Worcester 28 CDi pressure problems

[Phil29]

Hi guys,

First time posting on the forum and could do with a little help please.

We have a Worcester 28CDI combi boiler and over the last few weeks the pressure has been rising by itself to about 3 bar then lowering a little, then back up to 3 bar and so on. I released some pressure out so that it was back down to about 1.5 bar but then it steadily crept up again.

So i started searching through the forum for answers and it seems that the this would suggest that the water to water heat exchanger needs replacing. Could anybody confirm these symptoms suggest that please?

Also while searching for info and prices of the part over the last week i noticed that the pressure had dropped to 0.8 bar. So i refilled to 1.5 and then it seemed to hold steady over the next few hours which puzzled me somewhat. Then as leaving the house that day i noticed the overflow pipe outside was dripping water out. So it seems that the Pressure relief valve is letting pressure out at whatever pressure the system seems to be at.
Would this suggest that the Pressure Relief Valve has become faulty after venting the built up pressure for a week or so?

Any Advice appreciated guys.

Thanks, Phil.

 

[Aquaheat]

Hi Phil,

Increasing pressure could be one of two things.

1/ filling loop passing

2/ DHW heat exchanger pin holed.

If i remember correctly then the 28cdi has an integral filling loop which requires a key to allow filling. If you have no external filling loop and the key is removed then this will negate a passing loop.

This will then lead to the heat exchanger. To test this you can isolate the cold supply to the boiler and if the pressure then remains the same then this will prove the faulty HEX.

As for the PRV it does stand a good chance that this will require replacing aswell as they do tend to refuse to seat properly again.

Shouldn't cost too much to repair.

 

[Phil29]

Thanks for the quick reply jonjb.

Yes it does require a key to refill the system this however was removed when the pressure was increasing. I will try the test you suggested although with the PRV releasing pressure all the time now it's hard to see what's really going on due to the pressure rising when on and then falling when off.

I'm by no means experienced when it comes to changing parts on boilers but i am a pretty hands on person so are these easy enough to change? i.e any possible breakages or problems with changing them or is it pretty straight forward.

I take it the system will need draining down too? Could you tell me how to refill the system properly without creating airlocks in the system or boiler after the replacement parts are fitted? Is there a specific order of things on this type of system?

Again any help is much appreciated. Thanks in advance for your time.

Phil.

 

[Aquaheat]

They are a pretty easy change and the system does not always have to be drained down to replace although depending on the condition of the isolation valves on the flow and return to the boiler it may be beneficial to drain down. (they have a tendancy to leak afterwards).

Its quite hard to airlock a pressurised system so i wouldn't worry too much on that side of things but do remember to put inhibitor back into the system when you refill. If you should have any problems with that then there are numerous posts regarding airlocks etc it would depend on what was airlocked as to how you would proceed.

Depending on whereabouts in Staffs you are then if you have any problems you can feel free to contact us for advice or help. (details in profile)

 

[Phil29]

Thanks again for you help jonjb.

I'll do the test and then proceed with changing the Heat exchanger & PRV if it all checks out.

I'll do a search for how to add in the inhibitor you mentioned. Glad you mentioned that as i may not of found out that it needs to be added into the system.

Will let you know how i get on with it.

Thanks again, Phil.

 

[nickso]

do not use the isolation valves on a cdi to isolate it. that way lies certain doom. you might get away with using the cold mains isolation if you have just stepped in ****. i would do anything to avoid closing a cdi iso valve.

drain the boiler down using the boilers draincocks or the PRV if they are blocked. be prepared to change the PRV if you use it (easy job)

 

[namsag]

YEP as nickso says dont use the isolation valves ,

 

[Aquaheat]

Did we not just cover that?

 

[nickso]

They are a pretty easy change and the system does not always have to be drained down to replace although depending on the condition of the isolation valves on the flow and return to the boiler it may be beneficial to drain down. (they have a tendancy to leak afterwards).

Did we not just cover that?

not really. you seem to be saying that using the iso valves might be ok depending on their condition, whatever that means.

 

[Aquaheat]

Well its about 50/50 with them, some do some don't, you can tend to tell whether they are going to by how stiff the buggers are to move to start with. There are also signs of a little water egress sometimes aswell, never occuring as a drip as it is too faint but a little scale build up can sometimes be seen.

As you can read, i suggested that a drain down is not always needed but offered what the results may be if the OP were not to go down that road.

 

[nickso]

Well its about 50/50 with them, some do some don't, you can tend to tell whether they are going to by how stiff the b*****r are to move to start with. There are also signs of a little water egress sometimes aswell, never occuring as a drip as it is too faint but a little scale build up can sometimes be seen.

As you can read, i suggested that a drain down is not always needed but offered what the results may be if the OP were not to go down that road.

if you had offered the real results of a cdi iso leaking you would have told him they are a complete **** to replace, certainly not a job an novice wants to take on.

 

[Aquaheat]

Your not wrong there!!

 

[ollski]

The only fairly safe way to avoid them leaking is to take out all the pressure before you turn them....but I would never touch them, had a frigger the other month that leaked from the iso valve o ring after changing the expansion vessel.... was there 9 hours!!

 

[nickso]

The only fairly safe way to avoid them leaking is to take out all the pressure before you turn them....but I would never touch them, had a frigger the other month that leaked from the iso valve o ring after changing the expansion vessel.... was there 9 hours!!

forget the diverter, the vessel is the worst job on these turds. i only replace them if they are leaking, otherwise an external one gets fitted.

 

[ollski]

The only fairly safe way to avoid them leaking is to take out all the pressure before you turn them....but I would never touch them, had a frigger the other month that leaked from the iso valve o ring after changing the expansion vessel.... was there 9 hours!!

forget the diverter, the vessel is the worst job on these turds. i only replace them if they are leaking, otherwise an external one gets fitted.

It was leaking but the trouble was I had the genuine part ordered...if it were me I would have cut the pipe and capped it then fitted an external!
Took 5 drain downs as it was moving parts which had never been moved before and needed another full strip and couple of new o rings every time.....never again.