Worcester 28CDi combi boiler over pressurising

[djavery]

Hi,

I've read most of the posts on the site about Worcester combi boilers over pressurising and the main faults seem to be:

1. Mains pressure water inlet open (or faulty)
2. Water to water heat exchanger faulty
3. Expansion vessel faulty

I am confident it isn't the first one as I've turned the water off and it still happens, but what checks can I do for the other 2 and establish which one is causing my system to build up pressure? My system goes from 1 bar to 2.5 bar in around a week.

Any help would be most appreciated.

 

[gas_man_gaz]

sounds like the water to water heat exchanger to me..
does the pressure rise to 2.5 bar and stay there? or does it go up wen the heating is on then drop back down wen the heating is turned off?
do ya have a external filling loop? or do you jus have the one built into the boiler with the white key?

 

[davebgas]

Unlikely its the exp vessel if it takes a week - wozzas seem prone to plate to plates passing, done shed loads.

P.S has the customer had a water meter fitted?? Getting carp loads after one has been and there is no hot water exp vessel fitted, northumbrian water are advising companies that the non return on the meter is causing bot loads of trouble....

 

[djavery]

Thanks for the fast response guys .

Yes, the pressure rises and rises and never drops. It rises when the CH is on, but doesn't return to normal (as it used to). I let the pressure out using the pressure relief valve, but some other posts say this is not a good idea (leaking seat) and I should be using the white drain valves.

I have an internal filling loop (uses a plastic key and you turn a grey coloured knob).

I had a water meter fitted 5 years ago, but the boiler has only been playing up the last few weeks or so.

I've had a look on the Internet and I can get a new HE (part no 8716 142 903 0 - which I hope is the right one for the 28CDI) for £62.50 from Ezypart. I guess this is a good price?

Cheers guys, Dave

 

[bengasman]

Do you take the filling key completely out when not topping up?

 

[Gas2Air]

In your first post what do mean by eliminating the number 1 fault you turned the water off? for how long? This would also eliminate a p2p passing.

 

[djavery]

Yes, I take the key out each time.

I turned the water off to the boiler. Not for long, granted, but I could see no change. But I can see now, this might mask the other fault.

This brings me back to my original question of establishing what the fault actually is - what tests should I do in order to correctly fault find?

Thanks for the help so far.

 

[bengasman]

If it takes a week for the pressure to go up, you should leave the water off for a week, which is not practicable.
All you can do is eliminate other causes like gas developing in the rads.

 

[SteveZodiac]

If I understand correctly the pressure continues to rise even with the heating off AND mains water inlet off (not just the key removed from the filling loop) so water cannot be getting into the system. What about a faulty pressure gauge?

 

[Agile]

Pressurise to 2.5 Bar and disconnect cold in and hot out.

If its the plate to plate then water will continue dripping out of the connections.

Its not much fun changing the HE on your model and may be better to get Worcester to do a fixed price repair.

If you were changing it its better to replace the diverter valve at the same time!

Tony

 

[kevindgas]

sounds more like a split diaphragm in diverter valve if pressure rises with boiler turned off.

 

[Agile]

How could you explain that Kevin?

 

[djavery]

Sorry, I may have confused matters. With the water off I could see no noticeable increase in pressure (well for a few hours anyway). With Central Heating on pressure rises (as normal) but fails to drop down again fully when heating goes off. Day by day the pressure keeps going up and up.

 

[WSPH]

i've come across few of these, i changed the guage and diaphragm and it sorted it. based on the info you given i think it might be same sort of senario.

 

[gas_man_gaz]

i've changed loads of water to water heat exchanges (mainly on the cdi) for this exact fault and iam 90% sure it will be at fault in this case..
cant see how a diaphragm or even the the whole diverter on a cdi could create this fault