worcester 28cdi pressure problem

[Chris D F]

Can someone help please?
I have a Worcester 28cdi boiler which is 10 years old. Very few problems over the years but a recurrent problem over the past two or three years concerns unequal pressures on the two functions (heating and water demands). If the water draw pressure is set at 1 bar for example the heating pressure shoots up to nearly 3 bar. If I set the heating pressure at say 1.5 bar, the water draw pressure drops to about .5 bar. The pressure on both cycles then starts to fall over the next few days. As the pressure falls there develops a loud knocking in the boiler whenever a hot water outlet in the house is turned off. I can find no evidence of leaks anywhere in the heating system, but I don't think this is a problem associated with the heating cycle. The boiler is regularly serviced and my maintenance engineer says that he has not come across this before.
Any suggestions would be very welcome.

Chris F.

 

[45yearsagasman]

I assume you've checked the expansion vessel charge? Do that first. Incorrectly charged vessels can cause your symptoms.

 

[Chris D F]

Thanks for replying. Boiler serviced last week! Expansion vessel charged to just over 1 bar. Is this sufficient?

 

[45yearsagasman]

Yes 1 bar is ok. I hope he charged that with no pressure in the system?

 

[Chris D F]

I assume so.

 

[Agile]

Never assume anything.

So many just add a little air and think that will be alright.

Well it isn't.

The system pressure needs to be at zero when adding air.

Tony

 

[Chris D F]

Thanks for that. Should I now release all the pressure in the system and check/recharge the pressure in the expansion vessel?

 

[45yearsagasman]

Yes, that is the correct way to recharge a vessel ie with zero pressure in the system.