Worcester Bosch Combi Boiler Fault

[kevgh]

I had a qualified Corgi reg. plumber install me a Worcester Bosch Combi CDi35 boiler abour 8 years ago. It has recently developed a fault where the hot water goes cold mid-stream when running any tap/outlet. Sometimes it reverts back to hot but after a number of minutes, and sometimes I need to shut of the tap/outlet and open it once again to get the hot water back. I've also noticed that it now takes much longer before the water becomes hot when turning on any of my hot water taps/outlets.
Can anyone please diagnose what may be causing this fault.
Thanks very much.

 

[Agile]

Diagnose from a compter so you can change a part?

Proper diagnosis involves tests!

Anything else are just guesses.

Might be the DHW flow switch!

Best to get a competent engineer.

Tony

 

[ollski]

Diagnose from a compter so you can change a part?

Proper diagnosis involves tests!

Anything else are just guesses.

Tony

The whole point of the forum you're on ??

 

[Agile]

The whole point of the forum you're on ??

To give the best advice!

If you can diagnose the problem from the information given then tell us.

Otherwise, the best advice is to check the flow switch if he knew how to do that and if not to call an engineer.

Tony

 

[Jamespl]

I'm a Worcester engineer. It all depends on what version of the 35cdi you have to diagnose the fault MK I or MK II One thing to try is run the hot tap and check the lights on the front, the hot tap light should come on followed by the burner light. Now keep it running and check to see if the hot tap light stays on but the burner light goes off then comes back on after a short time, if the boiler is doing that then it could be the hot water heat exchanger that's blocked but as said I need to know which version you have.

 

[kevgh]

Hi James,
Thanks for responding. It is a mark II combi.
Regards,
Kev

 

[Jamespl]

Ok, you can do the same check as said above for the MK I and MK II. Another thing to check is run the tap and make sure the hot tap light stays illuminated the whole time a tap is running, if it goes out it will most likely be the diverter valve. If you take the white cover off and look at the back of it, it will help you diagnose a fault by telling you what light mean what.

 

[Agile]

Thats odd!

If the DHW demand light went out then I would expect it to be the flow switch not being activated rather than a diverter valve!

 

[Jamespl]

The diverter valve has a diaphram in it which has a lever on it so when the hot tap is opened the lever makes the micro switch so if the diaphram is nackered the micro switch won't engage so the hot tap light won't light up.

 

[Agile]

Yes, but if the diaphragm had managed to overcome the hysteresis and extend enough to operate the switch then its unlikely to then retract again unless the flow rate was greatly reduced.

In any case if it switches on at about 2 li/min then its bound to hold on at the usual DHW use range of 5-10 li/min

OP you could test that. SLOWLY turn up the DHW flow rate until it just turns on and then measure that flow rate in litres/minute. Should be close to 2 li/min

Tony

 

[kevgh]

Thanks Guys,

I will check these out.

Kev

 

[MJN]

Yes, but if the diaphragm had managed to overcome the hysteresis and extend enough to operate the switch then its unlikely to then retract again unless the flow rate was greatly reduced.

The usual problem is that a perishing diaphragm does indeed fail to remain in the open position, particularly if in the case of a thermostatic shower cartridge the demand for hot water reduces as the heated water reaches the outlet.

It is an extremely common failure mode for this type of diverter. I wrote a page a few years back giving an overview of the steps involved in replacing the diaphragm/diverter and get several e-mails a week from owners with exactly this problem.

Observing the lights as James suggests is a good starting point to help identify this as being the cause. Other failures can give rise to similar symptoms however this particular cause is so common it is a very good place to start the diagnosis.

Mathew

 

[Agile]

I agree that can happen with a thermo shower. but the OP says it happens with any tap.

Furthermore, he says it sometimes comes back on. However, measuring the min flow rate will show if the diaphragm is a problem.

Tony

 

[MJN]

That was just an example to illustrate the point; it is a very common failure mode which manifests itself in a variety of scenarios and usage and hence nothing 'odd' about the symptoms that the OP is describing.

Forget theoretical analysis in relation to measuring flow rate - the signs and symptoms are right there on the boiler and readily observable.

Mathew