pressure loss in Vailant combi

[frank12]

Hi. I have a combi vailant boiler, type vw20. Apart from one gasket and two thermocouples the applance has been very reliable for 20 years. However a new problem has arisen. The pressure drops very quickly. A couple of days ago a service fitter re-pressurised the expansion vessel. which he said was full of water. The appliance worked Ok for a while but the pressure dropped as soon as the "call for " heat was satisfied.

Do I need a new expansion vessel? I note that there are two available. One at about 20 pounds the other at 100 pounds. What is the difference?
How easy is it to change the vessel?
Any information would be grateful.
Thanks Frank

 

[Stinkwheel]

Well i dont knw anything about your particular boiler or which vessel you should use Im afraid but ...........

If you expansion vessel has filled with water its no good, they should be half water and half air seperated by a diaphragm. Water filled means knackered diaphragm.

As i understand it you do not need to replace the eactual expansion vessel, you just need one in the same pipe-line.

 

[chrishutt]

Water filled means knackered diaphragm.

Not so. The air charge may have been lost due to very minor leakage over many years. the diaphragm may be perfectly serviceable.

You say that your pressure is dropping quickly after the heat up period, but you don't say if it increases first during heat up. Could you answer that question?