CH problem onWorcerter 28i junior

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This is not a condensing junior. It was installed sept 04. The HW works fine but on the CH side when a demand is created it takes about 10 to 12 mins to get up to temp( all rads get hot, actually only 5 rads) then kettling occurs in the ~HX the O/H stat kicks in and shuts the gas valve and the auto air vent opens.

The pump works , the primary CH ensor tested OK and was even replaced but still no joy, Just wondering whether its scaling in the HX or a control boar problem but everything seems ok on the control board. If its scaling on the HX why isn't it causing a problen on the HW side? When the boiler was replaced I don't know if the rads where flushed or cleaned so maybe it is FeO2 deposits rather than scale so causing the problem on the CH side and not the HW?

Any advice appreciated.

Oh I forgot to add, gas valve is modulating ok, when demand is created it goes up to min and after a set time ramps up to the setting in the Mis, just doesn't modulate down before kettling occurs.

 

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Bump??

 

[gas4you]

Dont know if these are twin pipe heat exchangers or a plate HE, but sounds like hot system water is going bto DHW side instead of rads.

One of thw WB experts will put me and you right on this

 

[Gasguru]

This boiler has a bithermal heat exchanger, pump is only used for central heating.

Have you got TRV's on all the rads?. Are these all shutting down. You shouldn't have. Maybe the boiler bypass is blocked.

Try the pump at speed 3 ?. When the Junior was launched Worcester technical were never sure whether it would be OK to run the pump at lower settings so you may find yours is set to speed 1 or 2.

When the kettling starts is there a large difference in temperature between the flow and return, if so perhaps you have a blockage.

Otherwise maybe another strange pcb fault.

 

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Thanks for the replies.

There are only 5 rads on this system, the one in the bathroom doesn't have any trv.

Looking at the manual I can't see any integral bypass, there is definately no bypass been installed under the boiler. It has run ok for 3 years(since it was fitted apparently.

When the kettling occurs there s not a lot of difference between the flow and return pipe temps.


I have turned the pump down to 1 and if I also turn the CH dial down low the heating stays on longer before the kettling begins, even then it modulates down and closes then modulates up again the first 3 or so times it starts kettling, it is only after this that it shuts down(presumeably because of OH stat kicking in) and goes into lockout( warning light flashing once per second).

Am I right in assuming there is scaling up in the CH part of the HX and that turning the pump speed down prevents the kettling occuring at as quickly as water isn't been pumped through as fast, as a consequence is it worth trying fernox boiler silencer before I think about new HX or do you still think it could be some weird PCB problem?

Btw any idea on the price on a HX for one of these boilers?

 

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bump

 

[Gasguru]

The boiler was never installed correctly. There should be a radiator fitted with two lockshield valves (to prevent user adjustment). This is normally installed in the hallway or perhaps the living room (ie a resonalbly large area). This room also has a room thermostat fitted to shut the boiler down when the system is up to temperature.

Set the pump speed back to 3 (as advised in the manual). The boiler has its own bypass but may not be sufficient hence requirement for minimum flowrate (see manual).

Just having a small bathroom radiator without a TRV may not provide sufficient flowrate through the boiler. What happens if you leave another large radiator with the TRV set to maximum temperature (ie to keep the flowrate through the boiler high)?

Boiler silencers may help a little with the kettling noise but I doubt they make any difference to increasing the reduced thermal transfer of scale.

No idea on cost for heat exchanger but wouldn't be surprised at £300 plus.

 

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Thanks for your reply gasguru, understand your reasoning re the size of the bathroom radiator and will check what effect having trv on lfully open on the lounge rad has but this system has apparently worked OK for 3 years so must be something new causing overheating and kettling in HX, either scaling or pcb fault or something else. I will try fernox boiler silencer tomorrow but at £300 it might be just as well to advise the customer to get a new condensing boiler.

Again thanks for your advice

 

[gas4you]

Are you in a hard water area? If so this would usually only affect the DHW side of things.

As this only happens on the ch I would suspect gunk in the system or maybe a failing pump.

 

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Well I took the I took the heads of 2 trvs in the lounge and dinng room but boiler still kettling and o/h stat looking boiler out.


This is not a hard water area, no scale on kettle at all so that shouldn't be a problem on a HX in a 3 year old boiler.

Customer says the system was cleaned and inhibitor put in when the boiler was changed and after draining it down it did look spotless, no sludge or disclolouration at all in the water.

Have checked pump(OK) and primary CH stat(OK_ I even replaced this) , have been on phone to worcester and they said will be blockage in HX, can't find anything else wrong so this must be it which means new HX at £150 +VAT or is removing it and flushing it worth giving a go?


I still can't understand how a blockage occurs bad enough to cause kettling on a 3 year old boiler on an extremely clean system??



ANy other ideas or suggestions anyone?

 

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Bump- opinions anyone?

 

[Agile]

First I dont assume anything unless I have checked it!

You can establish if there is a blockage in the system by a thermal survey.

What you have said ( if correct ) implies that the boiler is not modulating correctly. Have you checked the min/max gas valve settings???

At the end of the day the PCB is responsible for controling the boiler but few CORGIS can eliminate all the other aspects before changing it!

Tony

 

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Yep checked the modulation, hitting min and max values, undergoes min rate then ramps up in the required time Agile