Faulty Expansion Vessel??

[paspatout]

I have a Saunier Duval Thelia 623 combi boiler which has started to lose pressure recently. There is no obvious leak from rads etc but the pipework runs are buried in the screed and I can\\\'t rule out a pin hole leak on the buried pipework. Other sypmtoms are that when topped up to 1.0 bar cold and then operated, the pressure is increasing to 3 bar at operating temperature and the boiler is making a constant low knocking sound. I\\\'ve had a look at the overflow pipe outside and there is no obvious indication that water is being discharged from it.
I spoke to a boiler engineer who I encountered through work who said it may be the expansion vessel - either faulty or in need of recharging. He went on to say however that because I\\\'ve topped up the system maybe half a dozen times already, introducing unconditioned water, I\\\'ve probably already condemned what is an old boiler (13 years old) and that I should consider replacement now rather than pay for a repair and a power flush on an old boiler. If that is the case I\\\'m wondering if I\\\'ll be able to buy a bit of time and keep it going for one more winter topping it up a bit every few days or recharging the expansion vessel if that is a likely diagnosis.
I\\\'d be grateful for any confirmation or otherwise on the likley cause and advice as to how I could keep it going a bit longer.

 

[Gasguru]

Read the FAQ's on expansion vessels. I'm surprised at 3 bar the safety isn't dripping, but normally this junk leaks on the left hand side underneath.

 

[paspatout]

Update to this is that taking the side of the casing off gives easy access to the expansion vessel in which there was zero pressure, though no water coming out of the valve. I reduced the pressure in the system to zero and tried to put 1.5 bar into the vessel with a pump but each time I watched as the pressure just slowly leached back to zero so I'm thinking it's a leaky schrader valve.

 

[Balenza]

Whats with the \\\ in your vocabulary ??.

 

[chrishutt]

I .. tried to put 1.5 bar into the vessel with a pump but each time I watched as the pressure just slowly leached back to zero...

Where did you observe the pressure reading?

 

[paspatout]

The pump has a built in pressure gauge

 

[chrishutt]

Most likely a perforated diaphragm in the expansion vessel. in any case you need a new expansion vessel - an external one is cheaper to buy and easier to fit if you have a space for it near the boiler.