so bought another from Screwfix and hey presto problem gone way...
Thanks for all the help.